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February 8, 2022

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(Harte et al., 2011). In this sense, the individual, agency, and taxpayers/recipients of public services all benefit.

Recommendations for Action
Investing in wellness programs that focus on preventing illness and maintaining health influences medical costs and absenteeism. Such programs can go a long way in helping to creative a high-performance environment, lower health care costs, and reduce absences (Harte et al., 2011). Funding for fitness programs could be included within agency budgets or agencies could seek state funding per North Carolina General Statute 130A-224 as it pertains to public health. This statute provides a full mandate for public health issues, including injury prevention (Stier et al., 2012). It is a collaboration worth exploring further. A partnership with the North Carolina Public Health Department would be advantageous to incorporating injury prevention programs. To justify continued funding, it would be necessary to routinely evaluate training methods (Ferguson et al., 2011). Evaluation results can be useful by also pointing out deficiencies or areas where improvements can be made. It is the obligation of criminal justice practitioners and researchers alike to render professional judgment to improve the well- being of those in the field (ACJS, n.d.). Poor health and fitness carry higher risks of injuries and absences, which in turn cost agencies and taxpayers more money. Therefore, to justify standards and funding, evaluation is a necessary component. Data collection that provides evidence-based results would help to justify the continuance (or elimination of) such programs in the future.

The main objectives of fitness policy include reducing the risks of both work- related injury and absenteeism (and therefore increasing the health benefits of officers)

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and reducing agency and officer costs by way of fewer medical expenses and less absenteeism. The stipulation is that data should be collected and funding should be contingent upon evidence-based results. This could also be a beneficial form of accountability.

The purpose of providing alternatives is to offer optional approaches in case the initial, intended proposal does not completely work (Bardach, 2012) or needs to be adjusted in some way. The current interest lies in reducing workplace injuries and absences. Employers are aware that health and work are interrelated and that employers have a role in facilitating job retention and wellness (Irvine, 2011). Therefore, they are key players in policy change.

Recommendations for Further Study
The results of this study are limited to what was found among six law enforcement agencies within one region of North Carolina. The intent was to concentrate mainly on the patrol function within policing; however, with OSHA 300A summary statistics, it is difficult to do that. Therefore, it is suggested that actual OSHA 300 forms be analyzed to select out how injuries and absenteeism among patrol officers might differ compared to other sworn or nonsworn positions within an agency. This analysis would require more time and resources because those reported on the OSHA 300 form as injured or absent are listed by name; therefore, confidentiality would first have to be considered, and with the assistance of human resources a role distinction would have to be made among names as to pull out patrol officers. Alternatively, perhaps those patrol officers could be contacted and interviewed to determine whether the injury or absenteeism was indeed work-related.

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It might also be interesting to compare differences in fitness levels and requirements for specialty teams (for example, SWAT, K-9) versus patrol as specialty teams generally require more strict fitness levels. Though the responsibilities differ, there may be a significant difference in injury and absenteeism rate when compared to patrol officers. Also, because more stringent requirements are often a condition of belonging to such specialty teams, how or why this policy was rationalized and decided might be explored to further test RCT or to test additional decision-making theories.

Conclusion
This study was conducted to address the gap in the literature regarding why police academy cadets have required fitness standards but not all law enforcement agencies require a maintained fitness level as a condition of employment in North Carolina. In the hiring process, a critical qualifying factor is the use of fitness testing; however, if testing is used agencies need to understand fitness assessments if they are to implement required standards (Cooper Institute, 2014). During this research, very limited studies were found specific to North Carolina agencies and fitness policy, and none were found that specifically examined OSHA-reported injuries and absenteeism compared to various agency fitness policies.

Providing quality public services to the community is demanding and often agency resources are stretched thin. Agencies are trying to find that delicate balance with limited resources, whether dollars, manpower, or time. Agencies discussed varying reasons for having or not having physical fitness policies in place, but regardless; all stressed the importance of officer safety which can be increased by paying attention to officer wellness (Office of Community Oriented Policing Services, 2015).

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Research continues to support the importance of a police officer’s physical and psychological health, but fitness is only one piece of a larger puzzle. Improving workplace productivity calls for a comprehensive strategy that includes integrating evidence-based measurement because it is important to show that the health and productivity of the workforce directly relates to the health and well-being of the organization (Harte et al., 2011; Office of Community Oriented Policing Services, 2015).

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Appendix A: North Carolina Regions

Reference: Piedmont Triad Council. (2012). Retrieved from http://www.ptrc.org/

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Appendix B: Piedmont Triad Region Law Enforcement Agencies

Agency Population 50,0000 – 500,000
Alamance County yes
-City of Burlington yes
Caswell County no
Davidson County yes
Davie County no
Forsyth County yes
-City of Winston-Salem yes
Guilford County yes
-City of Greensboro yes
-City of High Point yes
Montgomery County no
Randolph County yes
Rockingham County yes
Stokes County no
Surry County yes
Yadkin County no

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Appendix C: Population of Midsize Law Enforcement Agencies in the Piedmont Triad Region of North Carolina

Agency 2010 census population
Alamance County Sheriff’s Office 151,131
Burlington Police Department 49,963
Davidson County Sheriff’s Office 162,878
Forsyth County Sheriff’s Office 350,670
Winston-Salem Police Department 229,617
Guilford County Sheriff’s Office 488,406
Greensboro Police Department 269,666
High Point Police Department 104,371
Randolph County Sheriff’s Office 141,752
Rockingham County Sheriff’s Office 93,643
Surry County Sheriff’s Office 73,673
Note. From “2010 Census Interactive Population Search,” by U.S. Census Bureau, n.d. (https://census.gov/). N = 11. Midsize = areas with 50,000-500,000 population.

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Appendix D: Sample Agency Fitness Policy

7.4.1 OBJECTIVE
The objective of the xxxxxxx Police Department is to develop and maintain a level of fitness in police personnel and to ensure their ability to accomplish assigned duties and provide satisfactory job performance without undue risk of injury or fatigue.

7.4.2 RESPONSIBILITIES
The Training Division is primarily responsible for the coordination and execution of all tasks pertaining to departmental directive 8.1.3.

7.4.3 COMPONENTS
The fitness services, provided at no charge by the department, includes:

1. An optional nutritional assessment. Officers complete a journal about their present health/eating habits.

2. A physical fitness assessment. The assessment consists of the following:
 Age, Height measurement, Weight measurement
 Body composition (Body Fat Percentage) – is measured by using skin fold calipers.  Blood pressure and heart rate - are measured by a sphygmomanometer.  Aerobic fitness - is measured by both a 1.5 mile walk/run and a 300-meter walk/run.  Strength fitness - is measured by:
 Maximum one-time bench press
 Maximum number of push-ups within one-minute
 Maximum number of sit-ups within one-minute
Once all tests are computed, each participant is categorized into one of five levels of fitness (age adjusted):
1) Very Poor 2) Poor 3) Fair 4) Good 5) Excellent 6) Superior Each participant receives a synopsis of his/her results.

7.4.4 ANALYSIS OF RESULTS
Each officer will be given a copy of their physical assessment synopsis if requested. They may at any time schedule an appointment with the appropriate training coordinator for a free consultation concerning their results.

7.4.5 DUTIES
The Training Division will ensure that staff members performing tasks assessments are capable of properly conducting physical fitness assessments, providing fitness counseling and providing a written printout of results on each participant. Testing will be conducted at the Public Safety Training Facility utilizing Departmental equipment.

Retesting – Any participant can be retested upon request of the participant. Portions of any segment or the entire program can be retested.

7.4.6 FILE MAINTAINANCE
Any information obtained which pertains to the health and wellness program will be maintained in the Training Division and monitored by the appropriate training coordinator.

STANDARD OPERATING PROCEDURE UNIT: Training Division
SUBJECT: HEALTH AND FITNESS OF SWORN

OFFICERS
NUMBER 7.4
EFFECTIVE DATE: 8/1/04 PAGE 1 OF 1
REVISION HISTORY: (Adopted 1/1/96) R1-7/1/99 R2 8/1/04 R3 8/10/07 R4 3.6.09 R5 10.14.13

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Appendix E: OSHA 300A Form

316
Appendix F: Participating Agency OSHA-Reported Injuries in Order of Fitness Policy Stringency

Agency Year # of
employees
Days away
from work
*Absent rate
(per 100)
Total
injuries
*Injury rate
(per 100)
(1) 2011 228 2 .88 12 5.26
2012 248 117 47.18 15 6.05
2013 254 154 60.63 10 3.94
2014 254 0 0.00 14 5.51
2015 262 12 4.58 12 4.58
(2) 2011 missing 55 missing 23 missing
2012 1206 248 20.56 61 5.16
2013 892 411 46.08 65 7.29
2014 1023 305 29.81 79 7.72
2015 954 389 40.78 66 6.92
(3) 2011 248 138 55.65 31 12.50
2012 261 191 73.18 38 14.56
2013 268 19 7.09 30 11.19
2014 266 75 28.20 12 4.51
2015 279 43 15.41 18 6.45
(4) 2011 901 426 47.28 66 7.33
2012 900 128 14.22 66 7.33
2013 929 471 50.70 65 7.00
2014 881 363 41.20 56 6.36
2015 881 266 30.19 38 4.31
(5) 2011 230 3 1.30 8 3.48
2012 238 164 68.71 12 5.04
2013 240 2 .83 8 3.33
2014 243 0 0.00 2 .82
2015 235 18 7.66 7 2.98
(6) 2011 164 15 9.15 17 10.37
2012 182 5 2.75 18 9.89
2013 171 25 14.62 15 8.77
2014 172 52 30.23 17 9.88
2015 171 267 156.14 20 11.70

* Absenteeism/injury rate is total number of days away from work (or total number of injuries) divided by total number of employees times 100 (rounded to nearest hundredth)

317
Appendix G: Agency Questionnaire
Agency:

# of sworn officers:

Title:

Does your agency have a physical fitness policy? (please include copies of years 2011- 2015)

If YES

is it mandatory or voluntary?

how long has the policy been in place?

What is the rationale behind having/not having a fitness policy in place, or for changing policy?

Who are involved decision makers in fitness policy? Who ultimately decides?

What factors determine or affect fitness policy? (ie: resources, manpower, cost, etc.)

Was policy intended to address work-related injuries and illnesses?

Are you aware that North Carolina provides a full public health mandate that includes injury prevention?

If no, would you want more information?

If yes, is that part of the rationale for having/not having a fitness policy?

318
R E S E A R C H Open Access
Accuracy of peak VO2 assessments in career
firefighters
Dana C Drew-Nord 1*, Jonathan Myers 2, Stephen R Nord 3, Roberta K Oka 4, OiSaeng Hong 1 and Erika S Froelicher 5 Abstract
Background: Sudden cardiac death is the leading cause of on-duty death in United States firefighters. Accurately assessing cardiopulmonary capacity is critical to preventing, or reducing, cardiovascular events in this population.

Methods: A total of 83 male firefighters performed Wellness-Fitness Initiative (WFI) maximal exercise treadmill tests and direct peak VO 2 assessments to volitional fatigue. Of the 83, 63 completed WFI sub-maximal exercise treadmill tests for comparison to directly measured peak VO 2 and historical estimations.

Results: Maximal heart rates were overestimated by the traditional 220-age equation by about 5 beats per minute (p < .001). Peak VO 2 was overestimated by the WFI maximal exercise treadmill and the historical WFI sub-maximal estimation by ~ 1MET and ~ 2 METs, respectively (p < 0.001). The revised 2008 WFI sub-maximal treadmill estimation was found to accurately estimate peak VO 2 when compared to directly measured peak VO 2.

Conclusion: Accurate assessment of cardiopulmonary capacity is critical in determining appropriate duty assignments, and identification of potential cardiovascular problems, for firefighters. Estimation of cardiopulmonary fitness improves using the revised 2008 WFI sub-maximal equation.

Background
Every 23 seconds a fire in the United States requires the services of a career or volunteer fire department [1].

Sudden cardiac death is the most common cause of on-
duty death among firefighters and occurs at higher rates than those found in similar occupations, such as police and emergency medical services [2].

A joint task force of the International Association of
Firefighters (IAFF) and International Association of Fire Chiefs developed the Fire Service Joint Labor Manage-
ment Wellness-Fitness Initiative (WFI) in 1997. Revi-
sions in the 1999 and 2008 WFI recognize the
firefighter as the “most important asset ” i nt h ef i r es e r - vice, and its intent is to improve firefighter function, on- duty effectiveness, and overall quality of life, while redu- cing morbidity and mortality r elated to fire fighting [3].

A major component of the WFI is assessment of fire-
fighters ’ cardiopulmonary capacity, with a stepmill test, sub-maximal, or a maximal exercise treadmill test. The
WFI mandates that firefighters have a maximal exercise
test at age 40 and every other year thereafter. The maxi- mal exercise test is intended to measure peak VO
2 (measured as ml/kg -1·min-1), which is an objective, clini- cal measure that defines the limits of cardiopulmonary
function. Peak VO
2 reflects an individual ’sa b i l i t yt o
increase their heart rate and stroke volume, and redirect oxygenated blood to muscles for work on demand. Exer-
cising at levels beyond which the cardiopulmonary sys-
tem can adequately supply oxygen (commonly termed
the anaerobic or ventilatory threshold, or VT) involves progressively greater degrees of oxygen-independent
muscle metabolism, which is dramatically less efficient than aerobic metabolism, and can compromise cardio-
vascular function [4].

Quantifying the energy demands of firefighting during
fire suppression is difficult due to the inherent dangers of fire suppression tasks. Most efforts to define the
arduous physical work demand requirements during
firefighting have been focu sed on establishing the level of metabolic equivalents (METs) (1 MET ≈ 3.5 ml of
O
2/kg/min) using simulated tasks. A MET is a multiple
of the resting metabolic rate and is commonly estimated using standardized equations [4]. 10 METs is roughly
equivalent to jogging a 10-minute mile; 14 METs is
* Correspondence: [email protected]
1Department of Community Health Systems, School of Nursing, University of California, 2 Koret Way, San Francisco, California 94143, USA Full list of author information is available at the end of the article Drew-Nord et al . Journal of Occupational Medicine and Toxicology 2011, 6:25 http://www.occup-med.com/content/6/1/25
© 2011 Drew-Nord et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

319
similar to many extended competitive activities such as running or rowing competitively, or bicycle racing at a high level [5]. The estimated METs proposed for fire-
fighting range from 9.6 [6] to 14 [7] (a peak VO
2 range
of 33.6 ml/kg -1 min-1·to 49 ml/kg -1·min-1). Recent analy- sis of physical aptitude tests a mong firefighter recruits demonstrated that male recruits ’ average VO 2 require- ment was 38.5 ml/kg -1·min-1 (11 METs) to complete a
timed simulated firefighting assessment course [8]. Mea- surement of functional capacity in 23 firefighters sug- gested that a mean of 41.54 ml/kg
-1·min-1 (11.9 METs)
is required to complete stan dard fire suppression tasks while wearing personal protective equipment [9].

Firefighting work demands can be extreme and accu-
rate assessment of cardiopulmonary status, as well as
detection and treatment of any underlying cardiovascu-
lar disease, is critical to insure firefighter fitness for duty and prevent on-duty cardiac events or death. The 1999
WFI sub-maximal exercise te st was found to overesti-
mate true peak VO
2 in individual firefighters [10]. Con-
cern about overestimation led to a revised equation for estimating peak VO
2 from sub-maximal exercise tread-
mill tests in the 2008 WFI.

Materials and methods
Given that previous sub-maximal exercise test results in the WFI were shown to overestimate peak VO
2,a n dt h a t
the WFI maximal exercise t readmill protocol has not
been validated for accuracy in the literature, this study was undertaken to assess the validity of both the maximal and revised sub-maximal exercise treadmill peak VO 2
estimates in firefighters. Specifically, the present study tested the following comparisons: (a) estimated maximal heart rate (220 - age) to actual measured maximal heart rate; (b) WFI maximal exercise estimated peak VO
2 to
directly measured peak VO 2;c )a v e r a g e dp r e - r e v i s i o n sub-maximal estimated peak VO 2 to revised sub-maximal
estimated peak VO 2; and (d) directly measured peak VO 2 to revised WFI sub-maximal estimated peak VO 2.

Study Setting and Participants
The study setting was a medium-sized suburban fire
department in the eastern region of the San Francisco
Bay Area in northern California. This department serves approximately 163,000 citizens and covers 46 square
miles. All firefighters (N =1 0 5 )a s s i g n e dt os u p p r e s s i o n duties were recruited, including firefighters, firefighter/ paramedics, firefighter/engineers, firefighter/captains and battalion chiefs. There were no women suppression fire- fighters in the department studied. This is consistent
with national career firefighter statistics as women only represent approximately 4.5% of the fire service [11]. All testing took place during a five-week period between
December 2008 and January 2009.

Inclusion criteria for participation required that each participant had successfully completed a WFI examina-
tion within the previous nine months and achieved a
minimum of 10 METs (peak VO
2 of 35 ml/kg/min), on
either a sub-maximal (using the pre-2008 equation), or
maximal exercise treadmill test. Exclusion criteria
included injury, illness, or scheduling conflicts that pre- cluded testing during the study period. The final study population consisted of 83 male career firefighters from all suppression ranks in this department.

The study was conducted with approval of the Univer-
sity of California San Francisco Committee on Human
Research. Signed informed consents were obtained and
all testing was conducted during on-duty hours with the approval of the department and union local.

Testing occurred at an occupational health clinic
where previous WFI examinations for this fire depart-
ment had been conducted. A physician board certified
in internal medicine and occ upational medicine, and a
nurse practitioner experienc ed in exercise testing, per- formed all treadmills and direct VO
2 measurements.

Participants arrived on the day of scheduled testing with their assigned duty crew, with gym clothes and running
shoes appropriate for completing a maximal exercise
test.

Measurements
Data collection consisted of medical record abstraction for demographics, cardiovascular risk factors and exer- cise test information. Demographic characteristics
included age, rank, and years o f fire service. Definitions of cardiovascular risk factors were obtained from the
American Heart Association, Adult Treatment Panel III
(ATP III), The Seventh Report of the Joint National
Committee on Prevention, Detection, Evaluation and
Treatment of High Blood Pressure (JNC7), and the Cen-
ters for Disease Control and Prevention [12-15]. All
serum samples were analyzed at the same hospital-based
certified laboratory (Centers for Medicare and Medicaid Services Clinical Laboratory Improvements Amend-
ments (CLIA)). Cardiovascular risk factors of the partici- pants are summarized in Table 1.

Maximal Heart Rates
Maximal estimated heart rates were calculated as 220-
age. Directly measured maxi mal heart rates were deter- m i n e df r o mt h ee l e c t r o c a r d i o g r a ma tt h ep o i n to fv o l i - tional fatigue as determi ned by the firefighter and
corroborated by the direct VO
2 assessment indicating
that they had crossed the VT.

Maximal Exercise Treadmill with Direct Peak VO 2
Assessment
All 83 participants completed a maximal exercise test
using the 2008 WFI Protocol with concurrent direct
peak VO
2 measurements. Maximal exercise treadmill
Drew-Nord et al . Journal of Occupational Medicine and Toxicology 2011, 6:25 http://www.occup-med.com/content/6/1/25

320
tests were considered complete when the firefighter
indicated volitional fatigue ( n = 83, see above), or if ter- minated by the testing physician due to concerns about
cardiopulmonary distress ( n = 0). The WFI protocol is a modified ramp protocol comprised of a 3-minute warm-
up period at 3 mph - 0% grade, followed by fifteen 1-
minute stages. Stage 1 begins at 4.5 mph and 0% grade,
with the treadmill incline increasing 2% and speed
increasing by 0.5 mph alternately in stages 2 through

15. The WFI maximal exercise treadmill estimates peak
VO
2 based on the American College of Sports Medicine
metabolic equation for running [16].

Peak VO 2 was obtained using the Cardio Coach
CO2™ VO2 Fitness Assessment System, Model 9001-
RMR (Korr Medical Technologies, Salt Lake City,
Utah). The Cardio Coach CO
2™ is an economical,
portable metabolic testing device that is feasible for
use in a clinic and has been previously validated for
measurement of peak VO
2 levels [17,18]. The Cardio
Coach CO 2
TM is a dual gas analyzer (O 2 and CO 2)t h a t
automatically calibrates to standard temperature and
pressure, dry at the beginning of each testing cycle.

The Cardio Coach CO
2™ measures heart rate using
the Polar T-31 heart monitor (Polar, Inc., Lake Suc-
cess, NY). Heart rate and VO
2 (ml/kg -1·m i n-1), VCO 2
(ml/kg -1 ·m i n -1 ), VE/VO 2, VE/VCO 2,V Ei nL / m i n , FeO 2%, Fe CO 2%, and respiratory exchange ratio are
graphically reported every 15 seconds. The Cardio
Coach CO 2™ uses the ventilatory equivalents method
(Ve/VO 2) to detect VT (Korr Medical Technologies,
2009).

Revised Sub-maximal Exercise Treadmill Assessments
In the latter part of 2008, the WFI introduced a revised equation for estimating peak VO 2:p e a kV O2 = 56.981 + (1.242 × TT) - (0.805 × BMI), where TT is the test time required to achieve target heart rate, and BMI is Body
Mass Index. The 2008 WFI calculates target sub-maxi-
mal heart rate (208 - (0.7 × age) × 0.85, whereas pre-
vious sub-maximal heart rates were based on (220-age)
× 0.85 [3,19].

Of the 83 firefighters who volunteered for the maxi-
mal exercise treadmill tests and directly measured peak VO
2, 63 subsequently completely their annually sched-
uled WFI examination, which included a sub-maximal
exercise treadmill test, within the subsequent four to
eight weeks. These subsequent WFI sub-maximal exer-
cise treadmill tests, using t he revised equation, took place under identical conditions as the study WFI maxi- mal exercise treadmill tests but without the direct VO
2 measurement. The sub-maximal test uses the WFI
treadmill protocol (see above) but terminates 15 seconds after the firefighter reaches their target heart rate.

Pre-revision Sub-maximal Exercise Treadmill Assessments Prior to the 2008 WFI revision there was no published
equation for the estimation of peak VO 2 from the sub-
maximal exercise treadmill. The estimated peak VO 2
was determined by duration of the test and stage
achieved [19]. Between one and seven historical sub-
maximal test results were available for each of the 63
participants, and were averaged to create comparative
historical variables.

Procedure
Participant ’s height, weight and resting blood pressure was measured. A resting electrocardiogram (ECG) was
completed, using the Welch-Allyn Schiller AT-10 6-
Channel electrocardiograph/treadmill (San Diego, Cali-
fornia). Upon completion of the resting ECG the Mason-
Likar lead configuration was modified to accommodate
the exercise treadmill [4]. The participant was then fitted with the appropriate 2-way non-rebreathable mask
(Hans-Rudolph, Inc., Shawnee, Kansas). The mask com-
pletely covered the nose and mouth of the participant
and was checked for air leaks to eliminate extraneous
room air from affecting the interpretation of peak VO

2. A standing electrocardiogram was obtained and the
treadmill was initiated. At test termination the firefighter recovered in the supine position. Available data from the maximal exercise treadmills is detailed in Table 2.

Statistical Analyses
Prior to all analysis all data were examined using stem and leaf plots and found to have normal distribution.

Table 1 Participant Cardiovascular Risk Factor Profile - Maximal Exercise Treadmill-Peak VO 2 Assessment (n = 83) Risk Factor Mean, SD
Body Mass Index (kg/m 2) 28.2 (± 3.9)
Systolic BP 117 (± 10)
Diastolic BP 69 (± 7)
Total Cholesterol* (mg/dL) 197 (± 38)
HDL** (mg/dL) 47 (± 11)
LDL** (mg/dL) 126 (± 36)
Cholesterol/HDL Ratio 4.35 (± 1.17)
Triglycerides (mg/dL) 118 (± 70)
*-fasting
**- HDL - high density lipoprotein; LDL - low density lipoprotein Table 2 Maximal Exercise Treadmill Data (n = 83)
Minimum Maximum Mean, SD
Resting Systolic 102 164 122 (±10)
Resting Diastolic 60 100 73 (±8)
Resting Heart Rate 42 91 63 (±10)
Maximal Heart Rate 130 194 174 (±10)
Peak VO 2 Actual 26.3 69.5 43.6 (±9.1)
RER* - Peak Exercise 0.90 1.28 1.09 (± .07)
* - Respiratory Exchange Ratio
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321
Dependent t-tests were conducted on all 83 participants to test for differences between:
1) Estimated maximal heart rate (220 - age) and
directly measured maximal heart rate.
2) WFI maximal exercise treadmill estimated peak
VO
2 and directly measured peak VO 2.

Additional dependent t-tests were conducted on the
results of the 63 participants who subsequently per-
formed a revised WFI sub-maximal exercise treadmill
test for differences between:
1) Averaged pre-revision WFI sub-maximal exercise
treadmill estimated peak VO
2 mean (converted to
METs) to revised WFI sub-maximal exercise tread-
mill estimated peak VO 2 (converted to METs).
2) Directly measured peak VO 2 (converted to METs)
to revised WFI sub-maximal exercise treadmill esti-
mated peak VO
2 (converted to METs).

All dependent t-tests were two tailed, with a =0 . 0 5
used for statistical significan ce. Statistical analyses were performed using SPSS Version 15.0 (SPSS, Inc., Chicago, Illinois).

Results
There were 105 active suppression male career firefigh- ters eligible for participation in the study. Of those, five were new hires who had not completed a WFI examina-
tion. Six firefighters chose not to participate; of the 94 choosing to participate 11 could not be scheduled for
maximal exercise tests due to injury, illness or schedul- ing conflicts resulting in an n =8 3f o rt h i ss t u d y .T h e participants ’ ages ranged from 26 to 57 years with a
mean of 41.1; 94% of the participants were Caucasian,
and 6% were Hispanic or African-American. The years
of firefighting ranged from 2 to 34 with a mean of 15.6.

Maximal Estimates and Measurements
The traditional maximal heart rate estimation (220 -
age) was significantly higher than measured maximal
heart rate (178.6 vs. 173.6 with a mean difference of

4.96 beats/min, p < 0.001, 95% CI: 3.03, 6.90). Estimated peak VO
2 was significantly higher than directly mea-
sured peak VO 2 (47.7 vs. 43.6, with a mean difference of

4.06 ml/kg/min, (1.16 METs) p < 0.001, 95% CI: 2.88,

5.23).

Sub-maximal Estimates and Measurements
Within four to eight weeks of the maximal exercise
treadmill tests 63 participants completed a sub-maximal exercise treadmill test (using the revised 2008 WFI
equation). Their average age was 40.19 years (± 6.9) and average years of firefighting was 14.4 (± 6.8). All firefigh- ter suppression ranks were represented in this sub-
group. The subsequent examination allowed for compar-
ison of the revised sub-maximal exercise treadmill peak VO
2 estimate to an averaged pre-revision (comparative
historical variable) sub-max imal exercise treadmill peak VO2 estimate and the recently obtained directly mea-
sured peak VO 2. For simplicity in reporting sub-maxi-
mal results all peak VO 2 results were converted to
METs (peak VO 2/3.5).

A statistically significant difference was found between pre-revision sub maximal exercise treadmill peak METs
mean estimates and revised sub-maximal peak METs
estimates (14.81 vs. 12.58, with a mean difference of

2.23 METs, p < 0.001, 95% CI: 1.86, 2.59) These findings support previous research determining that WFI sub-
maximal peak METs estimates prior to the 2008 revi-
sion were overestimated [10]. Revised sub-maximal
treadmill METs estimates did not differ from directly
measured maximal exercise treadmill METs, indicating
that the revised 2008 estimating equation is a reasonable estimate of METs (12.64 vs. 12.58 with a mean differ-
ence of .07 METs, p ≤ .76, 95% CI: -.39, .54) This repre- sents additional validation of the accuracy of the new
estimating equation [3]. All maximal and sub-maximal
comparisons are summarized in Table 3.

Discussion
Fire departments often struggle to determine fitness for duty for their members who return from an injury or ill- ness, prepare to embark on wildland strike teams, heavy rescue missions, or for daily work assignments. There
are ongoing efforts to define minimally acceptable and
safe fitness levels; levels that should be informed by the energy requirements needed during a firefighter ’st o u r of duty. Maximum directly measured METs for the fire-
fighters in this study ranged from 7.5 to 19.9, indicating that some participants might have a difficult time meet- ing the demands of the job while others appear ade-
quately fit. Four different methods of cardiopulmonary
assessment are compared here: direct measurement of
peak VO
2, estimated peak VO 2 derived from a maximal
exercise treadmill equation, historical average of pre- revision estimated peak VO
2 sub-maximal exercise
treadmills, and estimated peak VO 2 derived from the
revised (2008) sub-maximal exercise treadmill equation.

Directly measured peak VO 2 is the most objective and
considered the “gold standard ” of the four methods [4].

The difference observed in maximum heart rate
between directly measured maximum heart rate (while
wearing a non-rebreathable mask), and a 220-age esti-
mated maximum heart rate (part of the maximal exer-
cise treadmill estimation equation) provides some
explanation for the over-estimation. Estimated maximal
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322
heart rates were about 5 beats per minute higher than
those measured during peak exercise. Heart rates are a
method used on the fire ground to evaluate a firefigh-
ters ’ capability to re-enter the fire scene. Using target heart rates that exceed true maximums, or percentages
of estimated maximum heart rates that are inaccurate,
could result in dangerous duty assignments.

Assessment of direct peak VO
2 and maximal exercise
treadmill results indicate that the equation utilized by the WFI maximal treadmill over-estimates peak VO 2 by
an average of 4.06 ml/kg -1 ·min -1, or approximately 1 MET. If a firefighter’ s fitness level is less than optimal, or if they have underlying cardiovascular disease, this overestimation could lead to on-duty clearances that
could prove compromising.

Revised sub-maximal exercise treadmill peak VO2
estimates were compared to averaged pre-revision his-
torical sub-maximal exercise peak VO
2 estimates. The
average overestimation o f the historical mean was
approximately 2 METs. This finding supports the Mier
a n dG i b s o nr e p o r t( 2 0 0 4 )t h a tt h ep r e - r e v i s i o nW F I sub-maximal treadmill equation overestimated peak
VO
2, and that those equation results should be used
with caution for duty assignment decisions.

The comparison of directly measured peak VO 2 to the
revised sub-maximal exercise treadmill peak VO 2 esti-
mates (n = 63) found that there were no differences
between the two assessment methods. When comparing
revised WFI sub-maximal e xercise treadmill peak VO 2
estimates to previous years of testing, or to reports in the literature, careful consideration must be given to
which estimation method was used. The same task,
measured with different estimating equations, can result in different results as demonstrated herein.

Limitations and Strengths
The limitations of our study include the self-selection bias of the participants, the limited gender and ethnic demographics of the group (all male, predominantly Cau- casian), and the range in number of historical sub-
maximal exercise treadmill VO 2 estimates, resulting in a less than ideal comparison group. While testing was
completed within a four month period, it included the
winter holiday season which may have had a seasonal
influence on fitness behavior (resulting in an increase or decrease in exercise intensity). The composition of the sample is reflective of the department in terms of gender and ethnicity. There is an average four to eight week gap between the direct measure peak VO
2 and the sub-maxi-
mal exercise treadmill peak VO 2 assessment without any documentation of fitness behaviors. However, any fitness improvement on the part of firefighters in the interim
would have directed the results towards the null.

The strengths of our study include the number of parti- cipants, their range in age, rank, firefighting experience, and their experience with the WFI protocol. The avail-
ability of seven years historical data can be viewed as a strength. Use of the mask to measure peak VO
2 was
familiar to the participants as they routinely work with self-contained breathing a pparatus. The ability to per- form all testing components while on duty encouraged
participation. There were no incentives offered for parti- cipation. All testing was completed in the same facility using the same equipment and personnel, thus increasing consistency of testing and inter-rater reliability.

Clinical Implications
Firefighters who have been tested using earlier estimation equations may require careful explanation as to a notice- able drop in test results when using the revised 2008 WFI equation. Participants are likely to be disappointed to see a reduction in their “fitness level” when they have not chan- ged their patterns, nor workout habits, between testing cycles. Again, if a fire fighter falls into the lower fitness categories, or has underlying cardiovascular disease, inac- curate estimates could contribute to cardiac compromise.

Conclusions
In order to protect firefighters from potentially life- threatening cardiac situations it is imperative that
Table 3 Comparisons: Heart Rate, Peak VO 2, Estimated METs n Mean SD SEM 95% CI Lower 95% CI Upper t d Sig(2-tailed) Estimated Max.

HR: Actual Max. 83 4.96 8.87 .97 3.03 6.9 5.09 82 .00
HR
Estimated peak
VO
2: Direct
measure peak VO 2
83 4.06 5.39 .59 2.88 5.23 6.85 82 .00
Pre-revision METs
Est.: Revised METs estimate 63 2.23 1.46 .18 1.86 2.59 12.14 62 .00 Direct METs:

Revised Sub-maximal METs estimate 63 .07 1.85 .23 -.39 .54 .31 62 .76 Drew-Nord et al . Journal of Occupational Medicine and Toxicology 2011, 6:25 http://www.occup-med.com/content/6/1/25

323
exercise testing results are accurate, whether the test is being used for duty assignment or part of a comprehen-
sive risk assessment. The results from the revised sub- maximal exercise treadmill estimation equation appear
to accurately reflect directly measured peak VO
2 results.

WFI maximal treadmill peak VO 2 estimates should be
interpreted with caution, especially as they appear to
o v e r - e s t i m a t eM E T sb ya na v e r a g eo f1 .G i v e nt h e potential for over-estimatio no ff i t n e s s ,p r o v i d e r sw h o make fitness-for-duty asse ssments should consider the
energy requirements of the job, any underlying cardio-
vascular risk factors, and the method of testing used
when recommending return to, or continuation of,
duties. These findings support the continuation and
further expansion of reliable exercise testing of firefigh- ters, within the context of a cardiovascular disease pre- vention program such as the WFI.

Performing measured peak VO
2 and maximal exercise
treadmill tests can be challenging for fire departments to accomplish due to limited resources. The 2008 WFI
sub-maximal exercise treadmill test can be safely admi- nistered outside of a medical setting using tools that are often available within the fire department (treadmill,
stopwatch, and Polar heart m onitor). Disadvantages of
the sub-maximal treadmill test are the limited means for assessing underlying cardiovascular conditions, and the inability to determine maximal cardiovascular perfor-
mance directly. However, the revised 2008 sub-maximal
treadmill peak VO
2 estimation equation is a valid tool
to assess interim progress in cardiovascular training
programs.

Acknowledgements
The corresponding author would like to thank the Livermore-Pleasanton Fire Department administration, suppression, and support staff for their trust, enthusiasm, and participation in this project; the National Institute for Occupational Safety and Health (Grant #T42 OH 008429) for its traineeship support; and the UCSF School of Nursing Century Club for its financial support.

Author details
1Department of Community Health Systems, School of Nursing, University of California, 2 Koret Way, San Francisco, California 94143, USA. 2School of Medicine, Stanford University, Palo Alto VA Health Care System, 3801 Miranda Avenue, Palo Alto, California 94304-1290, USA. 3Premier COMP Medical Group, Inc. 5635 W. Las Positas Blvd., Suite 401, Pleasanton, CA 94588, USA. 4Palo Alto VA Health Care System, 3801Miranda Avenue, Palo Alto, California 94304-1290, USA. 5Department of Physiological Nursing, School of Nursing, University of California, 2 Koret Way, San Francisco, California 94143, USA.

Authors’ contributions
All of the authors contributed substantially to the conception, design, data acquisition and analysis, manuscript drafts and revisions of this study. Each has given final approval for publication.

Competing interests
Dr. Drew-Nord and Dr. Nord own the occupational medicine practice where this research was conducted and contract with various fire agencies to provide WFI services. This relationship was determined to represent no conflict of interest by the Institutional Review Board of the University of California, San Francisco. The remaining authors declare that they have no competing interests.

Received: 6 October 2010 Accepted: 25 September 2011
Published: 25 September 2011
References

1. Firefighter Statistics. [http://www.usfa.dhs.gov/statistics/estimates/nfpa/ index.shtm], retreived August 3, 2011.

2. United States Fire Administration: Firefighter Fatality Retrospective Study, April 2002/FA-220. Edited by: Corporation T: Firefighter Fatality Retrospective Study 2006.

3. Fire Service Joint Labor Management Wellness-Fitness Initiative. [http:// www.iaff.org/HS/Well/wellness.html].

4. Froelicher VF, Myers J: Exercise and the heart. Philadelphia: Saunders Elsevier;, 5 2006.

5. Fletcher GF, Froelicher VF, Hartley LH, Haskell WL, Pollock ML: Exercise standards. A statement for health professionals from the American Heart Association. Circulation 1990, 82:2286-2322.

6. Sothmann MS, Saupe K, Jasenof D, Blaney J: Heart rate response of firefighters to actual emergencies. Implications for cardiorespiratory fitness. J Occup Med 1992, 34(8):797-800.

7. Malley KS, Goldstein AM, Aldrich TK, Kelly KJ, Weiden M, Coplan N, Karwa ML, Prezant DJ: Effects of fire fighting uniform (modern, modified modern, and traditional) design changes on exercise duration in New York City Firefighters. J Occup Environ Med 1999, 41(12):1104-1115.

8. Williams-Bell FM, Villar R, Sharratt M, Hughson RL: Physiological demands of the firefighter candidate physical ability test. Medicine and Science in Sports and Exercise 2009, 41:653-662.

9. Adams J, Roberts J, Simms K, Cheng D, Hartman J, Bartlett C: Measurement of functional capacity requirements to aid in the development of an occupation-specific rehabilitation training program to help firefighters with cardiac disease safely return to work. American Journal of Cardiology 2009, 103:762-765.

10. Mier CM, Gibson AL: Evaluation of a treadmill test for predicting the aerobic capacity of firefighters. Occup Med (Lond) 2004, 54(6):373-378.

11. Women in the fire service. [http://www.nfpa.org/itemDetail.asp? categoryID=955&itemID=23601&URL=Research/Fire%20statistics/The%20U.S. %20fire%20service&cookie%5Ftest=1], retrieved August 3, 2011.

12. Men and cardiovascular disease risk factors. [http://www.heart.org/ HEARTORG/Conditions/HeartAttack/UnderstandYourRiskofHeartAttack/ Understand-Your-Risk-of-Heart-Attack_UCM_002040_Article.jsp], retrieved August 3, 2011.

13. Healthy weight - it’ s not a diet, it’ s a lifestyle!. [http://www.cdc.gov/ healthyweight/assessing/bmi/adult_bmi/index.html#Interpreted].

14. Third report of the expert panel on detection, evaluation, and treatment of high blood cholesterol in adults (ATP III). [http://hp2010.nhlbihin.net/ atpiii/calculator.asp?usertype=prof].

15. National Institute of Health National Heart Lung and Blood Institute: JNC7 Guidelines. 2006.

16. American College of Sports Medicine: Guidelines for exercise testing and prescription. Baltimore: Lippincott, Williams & Wilkins;, 6 2000.

17. Jensky NE, Vallejo AF, Ong MD, Schroeder ET: Validation of the Cardio Coach for sub-maximal and maximal metabolic exercise testing. Medicine and Science in Sports and Exercise 2005, 37(5):S231.

18. Dieli-Conwright CM, Jensky NE, Battaglia GM, McCauley SA, Schroeder ET:

Validation of the CardioCoach CO2 for submaximal and maximal metabolic exercise testing. The Journal of Strength and Conditioning Research 2009, 23:1316-1320.

19. International Association of Fire Fighters: The Fire Service Joint Labor Management Wellness-Fitness Initiative. Washington, D.C.: International Association of Fire Fighters;, 2 1999.
doi:10.1186/1745-6673-6-25
Cite this article as: Drew-Nord et al .: Accuracy of peak VO2 assessments in career firefighters. Journal of Occupational Medicine and Toxicology 2011 6:25.

Drew-Nord et al . Journal of Occupational Medicine and Toxicology 2011, 6:25 http://www.occup-med.com/content/6/1/25

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Program Effectiveness

Case Study – Clovis
Police Department

325

Clovis Police Department

In 2008, the Clovis Police Department suspended their health screening and fitness testing program. As illustrated below, the number of claims increased until 2012 when Pinnacle Training Systems began a wellness program for these officers.

0 10
20 30
40 50
60 FY 2010 FY 2011 FY 2012 FY 2013 FY 2014 FY 2015 FY 2016 Police Department Number of Claims
Reported by Fiscal Year
$0
$100,000
$200,000
$300,000
$400,000
$500,000
FY 2010 FY 2011 FY 2012 FY 2013 FY 2014 FY 2015 FY 2016 Police Department Medical Paid by Fiscal
Year

326

There has been a 50% reduction in total dollars paid from 2012-2016.

Despite the increase in the number of claims in 2016 and the medical dollars paid per fiscal year (the first increase since 2012), the total number of dollars paid and the dollars paid for temporary disability is still lower since the implementation of our program. This demonstrates the effectiveness of a well implemented program. While claims might increase in any given year, fit officers will return to work more quickly than those who are unfit reducing indemnity dollars paid. The Clovis Police Department has implemented a non-voluntary, incentivized, yet potentially punitive MOU for officers based on their overall health and fitness levels. Since the implementation of this program, the culture within the department has changed and more officers are motivated to engage in healthy behaviors. Furthermore, 4 officer’
s lives have been saved due to identifying abnormal EKG rhythms.

Based on the cost of the program and the total dollars paid since 2012 to 2016, the return on investment for this
department is 17.5:1
$0
$200,000
$400,000
$600,000
$800,000
$1,000,000
$1,200,000
$1,400,000
$1,600,000
FY 2010 FY 2011 FY 2012 FY 2013 FY 2014 FY 2015 FY 2016 Police Department Total Paid by Fiscal
Year
$0
$100,000
$200,000
$300,000
$400,000
FY 2010 FY 2011 FY 2012 FY 2013 FY 2014 FY 2015 FY 2016 Police Indemnity for Temporary
Disability Paid by Fiscal Year

327
Original Contribution
The Association of Aerobic Fitness With Injuries in the Fire Service Gerald S. Poplin *, Denise J. Roe, Wayne Peate, Robin B. Harris, and Jefferey L. Burgess * Correspondence to Dr. Gerald S. Poplin, University of Virginia, Center for Applied Biomechanics, Department of Mechanical and Aerospace Engineering School of Engineering and Applied Science, 4040 Lewis and Clark Drive, Charlottesville, V A 22911 (e-mail: [email protected]).

Initially submitted April 24, 2013; accepted for publication August 6, 2013.

The aim of the present study was to understand the risk of injury in relation to fitness in a retrospective occupa- tional cohort of firefighters in Tucson, Arizona, from 2005 to 2009. Annual medical evaluations and injury surveil- lance data were linked to compare levels of aerobic fitness in injured employees with those in noninjured employees. The individual outcomes evaluated incl uded all injuries, exercise-related injuries, and sprains and strains. Time-to-event analyses were conducted to determine the association between levels of fitness and injury likelihood. Fitness, defined by relative aerobic capacity (V O2max), was associated with injury risk. Persons in the lowest fitness level category (V O2max <43 mL/kg/minute) were 2.2 times more likely (95% confidence interval:

1.72, 2.88) to sustain injury than were those in the hi ghest fitness level category (V O2max >48 mL/kg/minute).

T h o s ew i t haVO2max between 43 and 48 mL/kg/minute were 1.38 times (95% confidence interval: 1.06, 1.78) more likely to incur injury. Hazard ratios were found to be greater for sprains and strains. Our results suggest that improving relative aerobic capacity by 1 metabolic equivalent of task (approximately 3.5 mL/kg/minute) reduces the risk of any injury by 14%. These findings illustrate the importance of fitness in reducing the risk of injury in phys- ically demanding occupations, such as the fire service, and support the need to provide dedicated resources for structured fitness programming and the promotion of injury prevention strategies to people in those fields. aerobic capacity; fire service; injury
Abbreviation: V
O2max, relative aerobic capacity.

The work demands for fire service employees are well doc- umented as requiring considerable physical abilities. In 1992, Sothmann et al. ( 1) recommended that these workers have a relative aerobic capacity (VO2max) between 38 and 42 mL/kg/ minute in order to meet the measured workload demand for firefighters while also maintaining a reserve capacity to re- spond to other unanticipated events. Since then, most fire de- partments have adopted minimum fitness standards, often
emphasizing aerobic capacity as a definitive measure of over- all fitness. Standard fitness assessments for the general pop- ulation typically have a set of norms that are scaled to age and sex, and results can range from poor to superior (2).

Opportunities for injury in the fire service are diverse. Per- sons with higher aerobic capacities should be able to consume more oxygen than those with lower aerobic capacities, and their bodies are likely to be more efficient at circulating oxygen to all systems and producing energy. Persons in the top levels of a fitness spectrum may not be as susceptible to microtraumas and may recover better from injury than their less-fit counter- parts (3–9). Hence, those with higher V
O2max should have a
lower potential for fatigue and subsequent injury. Con- versely, a high fitness level may also be an indicator that a person has an increased risk of injury, as these persons likely have greater exposure time to exercising hazards. In somefire departments, exercise has been shown to be the leading activ- ity associated with on-duty injuries (10 ).

The objectives of the present study were to establish and understand the relationship between fit n e s ss t a t u sa n dt h e risk of injury in a 5-year occupational cohort of careerfire ser- vice members. We hypothesized that firefighters deemed to be on the lower end of the fitness spectrum would be more susceptible to injury than their more fit colleagues.
149 Am J Epidemiol. 2014;179(2):149–155
American Journal of Epidemiology
© The Author 2013. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: [email protected].

Vol. 179, No. 2
DOI: 10.1093/aje/kwt213
Advance Access publication:

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MATERIALS AND METHODS
Population description, data sources, and years
As previously described ( 10), the present study includes data from commissioned employees of a medium-sized met- ropolitan fire department in the southwestern United States.

Briefly, the fire department operates 21 fire stations and re- sponds to nearly 520,000 permanent residents (with seasonal increases nearing 720,000 residents). Like many other mu- nicipal fire departments, this fire department requires an an- nual physical examination to assess fitness levels and provide medical clearance for each commissioned employee. Be-
tween 2005 and 2009, data for this study were obtained from 2 sources: physical assessments from annual clinic visits and department injury surveillance reports. The present study in- cluded all commissioned (noncivilian) employees of the fire department who were employed at some point in time during the study period. Approval for and oversight of the use of hu- man subjects was provided by the University of Arizona’s In- stitutional Review Board.

Physical fitness measures
Information collected from annual exams included anthro- pometric measures (e.g., height, weight, body fat percent- age), V
O2max, muscular strength, muscular e ndurance, and
flexibility. Aerobic capacity can be de fined as the highest rate at which oxygen can be taken up and utilized by the body during rigorous exercise ( 11). Aerobic capacity is ex- pressed as a rate that is referenced in either absolute terms (L/minute) or by relative measures (mL/kg/minute) to ac- count for individual size variations. In this study, we refer to studied relative aerobic capacity. V
O2max was categorized
into 3 levels of fitness, using the 25th and 50th percentiles as the cutoff points between the “less fit” (<43 mL/kg/
minute) and “high fit” (>48 mL/kg/minute) aerobic capacity categories, respectively. The use of a percentile as the cutoff was also used in 1999 by Lee et al. (12 ), who studied the re- lationship between cardiorespiratory fitness and cardiovascu- lar disease in a large observational cohort of men. In addition, the 25th percentile closely relates to the recommended mini- mum level of aerobic capacity (42 mL/kg/minute) suggested by Sothmann et al. ( 1) that has been adopted by the National Fire Protection Association and most municipal fire depart- ments in the United States (including those in this study’ s population). The 50th percentile was chosen as the cut off for the high fit designation because of the distributional char- acteristics of the population ’s data and to remain in accord- ance with current annual physical assessment methods used by the department. Neither age nor sex was directly consid- ered when assigning aerobic fitness levels, as fire depart- ments institute a minimum standard for aerobic fitness to help ensure that all commissioned fire personnel are capable of performing the myriad of critical job tasks and responses regardless of age or sex. Maximum aerobic capacity was es- timated using the submaximal incremental treadmill protocol developed by Gerkin et al. (13 ) and guidelines suggested by the Wellness Fitness Initiative of the International Association of Fire Fighters and the International Association of Fire Chiefs (14). In brief, each participant is hooked up to a heart rate monitor and made to walk on a standard treadmill. The tread- mill’s speed and incline are increased at prespeci fied time points until the subject’ s target heart rate is reached and V
O2max can be estimated. Direct measurement of V O2max
using oxygen and carbon dioxide analyzers and monitoring of electrocardiogram output is generally limited to speci fic indications because of its higher cost.

Injury defined
Injuries that occurred on the job were recorded either if they were reportable to the Occupational Safety and Health Administration or if they were deemed nonreportable but were documented internally because of the potential for the injury to progress to the point of requiring an insurance claim (e.g., due to cumulative or repeated trauma). It should be noted that field personnel work 24-hour shifts. For the pur- poses of these analyses, reported injuries known to be only internally documented incidents with no loss of function or ability to perform duties (by review of injury report details) were excluded. In addition, cardiac events (e.g., stroke, heart attack), along with heat exhaustion, stress, and other medical issues, were excluded from injury analysis. These events were considered more likely to be indicative of an underlying set of symptoms, conditions, or diseases than to be related to an injury sustained on the job.

Statistical analyses
Data from annual physical examinations and injury sur-
veillance reports were merged utilizing unique identi fiers, which enabled a direct comparison of persons with and with- out injury. Quantitative methods, notably time-to-event re- gression models, were used to evaluate the relationship between aerobic fitness and injury. Analyses were conducted for 3 separate injury outcomes: 1) any recorded injury; 2) in- juries resulting from physical exercise; and 3) any reported sprain or strain.

For time-to-event (i.e., first injury) analyses, cumulative incidence was estimated using the life table and Kaplan- Meier methods, which allow for censoring (i.e., no injury).

Incidence rates were assessed with respect to established fit- ness measures. These levels were set using methods similar to those used for establishing aerobic fitness levels (as previ- ously explained); however, in this case, sex was taken into ac- count for measures of body fat percentage, grip strength, and flexibility. The time-to-event analysis utilized repeated mea- sures in which each time point (observation) corresponded to a person ’s annual medical examination until the occurrence of injury or censoring. This method accounted for variable observation periods because some employees were introduced later in the study period (e.g., new employees), whereas others dropped out (e.g., retired, transferred), and it enabled a single person to contribute time at risk to each of the fitness levels based on his or her most recent physical assessment. Survival analyses were completed using Cox proportional hazard regres- sion models. Statistical analyses were conducted using Stata software, version 11.2 (StataCorp LP, College Station, Texas). 150 Poplin et al.

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RESULTS
At the end of each calendar year from 2005 to 2009, there were between 577 to 694 commissioned employees within
this metropolitanfire service. During that time period, 799 em- ployees underwent at least 1 physical examination and follow- up until their first injury event or censoring. On average, the clinic database accounted for approximately 87% of the
workforce population, and the mean age was 39.2 (standard deviation, 9.6) years. Table 1 displays select summary fitness measures of the study population across the study period. The mean for V
O2max for the overall population across time was

49.6 mL/kg/minute, with a 12.2-unit difference between the minimum and maximum mean values. In total, 773 injuries were reported (Table 2). There were 357 persons who sus- tained at least 1 injury of any type, 174 who sustained at least 1 exercise-related injury, and 294 who sustained at least 1 sprain or strain injury. The median lengths of follow-up were 2.5, 3.2, and 2.8 years, respectively. A previous descrip- tive analysis of this population (for the years 2004 –2009) demonstrated that 67% of all injuries were sprains and strains, and that number increased to 89% among exercise injuries.

Thirty percent of all injuries were reported to have resulted in lost time on the job ( 10).

Kaplan-Meier analyses (data not shown) indicated that the incidence rate for injury decreased and the median time to in- jury increased with increasing age (stratified into 10-year cat- egories) (P < 0.001). There were no signi ficant differences identified between sexes; however, women acc ounted for
only 5% of the study population (as is common for the fire service (15 )). Increases in body fat percentage were related to increased incidence rate, driven most notably by those in the highest tier (>36% body fat).

Table 3 displays the general summary characteristics of the incidence of injury outcomes for V
O2max levels and repeated
measures modeling. Log-rank tests indicated that there were statistically significant increases in incidence rate with a de- cline in V
O2max for each of the 3 injury outcomes. In addition,
persons with lower V O2max levels were likely to sustain any injury sooner than were those who were more fit, as indicated by a median time to injury of 2.24 years in level III (least fit category) compared with 4.07 years for level I (most fitc a t - egory; P < 0.001).

Cox proportional hazard modeling
Results from Cox proportional hazards models are pre-
sented in Table 4. The dependent variables in these models utilized time to first injury as a function of fitness. The hazard ratios for fitness are shown with respect to 2 modeling strat- egies: 1) V O2max adjusted for age and sex and 2) V O2max ad- justed for other measures of fitness (i.e., resting heart rate, grip strength, flexibility, body fat percentage, number of con- tinuous sit-ups and push-ups, age, and sex). With a hazard ratio of 0.959 for all injuries, a 1-mL/kg/minute increase in V
O2max decreased the risk of injury 0.041 times ( P < 0.001).

The amount of work needed to complete a given task in rela- tion to the amount of energy expended during 1 minute of seated rest is referred to as the metabolic equivalent of task.

For V
O2max, a single metabolic equivalent of task is approx- imately 3.5 mL/kg/minute. Thus, these results s uggest that improving one’s aerobic capacity by 1 metabolic equivalent of task would reduce the risk for any injury by approximately 14%.

Table 5 shows the relationship between the categorical lev- els of aerobic fitness (V
O2max) for the repeated measures anal-
yses. For each injury outcome, persons with a lower fitness status (e.g., level III) had a higher hazard ratio for injury than did those in the most- fit category. For example, persons with a V O2max between 43 and 48 mL/kg/minute (level II) were 1.38 times more likely to sustain any injury than were those in the top category of VO2max (>48 mL/kg/minute). The risk of injury increased with decreasing fitness level, as those with a VO2max less than 43 mL/kg/minute (level III) were 2.2 times more likely to have any injury than were those in the top V
O2max fitness category. The hazard ratios were also found to Table 1. Mean Values of Person-Level Descriptive Statistics, Tucson, Arizona, 2005 –2009 Variable No. Mean Minimum Mean Maximum Mean Mean Difference VO2max, mL/kg/minute 782 49.6 43.6 55.8 12.2
Resting heart rate, beats per minute 797 62.9 57.2 69.2 12.0 Total grip strength, lbs a 797 229.5 211.4 247.5 36.1
Flexibility, inchesb 782 5.8 4.5 7.0 2.5
% body fat 790 18.3 15.4 20.9 5.5
Abbreviation: VO2max, relative aerobic capacity.
a 1 lb = 0.45 kg.
b 1 inch = 2.54 cm.

Table 2. Annual Frequency of Injury Outcomes, Tucson, Arizona, 2005–2009
Year No. of
Employees
No. of
Recorded
Injuries
No. of
Sprains and
Strains
No. of
Exercise-related
Injuriesa
2005 577 128 84 30
2006 625 148 100 50
2007 659 174 120 62
2008 694 199 128 81
2009 667 124 89 45
a Of all exercise-related injuries, 85.2% were sprains and strains.

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be greater when the event outcome was restricted to time to first reported sprain or strain.

Effect modification
To assess the potential of effect modi fication of the rela- tionship by age, a simple age-strati fied analysis was com- pleted for all injury outcomes, as well as for sprains and strains. Age proved to be a signi ficant modifi er of V
O2max
(P < 0.001). Table 6 presents the crude hazard ratios for the all-injury model and the sprain and strain model in relation to age (<30 and ≥30 years of age) and overall, strati fied by aerobic fitness level. For both outcome types, the risk of in- jury among those with decreased V
O2max was higher in per-
sons younger than 30 years of age than in those 30 years of age or older. Thirty years of age was chosen as our demarca- tion value primarily based on incidence rates resulting from the Kaplan-Meier analysis described above. In addition, per- sons 30 years of age or younger had a consistently increased incidence rate, regardless of the injury type. Our previous de- scriptive study of injury distributions demonstrated that fire- fighters (median age, 31 years) sustained the most injuries (30.7%), with lower injury rates for engineers, paramedics, captains, etc. (10). Given the strong relationship between age and rank and the results from Kaplan-Meier estimates, it seemed suitable to use 30 years of age as our cutoff value for assessing potential effect modification.

DISCUSSION
In the present study, we sought to better understand the as- sociation between levels of aerobic fitness and the incidence of injury using a retrospective occupational cohort. The find- ings were consistent with our original hypothesis that lower fitness levels, as defined by VO2max, would be associated with increased risk of injury. Furthermore, these increased risks were modified by age, with a larger association between fit- ness level and subsequent injury in those 30 years of age or younger. The reduction in injury risk was signi ficant for all injuries, sprains and strains, and physical exercise injuries.

These findings are especially noteworthy considering that one third of work-related injuries in this population resulted from exercise activities ( 10), further indicating the need for fitness programs with improved structure and management
relevant to the high physical demands of the job.

A number of studies have assessed the relationship be-
tween various measures of fitness and the performance of a given task, with varying results. However, in contrast to our present study, few studies have focused on assessing the as- sociation among fitness, performance, and injury risk.

Two published studies have demonstrated an increase
in injuries associated with fitness or the implementation of fitness programs (9, 16). After a baseline treadmill test to assess V
O2max, participants of the Aerobic Center Longitudi-
nal Study had their physical activity levels assessed over a Table 3. Time-to-Event Summary Statistics Between Fitness Levels and Injury Outcomes, Tucson, Arizona, 2005 –2009 Fitness
Levela
Injury Type
All Injuries Physical Exercise Sprains and Strains
IRb No. at Risk Years at Risk c IR No. at Risk Years at Risk IR No. at Risk Years at Risk I 17.5 460 921 7.3 532 1116 12.2 482 996
II 21.1 287 442 7.2 332 541 17.2 312 483
III 29.9 d 235 338 13.3 d 263 407 25.1 d 242 355
Abbreviation: IR, incidence rate.
a The relative aerobic capacity for each level was as follows: I, >48 mL/kg/minute; II, 43 –48 mL/kg/minute; and III, <43 mL/kg/minute. b Incidence rate per 100 person-years.
c Contributed time at risk (person-years).
d Statistical significance (P < 0.05) between levels using log-rank test for equality of survival functions.

Table 4. Cox Proportional Hazard Models for Assessing Aerobic Fitness and Risk of Injury, by Injury Type, Tucson, Arizona, 2005 –2009 Injury
Outcome
Model
VO2max
a VO2max Full Modelb
No. of Observations HR 95% CI No. of Observations HR 95% CI Allc 716 0.959 0.946, 0.972 710 0.953 0.939, 0.968
Exercise-related 718 0.960 0.941, 0.979 714 0.953 0.933, 0.973 Sprains and strains 718 0.952 0.937, 0.967 712 0.947 0.932, 0.963 Abbreviations: CI, confidence interval; HR, hazard ratio; V O2max, relative aerobic capacity. a Adjusted for sex and age.
b Includes independent variables: r esting heart rate, grip strength, flexibility, % body fat, number of sit-ups performed, number of push-ups performed, age, and sex.
c There were fewer subjects in this category because of lacerations that occurred during the first clinic visit, which precluded follow-up for those subjects.
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12-month period ( 16). Increased risk of musculoskeletal in- jury was associated with increases in cardiorespiratory fitness (as measured by a treadmill test), as well as increases in the amount of reported weekly physical activity. Strati fied anal- yses by physical activity type suggested that the association between cardiorespiratory fitness and musculoskeletal injury was potentially driven by unmeasured intensity levels of ex- ercise. After instituting a new fitness program among United States Air Force service members to increase fitness and participation in fitness-related activities, the mean relative V
O2max increased significantly (6.04 and 3.24 mL/kg/minute among men and women, respectively) over 3 years of the
program ( 9). The number of injuries also increased during that time, which was likely a result of increased participation in exercise activities with no embedded injury prevention program.

Two studies failed to find an association between fitness and injuries. During an 8-week basic military training regi- men, musculoskeletal injuries were assessed in relation to baseline body composition (or body mass index), aerobic fit- ness (determined by the time participants took to run 3,000 meters), health assessment measures, and age ( 17). Signi fi- cant associations were observed at a univariate level between injury and a variety of variables, including age greater than 23 years, increased body mass index, slow run times, and dys- function of back or lower limbs. Multivariate logistic regres- sions showed no relationship between injury and aerobic fitness level; however, increased body mass index, minor back and lower limb dysfunctions, and mental dysfunctions were predictive of injury. In a study of manual material han- dlers, McSweeney et al. ( 18) found no difference between exercisers and nonexercisers in terms of the likelihood of reporting an injury. However, the authors noted that in- creased or regular exercise was likely to reduce absenteeism occurrence and duration. In another study among male mate- rial handler employees at 3 separate facilities, no associa- tion was observed between injury occurrence and absolute aerobic capacity; however, a signi ficant increase in injury risk was related to a decreased V
O2max, in addition to in-
creased body fat percentage (19). It is important to note that none of the studies described above used repeated mea- sures of fitness, unlike the present study. The added strength of being able to capture time-series data for the population increased statistical power of our study and our ability to con- trol for confounding effects both within and across cohort members.

Table 5. Hazard Ratiosa for Injuries by Levels Aerobic Fitness in Repeated Measures Modeling, Tucson, Arizona, 2005–2009
VO2max
Levelb
All Injuries ( n = 716) Exercise Injuries ( n = 718) Sprains and Strains ( n = 718) HR 95% CI HR 95% CI HR 95% CI
I Referent Referent Referent
II 1.38 1.06, 1.78 1.20 0.81, 1.77 1.61 1.21, 2.13
III 2.22 1.72, 2.88 2.53 1.76, 3.64 2.63 1.98, 3.50
Abbreviations: CI, confidence interval; HR, hazard ratio, V O2max, relative aerobic capacity. a All models were adjusted for sex and age.
b The relativeaerobic capacity for each level was as follows: I, >48 mL/kg/minute; II, 43 –48 mL/kg/minute; and III, <43 mL/kg/minute.

Table 6. Age-Stratified Hazard Ratiosa for All Injuries and Sprains and Strains by Fitness Level, Tucson, Arizona, 2005–2009
Fitness Level by
Injury Typeb
Total Age <30 Years Age ≥30 Years
HR 95% CI HR 95% CI HR 95% CI
All injuries
I Referent Referent Referent
II 1.38 1.06, 1.78 2.28 1.41, 3.71 1.15 0.85, 1.57
III 2.22 1.72, 2.88 3.43 2.10, 5.58 1.86 1.36, 2.53
Sprains and
strains
I Referent Referent Referent
II 1.61 1.21, 2.13 2.27 1.32, 3.90 1.40 1.00, 1.95
III 2.63 1.98, 3.50 4.48 2.63, 7.64 2.10 1.49, 2.96
Abbreviations: CI, confidence interval; HR, hazard ratio. a All models were adjusted for sex and age within the strata. b The relative aerobic capacity for each level was as follows: I, >48 mL/kg/minute; II, 43 –48 mL/kg/minute; and III, <43 mL/kg/minute.

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Results from our study indicated that there was a modi fi- cation of injury risk based on age of the person. In particu- lar, younger employees (30 years of age or younger) with a V
O2max below that of the high fit group (i.e., VO2max <48 mL/ kg/minute) had a higher risk of injury than did their older, less fit counterparts. This effect modi fication may be due to changes in job rank (and presumably job duties and exposure to external conditions) with increasing age in firefighters.

Typically, younger personnel hold the rank of firefighter, whereas promotion or career progression tends to lead into ranks of paramedic, engineer, and captain. For most emer- gency responses, firefighters are the first to enter an emer- gency scene and are thus subject to greater hazards, known and unknown emergent threats, and time-limiting stresses.

One exception includes the risks to which paramedics are ex- posed during calls involving advanced life support. Never- theless, the hazard pro file and exposure risk for those with the rank of firefighter can be considered greater than those of their team counterparts (e.g., engineer, captain, chief ).

Limitations
Although injury events that occurred before the first ob- served clinic visit were removed to avoid left-censoring bias (a product of data merging), there was no knowledge of previous injury history. In addition, analyses were re- stricted to the first specified injury event; therefore, recurrent injuries were not assessed. Future studies on recurrent inju- ries should enhance the understanding of injuries in this pop- ulation by differentiating between the risks of repeated injuries (i.e., the same injury type suffered multiple times by a person) and those of the repeatedly injured (i.e., persons who suffer from multiple injury types). The data assessed for this study also did not permit evaluation of intrinsic factors, such as central motor control (i.e., balance), skeletal abnor- malities, alignment of joints, and ligamentous laxity ( 3–5, 20–29). Ideally, inclusion of these factors would improve fu- ture studies ( 30–32), as intrinsic risk factors each in fluence local anatomy and biomechanical limitations.

Although V
O2max is linearly related to heart rate and en-
ergy expenditure, it is an indirect measurement of an person’s maximal capacity to do work aerobically (33 ). In the present study population, VO2max was estimated using a submaximal test protocol that was previously validated and has been used widely in the fire service ( 13). Two recent studies, however, have indicated the potential for submaximal tests to overesti- mate true aerobic capacity (34, 35). If true, any overestimation of V
O2max should not in fluence the regression modeling be-
cause the potential bias would be nondifferential. Of note, the distribution of V
O2max values in our study is considered
higher than that in the general population. Standardfitness as- sessments classify midrange ( “good”) aerobic fitness at 40 years of age to be 35– 38 mL/kg/minute for women and
42–45 mL/kg/minute for men (2). The notion that firefighters are more fit than the general population is supported by the characteristics of this study ’s firefighting population, who had an average age of 39 years and a mean V O2max of 49.6 mL/kg/. These differences are likely due to the use of an em- ployed population and the active nature of the job. As previ- ously mentioned, the cutoff values established for the aerobic fitness levels in these analyses were based on the range of dis- tributions within this active population in addition to meth- ods used in other research.

When compared with the results from a previous study that described exercise-related injuries, which accounted for one third of all reported injuries, as the most common ( 10), the present findings may appear somewhat counterintuitive. It was not clear if the injuries sustained during exercise periods were the result of overexposure (i.e., fit people exercising too intensely or for too long while on duty) or if the types of ex- ercises being completed were not appropriately structured and evaluated in an effort to minimize the chances for over- exertion. The present study’s results regarding increased risks of injury among those deemed less fit suggests that the struc- ture and management of exercise within the fire service needs to be considered more intently and that employees without a physical training background should not necessarily be left to exercise without some level of appropriate programming, training, and oversight. Most professional fire departments promote or require some level of exercise among their em- p l o y e e si na ne f f o r tt oa s s u r etheir ability to complete job tasks with high physical demands (e.g., rescues). Persons in the fire service, much like the majority of the general popu- lation, can bene fit from exercise instruction and from re- sources aimed at maintaining or improving their functional fitness levels, thus reducing the potential injury loss. Given the limited financial and personnel resources, a challenge for all fire departments (and similar occupational settings) will be determining the best measures for assessing each component of functional fitness that are 1) consistent and re- liable and 2) feasible for implementation.

Conclusions
Findings from the present study provide empirical evi-
dence that lower fitness levels are associated with increased risks of injury among career fire service employees. Further- more, these increased risks were modi fied by age, which is likely due to the fact that rank, job task, and risk pro file are often associated with age in this population. As injuries con- tinue to be of relevant health concern in the fire service, the contribution of fitness to the likelihood of injury is signi fi- cant. Given that injuries are often the result of a multitude of factors and the effi ciency of every response activity in the fire service is dependent on the health and fitness of those re- sponders, comprehensive and multifaceted solutions need to be devised, applied, and distributed in order to prevent fur- ther injury loss.

ACKNOWLEDGMENTS
Author affiliations: Division of Epidemiology and Biosta- tistics, Mel and Enid Zuckerman College of Public Health, Uni- versity of Arizona, Tucson, Arizona (Gerald S. Poplin, Denise J. Roe, Robin B. Harris); Center for Applied Biomechanics, School of Engineering and Applied Sciences, University of Virginia, Charlottesville, Virginia (Gerald S. Poplin); and Division of Community, Environment and Policy, Mel and
154 Poplin et al.

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Enid Zuckerman College of Public Health, Tucson, Arizona (Wayne Peate, Jefferey L. Burgess).

This work was supported by grant 5R01OH009469 from
the Centers for Disease Control and Prevention, National In- stitute for Occupational Safety and Health.

We thank Emily Scobie, Becky Arnold, and all employees
of WellAmerica, who deserve our utmost gratitude for their enduring efforts throughout this project.

The contents of this article are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.

Conflict of interest: none declared.

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Firefighter Fitness: Improving Performance
and Preventing Injuries and Fatalities
Denise L. Smith, PhD
Abstract
Firefighting is dangerous work. Each year, approximately 80,000 fire- fighters are injured and about 100 firefighters lose their lives in the line of duty. Firefighters face multiple dangers in the course of their work; they encounter toxic fumes, dangerous products of combustion, high radiant heat loads, and a chaotic work environment. Despite the myriad dangers, the leading cause of line-of-duty death among firefighters is sudden cardiac event, accounting for approximately 45% of duty deaths. Firefighting requires high levels of aerobic fitness, anaerobic capacity, and muscular strength and endurance; however, data suggest that many firefighters do not possess high aerobic or anaerobic capacity.

Furthermore, many firefighters are overweight and have one or more modifiable risk factors for cardiovascular disease. The safety of the public and the health and safety of firefighters would be enhanced if firefighters followed well-designed fitness programs to improve overall health and fitness.

Introduction
Firefighters perform strenuous work in hostile, chaotic, and unpredictable conditions. Thus, firefighting is widely recog- nized as dangerous work. In 2009, there were 1.35 million fires in the United States, resulting in 3,010 civilian deaths, 17,050 civilian injuries, and an estimated total property loss of $12.5 billion (25). There are approximately 1.2 million firefighters in more than 30,000 departments providing
local communities with protection from fire and other haz- ards. Roughly 29% of the U.S. Fire Service is made up of career, paid firefighters, while the remaining 71% are vol- unteer firefighters (25).

Firefighters are a unique occupational group, and they
face multiple dangers in the course of their work; fire- fighters encounter toxic fumes, dangerous products of
combustion, high radiant heat loads,
and a chaotic work environment. In
addition to hazards encountered at the
scene of a fire, firefighters also perform
rescues, extrications, emergency medi-
cal system (EMS) calls, and respond to
natural disasters and hazardous mate-
rials spills. Attesting to the dangerous
nature of the job, approximately 80,000
firefighters are injured on the job each
year. More than 40% of the injuries
occur on the fireground, despite the
fact that firefighters spend a very small
percentage of their time engaged in fire
suppression activities. When expressed
relative to the type of call they are
responding to, approximately 23 to 25
firefighters are injured per 1,000 fires,
whereas only 0.6 to 0.7 injuries occur per 1,000 non-fire emergencies (24).

Firefighting also results in approximately 5.7 firefighter fatalities per 100,000 structure fires (13). A retrospective study, performed between 1995 and 2004, revealed that
1,006 firefighters had died in the line of duty during that period. Approximately 45% of those fatalities were the
result of cardiovascular events. While most people recog- nize that firefighting is dangerous, many believe that fire or the products of combustion account for most of the fatal- ities in the Fire Service. As seen in Figure 1, the percentage of fatalities attributed to sudden cardiac events far out- numbers the deaths due to burn or asphyxiation on a con- sistent basis.

Physical Demands of Firefighting
Firefighting involves a unique set of stressors (Fig. 2).

Firefighters perform strenuous muscular work; they must climb stairs and ladders, carry and use heavy tools, often above their head or in awkward positions, and they may be called upon to perform difficult rescue operations. Fire- fighters work in dangerous environments; they encounter extreme temperatures, toxic smoke (including carbon mon- oxide and hydrogen cyanide), and chaotic conditions that include loud noise and low visibility. Further, this work must be done with time urgency and is often performed
under the psychological stress of knowing that civilians are

TRAINING
www.acsm-csmr.org Current Sports Medicine Reports 167
Professor of Health and Exercise Sciences, Skidmore College, Health and Exercise Sciences, Saratoga Springs, NY; Research Scientist, University of Illinois, Fire Service Institute, Champaign, IL Address for correspondence: Denise L. Smith, PhD, Professor of Health and Exercise Sciences, Skidmore College, 815 N. Broadway, Saratoga Springs, NY 12866 (E-mail: [email protected]).
1537-890x/1003/167Y172
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335
in imminent danger. Additionally, firefighters must perform their work while wearing personal protective equipment
(PPE), equipment that is necessary to protect the firefighter but that also imposes a considerable physiological burden because of its weight, insulative properties, and restrictive- ness. The unique set of stressors that are encountered dur- ing firefighting results in substantial physiological strain, particularly to the thermoregulatory and cardiovascular systems.

Physiological Strain of Firefighting
Considering the work that is done and the environment
in which it is performed, firefighting is among the most arduous work that humans undertake. Not surprisingly
then, firefighting affects every system of the body. Figure 3 summarizes some of the major effects of firefighting on the body.

While firefighting results in significant physiological strain affecting nearly every system of the body, statistically the greatest risks to the firefighter come from the cardiovascular and thermal strain associated with firefighting. Strenuous firefighting activities lead to near maximal heart rates (HR) that can remain elevated for extended periods of time (1,38).

Stroke volume decreases following strenuous firefighting activity (38). Firefighting may result in high blood pressures that quickly drop below resting values following cessation of work (21).

Firefighting is associated with profuse sweating and hence a decrease in plasma volume. A 15% reduction in plasma
volume has been reported after 18 min of strenuous fire- fighting drills (38). The decrease in plasma volume contrib- utes to the reduction in stroke volume noted above and leads to hemoconcentration. Hemoconcentration causes a change in blood electrolytes and increases blood viscosity (39).

Platelet number increases (more than can be explained by hemoconcentration) and platelet aggregability increases fol- lowing firefighting activity (41).

Given that firefighters wear heavy, insulative PPE that often weighs in excess of 22 kg and are called upon to per- form strenuous muscular work in very hot environments,
it is no surprise that firefighting leads to thermal strain.

Challenges to the thermoregulatory system include elevated core temperature (hyperthermia) and dehydration. Hyper- thermia and dehydration are very serious problems in the Fire Service because these twin challenges can hasten the onset of fatigue and limit work time, add to cardiovascular strain, lead to fatal heat illnesses (including heat stroke), impair cognitive function, and increase the risk of injury.

Core temperature increases rapidly but does not reach
drastically high levels during short-term firefighting. Peri- ods of 18 to 20 min of firefighting have been reported to cause an average increase in body temperature of 1.5 to

2.5-F (21,40). Prolonged firefighting or repeated evolutions of training would cause greater elevations in body temper- ature. Firefighting, like other strenuous activity, leads to fatigue. The fatigue may be due to neural, metabolic, or muscular factors and is likely hastened by work in the heat.

Fatigue can impair the firefighting mission and may increase susceptibility to injury.

Performance Requirements
Firefighting requires high levels of aerobic fitness, anaer- obic capacity, and muscular strength and endurance. Addi- tionally, given the detrimental effects of excess body fat, firefighters also should possess an appropriate body compo- sition. Several studies have attempted to quantify the aerobic Figure 1: Firefighter casualty statistics from 1990 to 2008.

Based on data from The National Fire Protection Association (NFPA).

Figure 2: Unique job stressors. Stressors associated with firefighting include strenuous muscular work, heavy personal protective equipment (PPE), and a hot and dangerous environment.
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336
requirements of firefighting. A limitation to all of these stud- ies is that they ‘‘simulate’’ firefighting activity. Further, the work that must be performed varies greatly based on the work assignment (e.g., officer, venting, advancing a charged hose line, performing a rescue). Studies have reported
the estimated oxygen consumption associated with fire-
fighting to range between 33.6 and 49.0 mL Ikg
j1Iminj1
(17,26,28,44). Sothmann and colleagues (1990) devised
a simulated set of firefighting drills that took approxi- mately 9 min to complete (44). The participants had
an average V˙ O2max of 39.9 mLIkgj1Iminj1 a n dw o r k e da t approximately 76% of their V ˙ O2max to complete the
simulated drills (44). To validate a minimum V ˙ O2max
requirement, the researchers recruited another group of firefighters (V˙ O
2max range 26Y51 mLIkgj1Iminj1) and had
them perform the fire-suppression drills. Seven of 10 fire- fighters with a V ˙ O
2max of 33.5 to 41.0 mL Ikgj1Iminj1
were able to successfully complete the drills within the allotted time. All participants with a V˙ O
2max greater than
41 mLIkgj1Iminj1 were able to complete the fire-suppression activities within the acceptable timeframe. Based on re- search findings, the National Fire Protection Association (NFPA) Standard on Occupational Medical Programs for
Fire Departments recommends that firefighters have a mini- mal aerobic capacity of 42 mLIkg
j1Iminj1 (metabolic equiv-
alent of task score, 12) (31).

Firefighters also must have a high anaerobic capacity to perform certain job tasks. Strenuous firefighting relies on anaerobic energy sources (in addition to aerobic sources) and high lactate values (6Y13 mmol) have been reported fol- lowing demanding firefighting simulations (2,17,26).

Muscular strength and endurance also are important to
meet the physical demands placed on firefighters. Muscular strength and endurance are necessary for forcible entry, advancing the fire hose, chopping tasks, and victim rescues.

Sothmann and colleagues (2004) have sought to validate
minimally acceptable standards for muscular strength and endurance necessary to successfully perform firefighting tasks (43). Participants completed a simulated set of fire- fighting tasks and a battery of tests of physical abilities. The researchers found that physical ability tests (hose drag/high rise pack carry, arm lift, and muscle endurance) combined to significantly predict performance time on the simulated firefighting tasks. Rhea and colleagues (2004) investigated the relationship between several fitness scores and job per- formance (assessed by performance time on a simulated hose pull, dummy drag, stair climb, and hose hoist) (34). These authors reported high correlations between measures of
muscular strength, muscle endurance, anaerobic perform- ance, and performance times.

Fitness Profile of U.S. Firefighters
Given the heterogeneous nature of the U.S. Fire Service, it is difficult to get an accurate, generalizable assessment of the ‘‘typical’’ firefighter. Firefighter fitness profiles are likely to vary greatly depending upon region of the country, career or volunteer status, the age of the firefighter, and the hiring, medical, and fitness policies of the department.

Aerobic Capacity
Despite the physically demanding aspects of the job, sev- eral studies have reported that firefighters do not possess above-average aerobic capacity. Again, caution must be used when interpreting the literature because many articles report fitness values on a small number of firefighters, from a single department or small group of departments. Furthermore,
many of the aerobic fitness values reported in the literature are based on submaximal exercise tests. In general, reported aerobic fitness values for firefighters range from 35 to 56 mL Ikg
j1Iminj1 (9,35,37,49). An early study that ran-
domly sampled 150 firefighters from a large metropol-
itan city found that aerobic capacity significantly decreased from the 20- to 25-yr-old group (47.7 mL Ikg
j1Iminj1)t o
the 30- to 35-yr-old group (37.9 mLIkgj1Iminj1) and from the 35-yr-old group to the 40- to 45-yr-old group
(31.5 mL Ikg
j1Iminj1). In each case, however, the fire-
fighters’ aerobic capacity was similar to that predicted for sedentary individuals (37). In contrast, Davis and colleagues (2002) studied a small municipal fire department on the West Coast and found that firefighters in the 20- to 29-yr age Figure 3: Primary physiological responses to firefighting and the benefits of physical fitness. Firefighting results in considerable physio- logical strain. A well-designed fitness program provides multiple benefits and helps prepare the firefighter for the demands of firefighting. www.acsm-csmr.org Current Sports Medicine Reports 169
Copyright © 2011 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

337
group had an aerobic capacity of 55.9 mL Ikgj1Iminj1 and that the oldest group, 50 Y59 yr, had an oxygen uptake of

40.4 mL Ikgj1Iminj1 (9). In every decade-group, the fire- fighters’ aerobic capacity was higher than age-predicted values (9). While it is tempting to hope that the improve- ments in aerobic capacity cited in the more recent study reflect positive changes relative to fitness patterns among the Fire Service, other evidence suggests that firefighters possess modest aerobic capacity (7,9,49). A study of over 100 recruit firefighters in a metropolitan fire department found that recruit firefighters entered the fire service with an aerobic capacity of 35 mLIkg
j1Iminj1 (9). Encouragingly,
the study went on to document a 28% increase in aerobic capacity following a 16-wk fitness program.

Muscular Strength and Endurance
In general, firefighters have high levels of muscular
strength and endurance (2,9,29,34,35,43). Muscular strength and endurance are necessary to perform common firefight- ing activities, such as carrying ladders, advancing charged hose lines, and using heavy equipment. Firefighters rely on upper- and lower-body strength to perform their jobs. Fur- ther, core muscular strength also may serve to reduce the incidence of injuries (48). The high level of strength reported in the fire service also may reflect cultural values within the Fire Service.

Obesity
There is considerable evidence of a high prevalence of
obesity among U.S. firefighters (7,12,16,42,46,50). Clark and colleagues (2002) investigated the body mass index
(BMI) and health status of a group of municipal firefighters and reported that 80% of the firefighters were overweight or obese and that there was a significant increase in systolic blood pressure, diastolic blood pressure, cholesterol, and triglycerides as firefighters increased in fatness category (7).

Conversely, V˙ O
2max and METS decreased with increasing
fatness (7). A study involving a large group of career fire- fighters ( N = 332) found that at baseline testing, 53% of firefighters were overweight (BMI between 25 and 29.9), with an additional 35% classified as obese (BMI Q 30).

Additionally, obese firefighters were more likely to have hypertension and low levels of high-density lipoprotein- cholesterol (42). A study of more than 100 career and
volunteer firefighters with no known history of cardio- vascular disease, stationed throughout Illinois, found an average BMI of 28.1 and that 75% of the participants had a BMI greater than 25 (12). Furthermore, Fahs and col-
leagues (2009) reported that increased BMI was associated with increased arterial stiffness (12). In addition to the high prevalence of overweight and obesity in the Fire Service, there is evidence that firefighters are getting heavier over time. Soteriades et al. (2005) found that over a 5-yr follow- up period, the prevalence of obesity increased from 35% to 40%, and the proportion of firefighters with extreme obe- sity increased fourfold (from 0.6% to 2.4%) (42).

Cardiovascular Risk
Most studies that have compared the overall car-
diovascular mortality of firefighters with the general public have found that firefighters do not have an increased risk for cardiovascular death, except in instances where the careers of firefighters included significant time on the job before the use of respiratory protection (11). Studies in Boston, Connecticut, New Jersey, San Francisco, Seattle, Edmonton/Calgary and Florida all have found no associa- tion between cardiovascular mortality and occupation
(3,10,14,18,27,30,36,47). Similarly, a recent review found that firefighters have similar risk profiles for obesity, hypertension, and hyperlipidemia compared with the gen- eral population (11).

Mismatch Between Fitness Demands
and Fitness Profiles
Given that so much of a firefighter’s time is sedentary, perhaps it is not surprising that they do not differ from the general population in fitness, obesity, or other cardio- vascular risk factors. However, considering that their work is punctuated by periods of intense activity, these risk pro- files may explain why sudden cardiac events are the leading cause of line-of-duty deaths among firefighters; essentially, there is a mismatch between the fitness and health require- ments of strenuous firefighting and the current fitness pro- file of the U.S. Fire Service.

Fitness Recommendations
Current standards recommend that firefighters partic-
ipate in a fitness program (22,32), but it is the responsibility of each individual department to determine whether to
institute a fitness program. The NFPA 1583 Standard rec- ommends a program that is positive, nonpunitive, and does not set fitness standards. Of the 440 firefighter fatalities investigated by the National Institute of Occupational
Safety and Health (NIOSH) during the period from 1995 to 2004 (44% of fatalities during that period; 440 of 1,006), 39% of the departments offered a voluntary fitness pro- gram, but only 8% had mandatory participation (33).

Cardiovascular events are by far the leading cause of line- of-duty deaths among firefighters. Additionally, cardiac events are disproportionately related to fire suppression activities, with firefighters having a 10- to 100-fold in- creased risk of experiencing a fatal cardiac event after fire suppression versus normal duties at the station (23). Thus firefighters should have a high level of cardiovascular fit- ness in order to improve performance and decrease the risk of on-the-job fatalities associated with strenuous activity.

Additionally, nearly 80,000 firefighters are injured each year, with a large percentage of these injuries occurring during fireground operations. Clearly, fitness has an im- portant role to play in preparing firefighters for the stren- uous activity they encounter during firefighting activity.

Appropriate fitness programs can enhance overall health, improve performance, and lessen the risk of firefighter injury or fatality. Firefighters and the public they serve will benefit from more fitness programming in the Fire Service.

Fitness Prescription for Firefighter Health and Safety
Firefighting is strenuous physical work and places con- siderable strain on the body. In order to meet the unique physical demands of firefighting and to perform firefighting in a safe manner, firefighters must be physically fit. Like soldiers and elite athletes, firefighters should be physically 170 Volume 10 & Number 3 & May 2011 Firefighter Fitness Copyright © 2011 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

338
prepared to meet the unique physical challenges they face.

Figure 3 depicts the direct ways in which a fitness program can mitigate against the physiological strain of firefighting.

Fitness prescriptions for firefighters must meet certain criteria in order to adequately serve the U.S. Fire Service.

Prescriptions need to address the unique and specific phys- iological demands of firefighting. This is difficult to accom- plish because of the current diversity in fitness and health status of firefighters. Fitness prescriptions must recognize the unique structure and culture of volunteer and career fire departments. These prescriptions also must include indi- vidual and progressive programs to meet the individual
needs of low-fit to highly-trained firefighters.

Aerobic Training
Aerobic training provides several health benefits, includ- ing improved body composition, serum lipids, glucose
metabolism, and maximal aerobic capacity (20). While
moderate-intensity aerobic exercise (50% Y70% HR
max)i s
widely recommended for health benefits, research suggests that higher-intensity aerobic exercise training may promote weight loss and cardiovascular improvements to a greater extent (8). Given the physical demands of firefighting, and the high proportion of line-of-duty deaths attributed to cardiac events, it is essential that a training program for firefighters include endurance training.

Sprint Interval Training (SIT)
SIT is a type of high-intensity interval training (HIT) that is designed to improve endurance, increase anaerobic
threshold, and improve maximal performance. This type of training has been shown to be effective at increasing aerobic capacity (6), improving endurance capacity when working at 80% of aerobic capacity (4), enhancing aerobic metabo- lism (19), and increasing muscle glycogen content and the maximal activity of citrate synthase (5). Given the effec- tiveness and efficiency of these workouts and the degree to which they mimic actual energy expenditure during an
emergency, it is reasonable to include SIT in exercise pre- scriptions for firefighters. However, given the high intensity of the workouts and the heterogeneity of fitness levels in the Fire Service, it may be prudent to initiate exercise programs at a lower intensity and increase progressively.

Functional Training
Functional training targets movements that are necessary for activities of daily living (45). Functional training utilizes full-body, dynamic movements to increase muscular strength and endurance as well as aerobic capacity using equipment such as medicine balls, physioballs, and exercise bands to provide resistance. This type of exercise mimics the high- intensity demands of firefighting. In fact, functional training workouts have been gaining popularity among progressive Fire Departments. CrossFit workouts are now embraced by many members of the Fire Service.

Resistance Training
Resistance training increases muscle mass and function.

Muscle strength and endurance routinely have been found to predict performance on simulated firefighting activities and are unquestionably important for firefighters. Addition- ally, resistance training is associated with a decreased risk of all-cause mortality, the development and maintenance of lean muscle mass, and enhanced glucose metabolism
(15,20). Resistance training should be part of every fire- fighter’s fitness program. Not only will it improve work capacity, it is likely to provide protection against injuries, especially muscular strains, on the fireground.

Lifestyle Modifications
There must be a cultural change within the U.S. Fire
Service in order to improve fitness and decrease injuries and cardiac events. Changes should include a fitness program designed to improve aerobic capacity, muscle strength and endurance, and functional capacity. The fitness program and a sound dietary plan also should seek to promote
healthy weight for firefighters. The development of a social support system with adequate leadership and incentives
should promote healthy lifestyle changes. Each individual firefighter and Fire Department must set short- and long- term goals that are realistic and measureable as well as easy to implement within the constraints of space, equipment, and other duties.

Conclusion
Firefighting is strenuous and dangerous work with a
unique set of stressors. In order to meet the physical
demands of firefighting, firefighters must be physically fit.

Firefighters who possess high levels of cardiovascular and muscular fitness are better able to serve the public by per- forming their job more effectively. Fit firefighters have increased mobility, energy, and endurance, allowing them to better perform job duties efficiently and safely, and fit fire- fighters also are less likely to jeopardize the safety of their fellow firefighters or the public they serve.

The safety of the public and the health and safety of
firefighters would be greatly enhanced if firefighters fol- lowed well-designed fitness programs to improve overall health and fitness. Exercise scientists can play an important role in enhancing firefighter’s fitness, thereby improving public health and safety. Specific fitness programs that meet the needs of a broad range of individuals within the Fire Service must be developed that are tailored to the specific job requirements that firefighters face. These fitness pro- grams should be geared toward improving health, safety, and performance. In order to be adopted, these programs must be sensitive to the diverse needs in the Fire Service.

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Cardiovascular Strain of Firefighting and the Risk of
Sudden Cardiac Events
Denise L. Smith1,2, Jacob P. DeBlois1,S t e f a n o sN .K a l e s3,4, and Gavin P. Horn 2 1Health and Exercise Sciences, Skidmore College, Saratoga Springs, NY; 2University of Illinois Fire Service Institute, Champaign, IL; 3Department of Environmental Health, Harvard TH Chan School of Public Health, Boston, MA; 4Occupational Medicine, Cambridge Health Alliance/Harvard Medical School, Cambridge, MA SMITH, D.L., J.P. DEBLOIS, S.N. KALES, and G.P. HORN. Cardiovascular strain of firefighting and the risk of sudden cardiac events. Exerc. Sport Sci. Rev., Vol. 44, No. 3, pp. 90 –97, 2016. Approximately 45% to 50% of line-of-duty deaths in the fire service are caused by sudden cardiac deaths, which most often occur during or shortly after firefighting duties. We present a theoretical model linking the cardiac, vascular, and hematological responses of firefighting to the triggering of sudden cardiac death in susceptible individuals.

Key Words: firefighting, cardiac, vascular, coagulatory, cardiovascular disease, sudden cardiac death.

Key Points
 The leading cause of duty-related death within the fire service is a sudden cardiac event, accounting for ap-
proximately 50% of line-of-duty deaths.
 Sudden cardiac events are much more likely to occur af- ter firefighting activities than other duties and may be the result of primary arrhythmias or myocardial infarction.  Firefighting leads to significant cardiovascular strain, in- cluding alterations in cardiac function, vascular func- tion, and hemostasis.
 Most firefighters recover from the stress of firefighting without incident. However, the cardiovascular strain of firefighting may trigger a cardiovascular event in fire- fighters with an underlying disease.
 Increased cardiac work, vascular dysfunction, tissue is- chemia, and a procoagulatory state may be important
causal links that increase the risk of sudden cardiac
events in the vulnerable firefighter.

INTRODUCTION
Firefighters respond to multiple types of emergencies, such as fires, vehicle/machinery accidents, medical calls, calls for public assistance, technical rescue, and hazardous materials spills to protect the communities they serve. However, statistics show that firefighting (i.e., those activities directly related to fire suppression) results in the greatest risk of injury or fatality (8).

Firefighting activities often are conducted in immediately dan- gerous to life or health conditions that may expose firefighters to extreme environmental temperatures and to multiple chem- ical and particulate hazards. These conditions necessitate that firefighters wear heavy, insulated, and restrictive personal pro- tective equipment (PPE). Hence, it comes as no surprise that firefighting results in high levels of cardiovascular strain.

Fire service statistics reveal that despite all the acute trau- matic risks that firefighters face ( e.g., burn injuries, smoke inhalation, structural collapse), by far, the leading cause of line-of-duty death is cardiac related. In fact, approximately 45% to 50% of all firefighter duty –related fatalities are caused by sudden cardiac death (SCD) — ap r o p o r t i o nt h a ti sr e l a - tively stable and stubbornly high. This point is highlighted in Figure 1, which compares SCD with fatalities from burns and asphyxiation since 1990. The number of cardiac fatalities seems to be trending downward during the past 10 years. This encour- aging trend may be caused by efforts by the International Asso- ciation of Firefighters, the International Association of Fire Chiefs, and the National Volunteer Fire Council, all of which have undertaken rigorous campaigns to increase medical evalu- ations and to promote firefighter wellness and fitness. However, as the numbers indicate, considerable work remains to further reduce cardiac fatalities in the US Fire Service (Fig. 1).

Importantly, sudden cardiac events are disproportionately more likely to occur during or after a firefighting activity than other duties. Although firefighters spend a small percentage of their time (1%–5%) engaged in fire suppression activities, more Address for correspondence:Denise L. Smith, Ph.D., Health and Exercise Sciences, Skidmore College, 815 North Broadway, Saratoga Springs, NY 12866 (E-mail: [email protected]).

Accepted for publication: February 29, 2016.

Associate Editor: Bo Fernhall, Ph.D., FACSM
0091-6331/4403/90–97
Exercise and Sport Sciences Reviews
DOI: 10.1249/JES.0000000000000081
Copyright © 2016 by the American College of Sports Medicine Editor's note: Go online to view the Journal Club questions in the Supplemental Digital Content: see http://links.lww.com/ESSR/A19. 90 ARTICLE
Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

341
than 30% of cardiac fatalities occur during or shortly after firefighting activity, resulting in a 10 to 100 times greater risk of sudden cardiac events after firefighting versus station du- ties (19). Furthermore, there are approximately 17 to 25 duty- related nonfatal cardiovascular events (heart attacks and strokes) for every fatal event (8,15).

Cardiac events are devastating for individual firefighters and their families, but because firefighting relies on a coordinated team effort, duty-related cardiac events also can jeopardize job performance and the safety of other firefighters and may com- promise the ability of firefighters to protect civilians during emergencies. Cardiac injuries likewise often will require signifi- cant time away from the fire department at significant cost and burden to the local fire department. Hence, there is an impor- tant public safety concern about cardiovascular events in the fire service. We recently have published a review that proposed a theoretical model highlighting the interaction of occupa- tional, medical, and behavioral risk factors in contributing to underlying cardiovascular disease (CVD) (coronary heart dis- ease (CHD) and/or structural heart disease) and where the strenuous duties of firefighting may trigger a cardiovascular event (Fig. 2) (32). In the current review, we extend the prev- ious model by detailing results of translational research documenting the cardiovascular strain of firefighting and fur- ther highlighting the potential mechanisms by which cardio- vascular responses to firefighting may lead to pathophysiologic changes that can trigger fatal arrhythmias or myocardial infarc- tion, thus leading to a sudden cardiac event (Fig. 3).

FACTORS AFFECTING THE CARDIOVASCULAR

STRAIN OF FIREFIGHTING
Firefighting involves performing a complex series of physi- cally demanding tasks under psychologically stressful conditions within arduous environments. As depicted in Figure 3, the car- diovascular strain of firefighting results from multiple interact- ing factors including 1) sympathetic nervous system activation; 2) the strenuous physical work (aerobic and anaerobic); and 3) exposure to environmental conditions and pollutants contained in fire smoke.

Sympathetic Nervous System Activation
Sympathetic nervous activation begins with the alarm and continues throughout a fire call. The sudden sounding of an alarm increases psychological/mental stress, activating the sym- pathetic nervous system. While on the call, sympathetic activa- tion continues because the fire scene is a dynamic cluttered environment in which fire behavior can change rapidly and where loud noises, time urgency, and potential danger produce powerful sympathetic arousal. Several studies have shown a fivefold to sevenfold risk of sudden cardiac events during alarm response versus nonemergencies (19,20). The increased relative risk of a cardiac event during this period can be attributed largely to sympathetic arousal and a putative surge in catechol- amines because there is not yet exposure to the fire environ- ment or the requirements for large amounts of physical work.

Physical Work
Structural firefighting requires a combination of static work and aerobic exertion, such as stair and ladder climbing (while Figure 1. Line-of-duty deaths by major cause since 1990.

Figure 2. Theoretical model of occupational, medical and behavioral risk factors that contribute to sudden cardiac events following firefighting activities. Note that there is overlap between occupational and lifestyle risk factors. Arrows indicate that factors can be associated with progression or regressiono fr i s kf a c t o r s and subclinical disease. EtOH, alcohol; CHD, coronary heart disease; LVH, left ventricular hypertrophy; CVD, cardiovascular disease. (Reprinted from (32). Copy- right © 2011 Wolters Kluwer Health. Used with permission.) Volume 44  Number 3  July 2016 Firefighting and Sudden Cardiac Events 91 Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

342
carrying heavy equipment), forcible entry, victim search and rescue, building ventilation, and fire attack and suppression. Al- though it is known that different firefighting tasks require dif- ferent levels of energy expenditure, firefighting often results in oxygen consumption of greater than of 40 mL ∙kg
−1∙min−1
(24). The strenuous work of firefighting is performed while wearing heavy (≥25 kg) and fully encapsulating PPE that pro- vides range-of-motion restrictions, heavy insulation, and lim- ited breathability and adds to the metabolic demands of firefighting. In addition, fire fighters perform a great deal of upper body work (lifting and carrying heavy equipment, hose movement and control, chopping, ceiling overhaul), and upper body work (such as wood chopping or snow shoveling) leads to an exaggerated blood pressure response and an increased car- diac work and is associated with an increased risk of a sudden cardiac event (12).

Environmental Conditions
Radiant heat from a fire adds considerably to the heat stress experienced by firefighters. We have reported an increase in core temperature ( T
co) of approximately 1.5°C after short-
term (≤20 min) firefighting activity (30), and the rate of Tco change increased with subsequent firefighting activities (17).

Both heat stress and dehydration exacerbate cardiovascular strain associated with heavy work (9).

Although firefighters regularly use self-contained breathing apparatus to protect their airways while operating inside the structure, firefighters routinely are exposed to fire smoke (out- side of the structure and during overhaul/cleanup operations), which contains toxic and asphyxiant gases such as carbon mon- oxide, hydrogen cyanide, hydrogen sulfide, and particulate mat- ter among a myriad of other chemicals (5). When inhaled, carbon monoxide reduces the availability of oxygen, thus, po- tentially resulting in tissue hypoxia. Tissue hypoxemia caused by carbon monoxide and other asphyxiants (e.g., hydrogen cy- anide, hydrogen sulfide) may lead to myocardial ischemia in susceptible individuals. Fire smoke also contains particulate matter associated with the promotion of arrhythmias, decreased heart rate variability, and increased blood pressure (23). Such particulates also may increase the formation of free radicals leading to activation of proinflammatory and prothrombotic pathways that may cause endothelial dysfunction and increased blood coagulability.

CARDIOVASCULAR RESPONSES TO FIREFIGHTING
Firefighting operations are complex, and no fire scene is iden- tical to another. The magnitude of the cardiovascular strain is af- fected by the fuel that is burning (e.g., heat release rate, products of combustion), the size of the fire (amount of fuel involvement), dimensions and layout of the structure, the resources (human and equipment) that are available to fight the fire, the tactics that are used (e.g., interior, exterior, or combination attack), and the fire- fighter's assigned task. For example, a firefighter who is responsi- b l ef o ro p e r a t i n gt h ep u m po nt h ee n g i n ea tas m a l lf i r ew o u l d have a different level of cardiac strain than a firefighter who is performing search and rescue activities on the interior of a large structure during fire suppression activities. The following sections detail research efforts to characterize systematically the effects of firefighting on the various components of the cardiovascular system — the heart, vasculature, and hemostatic system.

Cardiac Responses
During strenuous fire suppression activities, heart rates (HR) increase and rise to maximal or near maximal levels andT co in-
creases rapidly (18,29). Studies that have focused on long- term firefighting activities that require more than 1 cylinder of air (supporting heavy firefighting work for approximately 12–16 min) have shown that the rate of increase in T
co is
Figure 3. Theoretical interplay between cardiac, vascular, and hemostatic responses to firefighting and sudden cardiac events in susceptible individuals.

RH, reactive hyperemia; BF, blood flow; Fxn, function; SEVR, sub-endocardial viability ratio; SV, stroke volume; RPP, rate pressure product; SS, shear stress. 92 Exercise and Sport Sciences Reviews www.acsm-essr.org Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

343
augmented in later bouts of activity, further exacerbating car- diovascular strain and leading to higher HR (17). HR responses to firefighting vary tremendously depending on the type of work the firefighter is doing and mu ltiple other factors (ambient temperature, length of time engaged, fitness level, etc.). Fur- thermore, firefighters' HR vary throughout a given emer- gency because of the intermittent nature of the work (17).

Although HR is the most frequently documented cardiovas- cular variable because of its ease of measurement, alterations in other cardiac variables are likely even more important to de- scribe the cardiovascular risk associated with firefighting. We have reported a 35% reduction in stroke volume (seated posi- tion) after three short bouts of strenuous firefighting activity that lasted a total of approximately 20 min (29). More recently, we performed a full echocardiographic examination (supine po- sition) before and after a 3-h training period that included mul- tiple training evolutions/drills, each lasting approximately 15 to 30 min (11). The firefighting training resulted in near maximal HR (mean, 192 ± 15 beats min
−1) and an increase in Tco from

37.1 ± 0.5°C to 38.9 ± 0.6°C. Echocardiographic measure- ments were obtained within 30 min of firefighting, and mean HR had decreased to 90 ± 13 beats min −1 at the time of mea- surement. In this study, we found a 13% reduction in stroke vol- ume (11). Furthermore, the repeated bouts of training resulted in significant reductions in left ventricular diastolic size and vol- ume, transmitral flow velocities, mitral E/A (the ratio of passive ventricular filling to active atrial filling of the ventricle), and left ventricular shortening fraction and ejection fraction. The changes in left ventricular function observed in this study may reflect changes in preload after the firefighting activity. However, there is some evidence that intrinsic systolic and diastolic func- tion also may be depressed after firefighting activity. TDI-E ′, an indicator of rate of ventricular relaxation, decreased by 19% when measured at the lateral wall but was unchanged when measured at the septal wall (11). The decline in the lat- eral wall TDI-E ′ is a marker of lusitropic function that is less load dependent than other measures of diastolic function. Al- though the clinical significance of the cardiac changes pre- sented above are not fully comprehended, it is important to consider these changes in the context of overall cardiovascular function, especially given that these results were noted in ap- parently healthy firefighters with no known CVD. Indeed, this study found that, in addition to decreased left ventricular func- tion, there was a decrease in systemic arterial compliance (11).

These findings raise the possibility that arterial-ventricular cou- pling may be altered with firefighting and reinforces the impor- tance of investigating vascular function after firefighting.

Vascular Responses
The myocardium and vasculature work together in a con-
certed effort to deliver blood to body tissues. Changes in vascu- lar function and/or the interaction between the myocardium and the vasculature may lead to tissue ischemia/hypoxia, pre- cipitating a sudden cardiac event. We have conducted several studies to investigate the effects of firefighting activities and heat stress on the vasculature and its function.

Yan et al. (35) reported changes in arterial-ventricular cou- pling after repeated bouts of firefighting during a 3-h period of live-fire training in a group of young (28 ± 1 yr) firefighters.

Arterial-ventricular coupling was measured on the right common carotid artery using wave intensity analysis, a hemody- namic index in which changes in blood pressure and blood flow provide insights into cardiovascular stress (25). In response to live-fire training, wave 1 amplitude, an indicator of cardiac con- tractility, was reduced by approximately 28%, suggesting that systolic function decreased (35). In a laboratory study designed to further investigate the role of heat stress in changing arterial- ventricular coupling, Smith et al. (26) examined 11 young (22 ± 3 yr) men before and after a 100-min exercise/rest tread- mill task (alternating 20-min walk/20-min rest in structural firefighting PPE) and found a nearly 32% increase in wave 2 amplitude. This measure of end-systolic/early-diastolic ventric- ular function from wave intensity analysis indicates increased afterload on the myocardium after heat stress (26).

We also have examined the influence of firefighting on vas- cular structure and function. Fahs et al. (6) examined the acute effects of live-fire training drills on arterial stiffness and limb blood flow in a group of 69 male firefighters (mean age, 28 ± 1 yr). After 3 h of live-fire training, hemodynamic proxies (wave reflection time and augmentation index) influenced by aortic stiffness increased by approximately 3%. Despite a possi- ble increase in aortic stiffness, a concomitant increase in pe- ripheral forearm vascular conductance (80%) and hyperemia (32%) also were observed (6). It is suggested that the unique combination of thermal, metabolic, psychological, and mental stress that occurs with firefighting activities might explain the increased aortic stiffness and forearm blood flow.

As a follow-up to live-fire studies, Lefferts et al. (22) con- ducted a laboratory-based study to isolate the influence of ther- mal stress on vascular and central hemodynamic changes.

Controlling for hydration status, Lefferts et al. (22) reported that exercise-induced moderate heat stress (T
co of 37.8 ± 0.1°C
caused by treadmill walking in firefighting PPE) did not change aortic stiffness (pulse wave velocity, pre: 5.0 ± 0.1 m·s −1 vs post:

4.9 ± 0.1 m·s −1; P =0 . 6 9 8 ) .H o w e v e r ,L e f f e r t set al. (22) did find alterations in myocardial work demands (rate-pressure pro- duct increased by 37%) and an indirect proxy of coronary per- fusion (subendocardial viability ratio (SEVR) reduced by about 27%). After a short 18-min bout of live-fire activity, Hornet al. (18) found a larger increase in rate-pressure product (80% – 95%) and similar reductions in SEVR (30% –35%). These re- sults, although they must be interpreted cautiously given lim- itations with SEVR, suggest a potential mismatch between myocardial oxygen demand and supply, which may result in is- chemia during strenuous work in the heat. This potential mis- match may be more pronounced after firefighting activities than exercise-induced heat stress.

Thus, firefighting activity and heat stress may result in vascular dysfunction and arterial-ventricular uncoupling as evidenced by increased central arterial stiffness, decreased vascular function, and a myocardial oxygen supply-demand mismatch. A potential linkage between vascular alterations and the risk of SCD may be related to firefighters' individual characteristics. We have found increased arterial stiffness in a group of obese ( ≥29.5 kg·m −2)
and overweight (25.9 –29.4 kg·m −2) firefighters compared with lean (<25.9 kg·m−2) firefighters, where carotid arteryβ stiff- ness was greatest in the obese group compared with the over- weight and lean groups (5.9 ± 0.3, 5.1 ± 0.3, and 4.9 ± 0.3, respectively) (7). Furthermore, we also have examined vascu- lar changes in older firefighters (40 –60 yr) before and after Volume 44  Number 3  July 2016 Firefighting and Sudden Cardiac Events 93 Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

344
firefighting activity and found that reactive hyperemia, a measure of microvascular blood flow, and pressure-controlled arterial stiff- ness (central pulse wave velocity/aMAP) increased significantly after firefighting (21). The cause of increased vascular stiffness after firefighting is not known. The increased central vascular stiffness observed after firefighting activities could be a compen- satory mechanism to counteract the profound skin vasodilation in an attempt to facilitate blood flow to the heart and brain or it could reflect a detrimental response to the stressors encoun- tered during firefighting. Addi tional studies are necessary to better understand the cause and consequences of arterial stiff- ness and to elucidate what populations of firefighters who, through underlying disease or CVD risk factors, may have a greater risk of experiencing acute vascular changes that may result in SCD.

Because changes in vascular function may be caused by re- active oxygen species or prostaglandin production, we had conducted studies that used antioxidant supplementation or as- pirin. We have investigated the effect of vitamin C supplemen- tation on vascular responses to firefighting in young healthy firefighters and found that 2 g of vitamin C did not affect mea- sures of vascular stiffness or microvascular function (6). We also have investigated the effect of aspirin supplementation on vas- cular changes associated with firefighting in older subjects and found that 2 wk of aspirin supplementation did not affect microvascular (reactive hyperemia) or macrovascular (arterial stiffness/central blood pressure) responses to firefighting (21).

Collectively, research has shown that firefighting and exercise- induced heat stress lead to vascu lar alterations , specifically, reduced vascular reactivity/endothelial function (6) and in- creased central arterial stiffness (7,22), which may play roles in arterial-ventricular uncoupling in response to firefighting activity and heat stress (26,35). The uncoupling of the arterial- ventricular system may explain the oxygen supply-demand mis- match observed after firefighting and exercise-induced heat stress (22), leading to ischemia. Increased arterial stiffness after firefighting may be detrimental because of increased cardiac work (increased afterload) or impaired vascular function (com- promised vasodilation), and this effect is likely more pro- nounced in individuals with underlying CVD. Furthermore, overweight/obese and older firefighters may be at increased risk for abnormal vascular responses associated with firefighting and heat stress because the vasculature of these persons is stiffer at rest without the external influence of occupational stressors (7).

Hemostatic Responses
Firefighting leads to an increase in blood pressure and sweating. Accordingly, we have documented a decrease of ap- proximately 15% in plasma volume after an 18-min bout of firefighting (30). In turn, this resultant hypovolemia decreases central venous pressure, causes hemoconcentration, and in- creases blood viscosity.

Firefighting likewise seems to disrupt hemostatic balance.

We conducted a large study (N = 114) to examine the acute ef- fects of firefighting on platelet number and function. Live-fire firefighting drills (~18 min) caused a modest change in T co
(0.7°C) and a peak HR of 167 beats min −1 and resulted in in- creased platelet number and decreased platelet closure time (in- creased aggregation) in young (29 ± 8 yr) apparently healthy firefighters (31). In a follow-up study, we evaluated the effects of firefighting activities on platelets, coagulation, and fibri- nolytic activity and documented the extent to which these variables recovered 2 h after completion of the firefighting ac- tivity in young (25 ± 5 yr) apparently healthy firefighters (27).

Platelet number, platelet activity, and coagulatory potential increased immediately after firefighting, and many variables (platelet function, activated partial thromboplastin time (aPTT) and factor VII) continued to reflect a procoagulatory state even after 2 h of recovery. Fibrinolysis also was enhanced immediately after firefighting but returned to baseline values 2 h after firefighting. Research with athletes has similarly indicated that strenuous physical activity acutely increases platelet number and platelet activation (4) and leads to a hypercoaguable state that is normally offset by simultaneous increases in fibrino- lysis (16). This hypercoaguable state persists longer into the postactivity recovery period than does the increase in fibrinoly- sis, potentially reflecting a “vulnerable period” after strenuous exercise (16). In situations of abnormal hemostatic balance, the exercise-induced procoagulatory state may result in in- creased incidence in cardiovascular events during and immedi- ately after strenuous physical activity. These data support the hypothesis that firefighting leads to a hemostatic imbalance that is primarily prothrombotic during the recovery period from firefighting activities.

INDIVIDUAL CHARACTERISTICS AND

UNDERLYING CVD RISK
As we have summarized in a previous review (32), individual factors may mediate the magnitude of the cardiovascular strain of firefighting. Although firefighting results in significant car- diovascular strain, most firefighters recover from firefighting ac- tivities with no untoward event. However, the cardiovascular strain of firefighting interacts with an individual's health and fitness status to determine whether responses to the strain of firefighting are limited to transient physiologic disruptions or re- sult in the activation of pathophysiologic pathways that may lead to sudden cardiac events (32). Underlying cardiovascular health status is determined largely by the prevalence of cardio- vascular risk factors. As shown in the Table, such risk factors greatly increase the risk of cardiacdeath in the fire service, with smoking, hypertension, diabetes, and old age all associated with a greatly elevated risk. However, the greatest risk was associated with previous CHD diagnosis (32). In addition, low fitness also likely contributes to the increased risk of sudden cardiac events (2) because fit individuals can do more work at the same level of cardiovascular strain or they experience less strain at the same level of work. Unfortunately, there is substantial evidence that firefighters often lack high levels of fitness and a large pecentage are overweight or obese (2,7,32).

A potential linkage between vascular alterations and the risk of SCD also may be related to individual factors, including obe- sity. Obesity (body mass index (BMI), ≥30 kg·m
−2) is a global
epidemic affecting all members of society, including first re- sponders. In the fire service, obesity has been found to increase the relative risk of an on-duty coronary heart disease fatality threefold (32). Obesity is a we ll-established risk factor for CVD and has been associated with reduced arterial function and increased aortic blood pressures (10). We examined the in- fluence of body weight on vascular structure and function in a group of 110 firefighters (30 ± 8 yr). Firefighters were evenly 94 Exercise and Sport Sciences Reviews www.acsm-essr.org Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

345
divided into tertiles according to BMI (<25.9, 25.9 –29.4, and ≥29.5 kg·m −2). We found increased arterial stiffness in the obese (≥29.5 kg·m −2) and overweight (25.9–29.4 kg·m −2)f i r e - fighters compared with that in lean (<25.9 kg·m −2) firefighters (7). Furthermore, carotid artery β stiffness was greatest in the obese group compared with the overweight and lean
groups (5.9 ± 0.3, 5.1 ± 0.3, and 4.9 ± 0.3, respectively). How- ever, measures of endothelial function did not differ by BMI classification (7).

We also have investigated the effect of obesity on coagu- latory response to acute firefighting activity (28). An 18-min bout of live-fire training caused a significant increase in co- agulatory and fibrinolytic markers, including an overall shift toward a procoagulatory state, as evidenced by a decrease in aPTT and an increase in platelet activity. We also found that obese firefighters exhibited lower baseline levels of fibrinolytic activity (lower tissue plasminogen activator and higher plas- minogen activator inhibitor-1), which is consistent with prev- ious literature. However, contrary to our hypothesis, we did not detect an increase in baseline coagulatory measures in obese firefighters nor did we find a greater coagulatory response among obese firefighters. Additional research is necessary to better understand how age, body composition, fitness, and car- diovascular risk factors affect vascular-hemostatic balance after firefighting because it is known that the vascular-hemostatic re- sponses to exercise differ between healthy individuals and those with underlying CVD or CVD risk factors (34).

TRIGGERING A CARDIOVASCULAR EVENT
Atherosclerosis and structural heart changes, such as left ven- tricular hypertrophy (LVH), generally develop during a period of many years — even decades. Although CVD may progress with or without symptoms for many years, a plaque rupture or arrhythmia can lead to the rapid onset of severe symptoms and even SCD. Understanding what precipitates, or triggers, such an event is of interest to researchers, clinicians, and those involved in health policy.

Classic studies have temporally linked heavy physical exertion/ strenuous work (such as running or snow shoveling) to the on- set of acute cardiovascular events, with the risk being con- centrated among individuals who are unaccustomed to such levels of exertion (1,12). Sympathetic nervous activation caused by emotional stress, such as excitement and frustration/ anger, also is associated with triggering cardiovascular events in individuals with known CHD (32). In addition, environ- mental conditions such as elevated levels of air pollution have been shown to be associated with increased rates of sudden car- diac events among susceptible individuals (23).

Although a series of complex pathophysiological processes are involved in SCD, pump failure usually caused by terminal cardiac arrhythmias is ultimately the cause of death. Terminal arrhythmias may be caused by a primary electrical conduction problem (primary arrhythmia) or as complications of plaque rupture and thrombus formation that causes a myocardial in- farction. SCD can be caused by many underlying conditions; however, a very large percentage of SCD is caused by CHD (i.e., atherosclerosis) and/or cardiomegaly/LVH (13,36).

Firefighting as a Trigger for Sudden Cardiac Events
The work of a firefighter is characterized by long periods of low-intensity work, such as inspection, chores, and public ed- ucation, unpredictably punctuated by episodes of strenuous work. During these intense intervals, firefighting activity in- volves heavy muscular work and requires high levels of oxygen consumption. Thus, during firefighting, multiple stressors may function independently or more likely synergistically to precip- itate acute CVD events among susceptible firefighters (32).

Multiple studies have provided compelling evidence that firefighting activities can trigger cardiovascular events in sus- ceptible firefighters (19,20). These studies found highly ele- vated and remarkably consistent odds for SCD and other
acute CVD events during emergency firefighting activities com- pared with nonemergency duties. The largest of these studies (19) investigated line-of-duty deaths (n = 449) attributed to CHD between 1994 and 2004 and found that, although
firefighting ( i.e., active fire suppression) represents between 1% and 5% of a firefighter's annual working time, firefighting accounted for more than 30% of line-of-duty CHD deaths. This resulted in a relative risk of SCD during fire suppression of roughly 10 to 100 times the risk encountered during nonemer- gency duties (19).

The aforementioned physiological disruption and cardiac strain associated with firefighting do not normally pose a signif- icant risk in healthy individuals. However, in susceptible indi- viduals, the stress of firefighting can serve as a trigger for SCD.

Most sudden cardiac events are likely caused by myocardial in- farctions in individuals with CHD or fatal arrhythmias in indi- viduals with CHD or cardiomegaly/LVH.

TABLE. Relative risk of cardiovascular outcome by risk factor in firefighters On-Duty CHD Fatalities,
OR (95% CI) (20)
Non-CHD Cardiovascular Retirements,
OR (95% CI) (16)
CHD Retirements,
OR (95% CI) (16)
Current smoking 8.6 (4.2 –17) 2.5 (1.2 –5.1) 3.9 (2.5 –6.2) Hypertension 12 (5.8 –25) 11 (6.1 –20) 5.4 (3.7 –7.9)
Obesity, BMI ≥30 kg∙m−2 3.1 (1.5–6.6) 3.6 (2.0 –6.4) 1.4 (0.96 –1.93) Cholesterol ≥5.18 mmol∙L−1 (200 mg∙dL−1) 4.4 (1.5–13) 1.1 (0.51–2.24) 2.4 (1.6–3.6) Diabetes mellitus 10.2 (3.7–28) 7.7 (2.9–20) 13 (6.1–28) Prior diagnosis of CHD 35 (9.5–128) NA 30 (9.1–96)
Age ≥45 yr 18 (8.5–40) 26 (13–51) 63 (35–111)
BMI, body mass index; CHD, coronary heart disease; CI, confidence interval; diabetes mellitus, defined as random blood glucose level more than 8.3 mm ol∙L−1 (150 mg∙dL−1), previous diagnosis, or receiving insulin or hypoglycemic medications; hypertension, defined as resting blood pressure of 140/90 mm Hg or higher, previous diagnosis of hypertension, or receiving anti- hypertensive therapy; OR, odds ratio. (Reprinted from (32). Copyright © 2011 Wolters Kluwer Health. Used with permission.) Volume 44  Number 3  July 2016 Firefighting and Sudden Cardiac Events 95 Copyright © 2016 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

346
Coronary Heart Disease
Studies that have examined autopsy findings of firefighters who suffered SCD have found that roughly 90% of victims had evidence of coronary atherosclerosis (13,20). CHD is characterized by atherosclerotic plaque in the arterial wall, which is preceded by endothelial dysfunction as the onset of the atherosclerotic process. As the plaque progresses, it results in stenosis and can cause ischemia. Death from a myocar- dial infarction frequently involves the rupture of vulnerable plaque, exposing blood to underlying connective tissue that is highly thrombotic. Platelets begin to adhere to the vessel and aggregate to each other to form a plug. Ultimately, this may re- sult in the formation of an occlusive thrombus that causes a myocardial infarction.

Firefighting leads to increased shear stress that may increase the risk of plaque rupture. Furthermore, firefighting increases platelet number and activity and leads to a procoagulatory con- dition that may make thrombus formation more likely. Thus, although most firefighters recover from the cardiovascular strain of firefighting without incident, an individual with underly- ing atherosclerotic plaque, especially vulnerable plaque, is at a greatly increased risk of plaque rupture and thrombus formation during firefighting activity.

Cardiomegaly/LVH
Cardiomegaly (increased heart size and mass) and LVH (in- creased wall thickness and mass) are structural abnormalities that increase the risk of SCD caused by arrhythmia. LVH is a powerful predictor of cardiovascular morbidity and mortality in population-based studies (3), and there is a strong graded as- sociation between left ventricular mass and increased cardio- vascular risk (14). LVH is frequently associated with fatal arrhythmias (33). Research has found that the increased risk as- sociated with LVH is independent of other factors such as age, sex, smoking status, diabetes, and serum cholesterol. In a major- ity of cases, LVH is typically a result of hypertension with or without obesity and/or CHD. Obstructive sleep apnea, which also is commonly associated with hypertension and obesity, is another risk factor for LVH. In cases of firefighting SCD, CHD and LVH are frequently comorbid (13,20,36). Myocardial fibrosis is thought to be an important mediator of increased risk of SCD associated with LVH, although the precise mechanisms by which LVH causes cardiovascul ar morbidity and mortality are not fully understood. However, vascular changes, such as in- creased arterial stiffness, altered wave reflections and arterial- ventricular uncoupling may lead to the development of LVH.

A recent retrospective study found that cardiomegaly/LVH is a frequent cause of SCD in the general public and is highly as- sociated with obesity and death at a younger age than CHD (33). There is mounting evidence that LVH/cardiomegaly is common among US firefighters and plays a major role in
CVD events in the fire service. Kales et al. (20) conducted a case-controlled investigat ion of on-duty CHD fatalities and found evidence for LVH in 76% of the CHD deaths
where the autopsy results were available. Subsequently, a larger follow-up case-fatality study was conducted to compare fire- fighters succumbing to on-duty CHD fatalities with firefighters suffering nonfatal CHD events leading to retirement. Among the fatalities, LVH/cardiomegaly was mentioned in summary reports of almost 60% of the available autopsies (13). In addition, Yang et al. (36) studied younger (<45 yr) firefighters and found a greater than 100-g difference in heart weight among SCD cases compared with trauma fatality controls. Fur- thermore, approximately 66% of cardiac cases had evidence of cardiomegaly (heart weight, >450 g), and this conveyed a five- fold increase in relative risk of SCD. These studies provide con- vincing evidence that LVH/cardiomegaly plays a role in a large percentage of firefighter fatalities.

As illustrated in Figure 3, firefighting may lead to several car- diac and vascular changes that increase the risk of arrhythmia, particularly in individuals with underlying vascular dysfunc- tion and/or structural heart abnormalities. Obviously, firefight- ing leads to increased cardiac work. Furthermore, our research suggests that firefighting also leads to increased arterial stiffen- ing and decreased arterial compliance. The combination of in- creased cardiac work and decreased arterial compliance may lead to ischemia that could provoke an arrhythmia, particularly in individuals with underlying CHD and/or structural heart changes that include myocardial fibrosis, which likely increases susceptibility to electrical abnormalities.

CONCLUSIONS
The proposed model suggests that the cardiovascular strain associated with firefighting may trigger a sudden cardiac event in a susceptible person through several biological pathways. In- creases in shear stress may cause rupture of vulnerable plaque, resulting in thrombus formation and the occlusion of coronary arteries, which may be exacerbated by hypercoagulability that is known to increase the risk of thrombotic events. Alternately, acute risks encountered during firefighting activities may in- crease the risk of fatal arrhythmias. Ischemia (caused by an in- crease in myocardial oxygen demand that exceeds myocardial supply) may result in electrical, mechanical, and biochemical dysfunction of the cardiac muscle. Exposure to environmental conditions (such as gaseous and particulate toxicants in smoke) also may increase susceptibility to arrhythmias (23), particularly in those with LVH and other forms of cardiomegaly or in the context of ischemia.

Additional research is necessary to better understand how in- dividual characteristics affect the cardiovascular responses to firefighting and the precise mechanisms by which firefighting leads to an increased risk of fatal arrhythmias and plaque rup- ture. Additional research also is needed to identify the types of physical fitness training programs that provide the greatest potential for reducing the risk of sudden cardiac events in the fire service. Firefighters accept great risks to protect their com- munities. Exercise professionals have a unique opportunity to help improve the health of this remarkable occupation, which is so important to our public safety.

Acknowledgments
The authors express their appreciation to Steve Ives and Kevin Heffernan who provided a review of the manuscript.

D. L. Smith and J. P. DeBlois were supported by FEMA-AFG EMW-2013- FP00749.

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R E V I E W Open Access
Prostate cancer in firefighting and police
work: a systematic review and meta-
analysis of epidemiologic studies
Jeavana Sritharan 1,2* , Manisha Pahwa 1, Paul A. Demers 1,2,3,4*, Shelley A. Harris 1,4,5, Donald C. Cole 4 and Marie-Elise Parent 6
Abstract
Objectives: We conducted a systematic review and meta-analysis to evaluate potential associations between firefighting and police occupations, and prostate cancer incidence and mortality.

Methods:Original epidemiological studies published from1980 to 2017 were identified through PubMed and Web of Science. Studies were included if they contained specific job titles for ever/never firefighting and police work and associated prostate cancer risk estimates with 95% conf idence intervals (CI). Study quality was assessed using a 20-point checklist. Prostate cancer meta-risk estimates (mRE) and corresponding 95% CIs were calculated for firefighting and police work separately and by variou s study characteristics using random effects models.

Between-study heterogeneity was evaluated using the I
2 score. Publication bias was assessed using Begg ’sa n d Egger’st e s t s .

Results: A total of 26 firefighter and 12 police studies were included in the meta-analysis, with quality assessment scores ranging from 7 to 19 points. For firefighter studies, the prostate cancer incidence mRE was 1.17 (95% CI = 1.08–1. 28, I2 = 72%) and the mortality mRE was 1.12 (95% CI = 0.92 –1.36, I2 = 50%). The mRE for police incidence studies was 1. 14 (95% CI = 1.02–1.28; I2 = 33%); for mortality studies, the mRE was 1.08 (95% CI = 0.80 –1.45; I2 = 0%). By study design, mREs for both firefighter and police studies were similar to estimates of incidence and mortality.

Conclusion: Small excess risks of prostate cancer were observed from firefighter studies with moderate to substantial heterogeneity and a relatively small number of police studies, respectively. There is a need for further studies to examine police occupations and to assess unique and shared exposures in firefighting and police work.

Keywords: Firefighters, Police, Occupation, Prostate cancer risk, Incidence, Mortality, Meta-analysis, Systematic review, Epidemiology
Background
Prostate cancer is one of the most commonly diagnosed
cancers in men worldwide but its etiology remains poorly understood [1 –5]. The only established risk factors for prostate cancer are older age, positive family history of prostate cancer, and African-American ethnicity [1, 2, 4, 5].

There is some evidence linking prostate cancer to differ- ences in socioeconomic status, increased height, increased obesity, reduced physical activity, and active smoking and alcohol use [3, 5–10]. There is growing evidence that occu- pation may be a risk factor, and previous studies have
shown increased risks associated with employment in agri- culture/farming, management and administration, rubber
production, metal work, and transportation [11–13]. Some studies have also suggested associations between prostate cancer risk and employment in protective services occupa- tions [11, 12, 15–17].

Protective services occupations include firefighting,
police, military, and other groups (eg. security guards).

Previous epidemiological studies have demonstrated
consistent associations between firefighting and different * Correspondence: [email protected]; [email protected]
1Occupational Cancer Research Centre, Cancer Care Ontario, 525 University Avenue, Toronto, ON M5G 2L3, Canada
Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Sritharan et al. Environmental Health (2017) 16:124
DOI 10.1186/s12940-017-0336-z

349
types of cancer, with some evidence for prostate cancer [14]. In 2007, the International Agency for Research on Cancer (IARC) classified firefighting as “possibly” car- cinogenic to humans (IARC Group 2B) [16]. IARC ’s
evaluation was based on evidence from 42 epidemio-
logical studies, including two previous meta-analyses on firefighting and cancer [14, 18]. Based on studies pub- lished at the time, IARC evaluated multiple cancer sites and identified statistically significant increased risks of prostate cancer, testicular cancer, and non-Hodgkin
lymphoma [16]. Since the IARC evaluation, 11 new stu-
dies have been published that included assessments of
prostate cancer risk in firefighters. Relatively less is known about prostate cancer risk in police occupations, as this group is often understudied and findings have
been inconsistent [11, 12, 15, 19, 20].

Only one meta-analysis, published over a decade ago,
focused on firefighting and cancer risks that included
prostate cancer [14]. This study found a significant asso- ciation with prostate cancer incidence (summary risk
estimate: 1.28, 95% CI: 1.15 –1.43) based on evidence
from 6 cohort studies [14]. Recently, a narrative review examined cancer risk in police work. Eight studies re-
ported on prostate cancer risk in police work, with
mixed findings [15]. The objective of the present syste- matic review and meta-analysis was to evaluate the qual- ity of the epidemiological evidence on firefighting and police employment in association with prostate cancer
incidence and mortality, and to conduct a quantitative
synthesis. Based on the availability of epidemiologic lit- erature, this meta-analysis focused on firefighting and police work, and not protective services as a whole.

Material and methods
Search strategy
A search was conducted on PubMed and Web of Sci-
ence to identify epidemiological studies published be-
tween January 1980 and December 2017 in English or
French about employment in firefighting and police
occupations, and risk of prostate cancer. Various combi- nations of MeSH terms were used to search for studies
that included firefighter and police occupations (fire- fighting OR firefighter OR fire fighter OR fire OR police OR police officer OR policeman OR policemen) and that
reported on associations with prostate cancer risk (pros- tate OR prostate neoplasm OR neoplasm OR cancer).

Cited references in individual papers and review papers that resulted from the search were used to identify any additional studies.

Inclusion criteria
To be included in the meta-analysis, articles must have reported results for original case –control or cohort stud- ies that contained specific job titles related to ever/never firefighting and police work and that examined associ-
ated prostate cancer incidence and/or mortality using
any type of relative risk estimator (hazard ratio (HR), odds ratio (OR), relative risk (RR), standardized mortal- ity ratio (SMR), or standardized incidence ratio (SIR)) with corresponding 95% confidence intervals. Reviews,
meta-analyses, editorials, and experimental studies were excluded. For any articles with overlapping study popu- lations, only the most recently published study with
prostate cancer incidence and/or mortality results was
included. Furthermore, studies were excluded if reported risk estimates were only based on internal comparisons
between different occupational groups rather than based on comparisons to the general population. Titles and
abstracts were initially screened for eligibility, and for those eligible, full-text articles were reviewed.

Data extraction
Information on author(s), date of publication, title, coun- try of study, study design, number of cases/deaths and
controls/non-cases, data collection method, effect sizes and 95% CIs for prostate cancer, and covariates was ex- tracted from and tabulated for each study included in
the meta-analysis. Effect sizes and 95% Cls recorded
from included studies were for ever vs. never firefighter or police employment in models that were adjusted for
the maximum number of potentially confounding
variables.

Quality assessment
The quality of each study included in the meta-analysis was independently assessed by two authors (JS and MP)
using a modified quality assessment checklist by Downs
and Black [21]. Checklist items that were irrelevant to observational studies were omitted, resulting in a max- imum of 20 achievable points for reporting (9 points),
external validity (2 points), internal validity (bias and confounding) (8 points), and power (1 point) [21]. Any
disagreement of ratings was discussed and a consensus
was arrived at mutually or by consulting a third author, if earlier consensus could not be reached.

Statistical analysis
Reported ORs, HRs, RRs, SIRs, and SMRs were consid-
ered as RRs in meta-analyses and used in forest plots. A random effects model was used to calculate meta-risk esti- mates (mREs) in all meta-analyses due to potential vari- ance in effect sizes between the included studies. mREs were calculated separately for firefighting and police occu- pations and prostate cancer risk. mREs were calculated for subgroups based on the following characteristics: inci- dence versus mortality, study design (i.e. cohort versus case–control, and administrative linkage-based studies, Sritharan et al. Environmental Health (2017) 16:124 Page 2 of 12

350
defined as large studies that used multiple linked adminis- trative databases, e.g. census data and tumour registries.

For each mRE, heterogeneity was evaluated using the
I2 statistic. The I 2 statistic is a percentage that describes the variation between studies that is not due to chance [22]. Two-sided p-values for the I2 statistic were re-
ported. Ninety-five percent confidence intervals for the I2 statistic were calculated to address small numbers of included studies ( N < 5) in some subgroup meta-
analyses. In addition, the Galbraith plot was used to
visualize if individual studies fell within or outside of the 95% confidence region. Studies outside of the 95% confi- dence region can contribute to high heterogeneity.

These studies were removed in sensitivity analyses to
evaluate the impact of decreased heterogeneity on
mREs [23].

Begg’s test and Egger ’s test were used to assess publi- cation bias. Begg ’s test uses the correlation between
ranks of effect sizes and variances, whereas Egger ’s test uses a funnel plot to plot the effect estimates against sample size [24, 25]. All statistical analyses were
performed using STATA version 14.2 (StataCorp LLC,
College Station, USA).

Results
The literature search resulted in 366 unique studies pu- blished in English or French. Based on the screening of titles and abstracts, 318 (87%) were excluded due to non- observational/non-human studies, missing job titles, miss- ing effect estimates for prostate cancer, duplicate studies, or irrelevancy to the objective of this meta-analysis. Of the remaining 48 studies that were obtained in full text, 17 were excluded because they did not include reports of rela- tive risks for prostate cancer with 95% CIs, had overlapping study populations, or were studies of military workers. As a result, 31 unique studies were included (Fig. 1).

Of these, 24 were cohort and seven were case –control
studies. Nineteen studies only included investigations of firefighters (Table 1) and five focused on police workers (Table 2); seven contained investigations of both fire- fighters and police workers (Table 3). In all studies that included firefighters ( N = 26), there were 5712 incident cases of prostate cancer and 428 deaths from prostate
cancer. In all studies that included police workers
(N = 12), there were 1510 incident cases and 49 deaths.

The characteristics of each included study are summa-
rized in Tables 1 (firefighters), 2 (police workers), and 3 (both). Covariates included in the risk estimates selected from each of the seven case –control studies are shown
in Additional file 1: Table S1.

Of all the firefighter studies, 2 pairs of studies (Ma et al., 2005 & Ma et al., 2006; Demers et al., 1992 & Demers et al., 1994) [26 –29] examined the same respective popula- tions but reported on different prostate cancer outcomes (incidence and mortality). In the meta-analyses of prostate cancer incidence and mortality in firefighters, respective results from both pairs of studies were retained and used.

Two studies [30, 31] published results for both prostate cancer incidence and mortality, and each estimate was
used [31, 32]. For the police studies, [28, 29] reported on the same populations with different outcomes of incidence and mortality, and each estimate was used. Each incidence and mortality outcome was used only in their respective categories and not included together for any meta-risk
estimates.

Quality assessment
The overall quality assessment of all 31 included studies ranged from 5 to 19 points (Table 4). Scores were similar for firefighter, police, and firefighter and police studies across the different quality assessment categories. The mean score for reporting was 6 out of 9, based on clear 366 unique studies identified in
PubMed and Web of Science
48 full-text articles obtained
31 included studies
318 titles and abstracts excluded based
on study design (non-observational/non-
human studies), missing appropriate job
titles, missing appropriate association
measures, overlapping study
populations, irrelevancy to objective
17 full-text articles excluded
- 12 military studies
- 4 missing prostate cancer risk estimates
- 1 overlap of study population
Fig. 1 Descriptive flow chart of study selection in this meta-analysis Sritharan et al. Environmental Health (2017) 16:124 Page 3 of 12

351
and detailed reporting of aims/hypotheses, outcomes
measures, participant information, confounder informa-
tion, and loss to follow-up. Studies were generally found to be externally valid, and there was minimal bias.

Studies of firefighters had higher scores for confounding factors than studies of police workers. Only one study
reported a power calculation making it difficult to eva- luate this category.

Table 1 Characteristics of included studies on firefighting and prostate cancer risk ( N = 19) Author/
Year
Location of Study Study
Design
Incidence or
Mortality
Follow-
up period
Number of
Cases/
Deaths
Cohort Size/Total
Number of
Cases
a
Prostate Cancer Risk Estimates
for Ever versus Never
Employment
b
Glass et al.
2016 [63]
Australia Cohort Incidence 1980 –
2011
478 30, 057 SIR 1.31, 95% CI 1.19 –1.43
Brice et al.
2015 [64]
France Cohort Mortality 1979 –
2008
17 10, 829 SMR 0.54, 95% CI 0.31 –0.86
Daniels
et al. 2014
[32]
USA Cohort Incidence;
Mortality
1950–
2009
1261 cases
282 deaths
29, 993
29, 993
SIR 1.03, 95% CI 0.98 –1.09;
SMR 1.09, 95% CI 0.96 –1.22
Pukkala
et al. 2014
[17]
Denmark, Finland,
Iceland, Norway
and Sweden
Cohort
(linkage)
Incidence 1961 –
2005
660 16, 422 SIR 1.13, 95% CI 1.03 –1.22
Ahn et al.
2012 [65]
Korea Cohort Incidence 1996 –
2007
9 33, 416 SIR 1.32, 95% CI 0.60 –2.51
Ma et al.
2006 [27]
USA Cohort Incidence 1981 –
1999
209 34, 796 SIR 1.10, 95% CI 0.95 –1.42
Ma et al.
2005 [26]
USA Cohort Mortality 1972 –
1999
21 34, 796 SMR 1.08, 95% CI 0.67 –1.65
Baris et al.
2001 [66]
USA, USA Cohort Mortality 1925 –
1986
31 7, 789 SMR 0.96, 95% CI 0.68 –1.37
Bates et al.
2001 [67]
New Zealand Cohort Incidence 1977 –
1995
11 4, 221 SIR 1.08, 95% CI 0.50 –1.90
Tornling
et al. 1994
[31]
Sweden Cohort Incidence;
Mortality
1951–
1986
28 cases
14 deaths
1, 116
1, 091
SMR 114, 95% CI 76 –165;
SMR 121, 95% CI 66 –202
Aronson
et al. 1994
[68]
Canada Cohort Mortality 1950 –
1989
16 5, 373 SMR 132, 95% CI 76 –215
Giles et al.
1993 [69]
Australia Cohort Incidence 1980 –
1989
5 2, 865 SIR 2.09, 95% CI 0.67 –4.88
Guidotti
1993 [70]
Canada Cohort Mortality 1927 –
1987
8 3, 328 SMR 146.1, 95% CI 63.1 –287.9
Beaumont
et al. 1991
[33]
USA Cohort Incidence 1940 –
1982
8 3, 066 RR 0.38, 95% CI 0.16 –0.75
Grimes
et al. 1991
[71]
USA Cohort Mortality 1969 –
1988
4 205 PRR 2.6, 95% CI 1.4 –5.0
Vena &
Friedler
1987 [72]
USA Cohort Mortality 1950 –
1979
5 470 SMR 0.71, 95% CI 0.23 –1.65
Tsai et al.
2015 [73]
USA Case–
control
(linkage)
Incidence 1988 –
2007
1397 3, 996 OR 1.45, 95% CI 1.25 –1.69
Kang et al.,
2008 [74]
USA Case–
control
(linkage)
Incidence 1986 –
2003
577 285, 964 SMOR 1.05, 95% CI 0.88 –1.24
Krstev et al.
1998 [75]
USA Case–
control
Incidence 1986 –
1989
12 981 OR 3.34, 95% CI 1.13 –9.91
acohort size represents the total sample size in only cohort studies, and the total number of cases is only applicable to case –control studies bHR – hazard ratio, SIR – standardized incidence ratio, SMR – standardized mortality/morbidity ratio, RR – relative risk, PRR – proportionate risk ratio, OR – odds ratio, NR – not reported
Sritharan et al. Environmental Health (2017) 16:124 Page 4 of 12

352
Firefighter and prostate cancer meta-analyses
There were significantly elevated prostate cancer risks for firefighting occupations for incidence outcomes, cohort studies, and administrative linkage-based studies. For inci- dence studies, the mRE was 1.17 (95% CI: 1.08 –1.28;
I2 = 72%, 95% CI: 55– 82%, p-value <0.001; 19 studies)
(Fig. 2); for mortality studies, it was 1.12 (95% CI: 0.92 –

1.36; I2 = 50%, 95% CI: 0– 76%, p-value = 0.04; 10 studies) (Fig. 3). In cohort studies, the prostate cancer mRE was

1.14 (95% CI: 1.03–1.26; I2 = 67%, 95% CI: 46–80%, p-value <0.001; 18 studies) (Additional file 2: Figure S1). The meta- analysis of case–control studies resulted in an mRE of 1.27 (95% CI: 0.95– 1.69; I 2 = 78%, 95% CI: 53– 90%, p-value <0.001; 6 studies) (Additional file 3: Figure S2). The mRE for census or administrative linkage-based studies was 1.19 (95% CI: 1.06–1.34; I
2 = 61%, 95% CI: 0–85%, p-value = 0.04;
5 studies) (Additional file 4: Figure S3).

Police work and prostate cancer meta-analyses
There were significantly elevated prostate cancer risks for police occupations by incidence outcomes and in case –
control studies. The mRE for prostate cancer incidence
studies was 1.14 (95% CI: 1.02 –1.28; I
2 = 33%, 95% CI: 0–
74%, p-value = 0.16; 9 studies) (Fig. 4) while the mRE for prostate cancer mortality studies was 1.08 (95% CI: 0.80 –

1.45; I2 =0 % ,9 5 %C I :0 %–90%, p-value = 0.62; 3 studies) (Fig. 5). The mRE for case –control studies was higher
compared to the mRE for cohort studies (case –control
studies: mRE = 1.22, 95% CI: 1.03– 1.44; I2 = 0% (95% CI 0%–85%, p-value = 0.42; 4 studies) (Additional file 5:

Figure S4) versus cohort studies: mRE = 1.10, 95% CI:

0.96–1.26; I
2 = 37%, 95% CI: 0– 79%, p-value = 0.15; 7 stu-
dies) (Additional file 6: Figure S5). There were no adminis- trative linkage-based studies of police workers and prostate cancer risk.

Table 2 Characteristics of included studies on police work and prostate cancer risk ( N =5 ) Author/Year Location of
Study
Study
Design
Incidence or
Mortality
Follow-up
Period
Number of
Cases/Deaths
Cohort Size/Total
Number of Cases
a
Prostate Cancer Risk Estimates for Ever
versus Never Employment
b
Vena et al. 2014
[19]
USA Cohort Mortality 1980 –
2005
31 3, 424 SMR 1.18, 95% CI 0.80 –1.67
Gu et al. 2011
[76]
USA Cohort Incidence 1976 –
2006
104 2, 234 SIR 0.88, 95% CI 0.72 –1.07
Finkelstein 1998
[20]
Canada Cohort Incidence 1964 –
1995
85 22, 197 SIR 1.16, 95% CI 0.93 –1.43
Forastiere et al. 1994
[77]
Italy Cohort Mortality 1973 –
1991
7 3, 868 SMR 0.77, 95% CI 0.31 –1.50
Bouchardy et al. 2002
[78]
Switzerland Case –
control
Incidence 1980 –
1993
129 9, 126 OR 1.20, 95% CI 1.00 –1.50
acohort size represents the total sample size in only cohort studies, and the total number of cases is only applicable to case –control studies bHR – hazard ratio, SIR – standardized incidence ratio, SMR – standardized mortality/morbidity ratio, RR – relative risk, OR – odds ratio Table 3 Characteristics of included studies on both firefighting and police work and prostate cancer risk ( N =7 ) Author/Year Location of
Study
Study
Design
Incidence or
Mortality
Follow-up
Period
Number of
Cases/Deaths
Cohort Size/Total
Number of Cases
a
Prostate Cancer Risk Estimates for
Ever versus Never Employment
b
Sritharan et al, 2017b* Canada Cohort
(linkage)
Incidence 1991 –
2011
165 firefighters;
325 police
1,100,000
1,100,000
HR 1.17, 95% CI 1.01 –1.36;
HR 1.22, 95% CI 1.09 –1.36
Zeegers et al.
2004 [11]
Netherlands Cohort
(linkage)
Incidence 1986 –
1993
709 firefighters;
693 police
58, 279
58, 279
RR 0.59, 95% CI 0.05 –6.33;
RR 1.62, 95% CI 0.62 –4.27
Demers et al.
1994 [28]
USA Cohort Incidence 1974 –
1989
66 firefighters;
28 police
2, 447
1, 878
SIR 1.40, 95% CI 1.10 –1.70;
IDR 1.10, 95% CI 0.70 –1.80
Demers et al.
1992 [29]
USA Cohort Mortality 1945 –
1989
30 firefighters;
11 police
4, 546
3, 676
SMR 1.34, 95% CI 0.90 –1.91;
SMR 1.02, 95% CI 0.51 –1.82
Sritharan et al.
2017a [79]
Canada Case –
control
Incidence 1995 –
1998
38 firefighters;
35 police
1, 737
1, 737
OR 1.67, 95% CI 0.94 –2.95;
OR 1.15, 95% CI 0.66 –1.99
Sritharan et al.
2016 [80]
Canada Case –
control
Incidence 1994 –
1997
53 firefighters;
12 police
760
760
OR 0.73, 95% CI 0.53 –1.01;
OR 0.82, 95% CI 0.41 –1.63
Sauve et al.
2016 [12]
Canada Case –
control
Incidence 2005 –
2009
26 firefighters;
45 police
1, 937
1, 937
OR 1.72, 95% CI 0.88 –3.37;
OR 1.60, 95% CI 1.00 –2.40
acohort size represents the total sample size in only cohort studies, and the total number of cases is only applicable to case –control studies bHR – hazard ratio, SIR – standardized incidence ratio, SMR – standardized mortality/morbidity ratio, RR– relative risk, IDR – incidence density ratio, OR – odds ratio *manuscript submitted and currently being revised for publication Sritharan et al. Environmental Health (2017) 16:124 Page 5 of 12

353
Between-study heterogeneity
There was high heterogeneity (72%) for the meta-analysis of all 19 firefighter incidence studies. As a sensitivity ana- lysis, the Galbraith plot was used and one study [33],
appeared outside of the 95% confidence region. Removal
of this study resulted in a minimal change in heterogeneity (72 versus 69%, respectively). For the meta-analysis of the 10 mortality studies, there was moderate heterogeneity
(50%). High heterogeneity was observed for the six case – control studies (78%), 18 cohort studies (67%) and the five administrative linkage-based studies (61%). When plotting these subgroups using the Galbraith plot, no studies ap- peared outside of the 95% confidence region.

For police studies, heterogeneity ranged from none to
moderate. Moderate heterogeneity was observed for the
nine incidence studies (33%) and seven cohort studies
(37%), but no heterogeneity (0%, 95% CI: 0 –90%) was ob- served for the mortality (three studies) and case –control (four studies) subgroups. I
2 values of 0% are biased and
imprecise, likely because of the small number of studies in these subgroups ( n < 5) [34]. Using the Galbraith plot, none of the police studies appeared outside of the 95%
confidence region.

Publication bias
There was no evidence of publication bias according to
Begg’s test ( p = 0.86) and Egger ’s test ( p = 0.11) for the meta-analysis of all 19 firefighter incidence studies. No publication bias was evident for the 9 police incidence studies (Begg ’s test: p = 0.60, Egger ’s test: p = 0.68).

There were also no statistically significant findings for Table 4 Quality assessment of included firefighter and police studies Quality Assessment Category Maximum Attainable
Score
Studies on firefighters
(n = 19)
Studies on police
workers (n = 5)
Studies on both firefighters
and police workers (n = 7)
All studies ( n = 31)
Range Mean Range Mean Range Mean Mean
Reporting 9 4–9 6.0 1 –8 5.4 4 –8 6.1 5.9
External Validity 2 1–2 1.8 0 –2 1.6 1 –2 1.6 1.7
Internal Validity: Bias 4 3–4 3.8 3 –4 3.8 4 4 3.8
Internal Validity: Confounding 4 2–4 3.2 1 –4 2.8 3 –4 3.6 3.2 Power 1 0 0 0 0 0 –1 0.1 0.0
Total 20 10–19 14.8 5 –18 13.6 12 –18 15.4 14.6
Fig. 2 Forest plot and mRE of all included prostate cancer incidence studies on firefighters Sritharan et al. Environmental Health (2017) 16:124 Page 6 of 12

354
publication bias for mortality studies, case –control, co- hort, and administrative linkage-based studies.

Discussion
In this meta-analysis of 31 epidemiological studies of pro- tective services workers, nearly identical and small statisti- cally significant excess risks of prostate cancer were found for ever working in firefighting and police work. Statisti- cally significant and borderline prostate cancer mREs were found for firefighters in separate evaluations of incidence studies, cohort studies, and administrative linkage studies, as well as in each meta-analysis of police worker incidence studies and case –control studies. Most studies were of average quality, with opportunities for improvement in
reporting and study power assessment. As expected, case – control studies compared to cohort studies generally had more information on variables that can act as potential confounders of the firefighter/police work and prostate cancer associations. All case –control studies reported prostate cancer risk estimates that were adjusted for age; Fig. 3 Forest plot and mRE of all included prostate cancer mortality studies on firefighters Fig. 4 Forest plot and mRE of all included prostate cancer incidence studies on police workers Sritharan et al. Environmental Health (2017) 16:124 Page 7 of 12

355
most were also adjusted for ethnicity. Fewer case –control studies adjusted risk estimates for family history of pros- tate cancer and potentially confounding variables such as socioeconomic status, physical activity, height, obesity, active smoking, and alcohol consumption. Overall, find- ings from this meta-analysis support positive associations found between prostate cancer risk and firefighting in the epidemiological literature, and indicate a potential rela- tionship with police work as well.

There are a few hypotheses that may explain why em-
ployment in protective services occupations could be as- sociated with increased prostate cancer risk. Firefighting and police jobs are inherently dangerous occupations
that involve stressful, and, at times, life-threatening, situ- ations with exposure to multiple hazards [14– 16]. Psy- chological stressors can influence biological processes and lead to decreased immune function, increased pro-
inflammatory cytokine secretion, and cancer progression [15]. Shift work, which is common in protective services work, was significantly associated with increased pros- tate cancer risk in a recent meta-analysis of eight case – control and cohort studies [35]. Firefighters are also ex- posed to toxins released by fire and smoke including
benzene, 1,3-butadiene, formaldehyde and at times can
be exposed to other compounds such as radiation, diesel exhaust, asbestos, metals (arsenic and cadmium), and
PAHs [14, 16, 27]. The chemical reactions during com-
bustion and the age and type of building or material on fire can contribute to exposure to these compounds
[16]. Police work involves fewer chemical exposures
compared to firefighting, although exposure to ionizing radiation from radar devices is a concern for overall can- cer risk [11, 12]. Firefighters and police workers may
also be exposed to air pollution on the job, as ambient concentrations of ultrafine particles and NO
2 have been
previously linked to prostate cancer risk [36, 37]. Of the described chemical exposures, only x and gamma radi-
ation, arsenic compounds, and cadmium compounds
have been linked to prostate cancer by IARC based on
limited evidence in non-occupational settings. However, IARC has classified benzene, ionizing radiation, diesel exhaust, asbestos, arsenic compounds, cadmium com-
pounds, and air pollution as all Group 1 carcinogens,
based on evidence for other cancer sites [38]. There is a need to further examine these chemical exposures in
both firefighting and police work to understand if these exposures are involved in prostate cancer risk.

Evaluating potential associations between shift work
and prostate cancer is an active area of ongoing
research [39 –41]. Shift work can disrupt the body ’s
endogenous circadian rhythm (sleep-wake cycle) and
contribute to increased susceptibility to acute and
chronic diseases. However, the biological mechanisms
t h a tm a yb ei n v o l v e di np r o s t a t ea n do t h e rc a n c e r s have not been established [18, 42]. One hypothesis is
that night shift work can lead to decreased melatonin,
which can then lead to continuous testosterone pro-
duction, influencing the growth and differentiation of
prostate cancer cells [16]. In addition, decreased sun- light exposure in night shift workers reduces the pro-
duction of vitamin D, thereby compromising the
effects of vitamin D on suppressing the production of
prostate cancer cells [16].

Psychological stress also has been linked to cancer
progression, but there is limited evidence for how this impacts cancer promotion [43]. Firefighting and police
work involve constant stressors that can potentially
affect cancer progression, particularly prolonged stress over years of employment in these jobs [44]. A recent
study on stress at work and cancer outcomes found that
Fig. 5 Forest plot and mRE of all included prostate cancer mortality studies on police workers Sritharan et al. Environmental Health (2017) 16:124 Page 8 of 12

356
the highest prevalence of stress at work was reported
among firemen when compared to other types of occu-
pations [45].

Another factor that may influence our meta-analysis
results is prostate cancer screening. Although prostate specific antigen (PSA) testing varies across different
countries and within countries, it is believed that pro- tective services workers have frequent and better access to health resources compared to other workers, includ-
ing access to cancer screening [32]. In North America,
for example, firefighters are provided with health infor- mation and recommendations on what to consider when
completing a health examination with their primary
physician, including recommendations for prostate can-
cer screening [46]. However, it is up to each fire depart- ment to disseminate this information and ultimately up
to each firefighter to request screening from their pri- mary physician. In this meta-analysis we found slightly lower mortality mREs compared to incidence mREs for
firefighters and police officers. As increased screening of prostate cancer leads to the identification of more early stage cases (increased incidence), this may be indicative of a screening effect. However, the mREs for both inci- dence and mortality were so similar that it was difficult to attribute these differences to screening. Also, prostate cancer screening may not be of high importance in fire- fighting compared to other cancers (ex. brain, bladder, and colon) and health conditions that have been consist- ently associated with firefighting. We evaluated study
estimates based on different follow-up periods defined
as pre-PSA period (prior to 1990 before the PSA test
was introduced), during the introduction of PSA testing (early 1990s), and after the introduction of PSA testing (late 1990s and onwards). Although we included studies
from different nations, most of the studies were North
American so we loosely defined the time periods based
on North America. We identified a number of pre-PSA
period firefighter studies and observed a meta-risk esti- mate of 1.26 (95% CI 0.96 –1.67) for these studies. For firefighter studies that had follow-up periods during and after the introduction of PSA testing, we observed a
meta-risk estimate of 1.13 (95% CI 1.02 –1.25). It was
challenging to define firefighter study follow-up periods as post PSA testing (late 1990s onwards) since most of
these studies had follow-up periods that overlapped the early 1990s when PSA testing began. We identified only
a few firefighter studies that had later follow-up periods (late 1990s and early 2000s) and observed a meta-risk
estimate of 1.58 (95% CI 1.09– 2.29) for these studies.

Overall, we observed an elevated risk for firefighter stud- ies that were conducted before the introduction of PSA
testing, and a statistically significant elevated risk for firefighter studies that took place during and after the introduction of PSA testing. These findings may be
representative of the increased screening that took place over this time period. We attempted to evaluate police
studies as well but were limited as almost all included police studies had follow-up periods overlapping periods with and without PSA testing.

Our findings of a slight excess risk of prostate cancer in firefighting and police services should be cautiously interpreted. As expected, there was considerable hetero- geneity between studies, particularly in subgroup meta- analyses of police workers and prostate cancer risk
that involved small numbers of studies. This makes it
challenging to interpret mRE values with precision
[34]. Heterogeneity was likely due to differences in
study design and populations studied, follow-up years,
occupational exposure assessment and job coding,
and adjustment of relative risk values for known or
potential covariates. Specifically, there were differ-
ences in how the study populations were defined, in
terms of paid or unpaid work, full time vs. part time,
and eligible employment duration. Some heterogeneity
may also be attributed to different follow-up periods
in each study, especially those overlapping the pre
and post PSA era. The variation in age distribution
across included studies could also contribute to het-
erogeneity based on differences in how studies strati-
fied by age. Some studies had relatively younger
populations than other studies and we observed a
similar elevated meta-risk estimate for these younger
population studies as we did for the overall estimates
Publication bias was also considered, but was not rec-
ognized as a significant factor as a majority of the
included studies were cohort designs. The cohort
studies generally looked at multiple cancer sites as
outcomes, so it is unlikely that publication bias would have been of concern based on solely prostate cancer
results.

A major strength of this meta-analysis is that it was
the first to assess prostate cancer risk in both firefighting and police work, replete with subgroup analyses and as- sessments of study quality, heterogeneity, and publica- tion bias. This meta-analysis captured all previously and newly published studies since the IARC evaluation of
firefighting in 2007, and also quantitatively evaluated prostate cancer risk in police studies which had not been done before. Firefighting and police work should be pri- ority areas for investigation because these occupations frequently involve exposure to multiple chemical, bio-
logical, physical, and psycho-social hazards. Exposure to some hazards may be associated with increased risk of
prostate cancer, although the strength and consistency
of associations varies across studies and there are sub- stantial research gaps. Altogether, this research can be used to help identify opportunities for further research on occupation and prostate cancer risk.

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Other occupations of interest with respect to prostate
cancer risk are military workers. While we initially sought to include military studies in this meta-analysis, they were ultimately not included because these studies were pri- marily based on specific historical events (ex. Gulf war) or internal comparisons between military groups [47 –62].

This made it difficult to compare findings to other studies that did not focus on single events or that compared
workers to the general population. Future assessments can separately consider military studies.

Conclusions
Overall, the slight excess risks of prostate cancer in fire- fighting and police services found in this meta-analysis of 31 studies were generally robust to subgroup analyses by outcome (incidence and mortality) and study design. Our findings are important as they show the importance of
prostate cancer incidence and mortality among protective services workers, and as this is the first meta-analysis to include both firefighting and police work and prostate
cancer risk. The observed findings suggest that screening may not entirely explain our findings, but further investi- gation into actual screening rates and screening behav- iours in firefighting and police work is warranted. Also, further investigations should be designed to assess specific exposures such as benzene, radiation, diesel exhaust, ar- senic and cadmium compounds, PAHs, asbestos, and air
pollution which are involved in firefighting. Little evidence on how they may relate to prostate cancer risk has been accrued. There is also a need for future studies to examine prostate cancer risk in police work given the small num- ber of police workers published to date. By addressing
these important issues in future studies, there will be bet- ter understanding on prostate cancer risk in firefighting and police work.

Additional files
Additional file 1: Table S1. Covariates adjusted for in firefighter and police case –control studies (DOCX 12 kb)
Additional file 2: Figure S1. Forest plot and mRE of all included cohort studies on firefighters. (DOCX 285 kb)
Additional file 3: Figure S2. Forest plot and mRE of all included case–control studies on firefighters. (DOCX 137 kb)
Additional file 4: Figure S3. Forest plot and mRE of all included administrative linkage-based studies on firefighters. (DOCX 153 kb) Additional file 5: Figure S4. Forest plot and mRE of all included cohort studies on police workers. (DOCX 175 kb)
Additional file 6: Figure S5. Forest plot and mRE of all included case–control studies on police workers. (DOCX 117 kb)
Abbreviations
CI: Confidence Interval; HR: Hazard Ratio; IARC: International Agency for Research on Cancer; mRE: Meta-risk Estimate; OR: Odds Ratio; PSA: Prostate Specific Antigen; RR: Relative Risk; SIR: Standardized Incidence Ratio; SMR: Standardized Mortality Ratio
Acknowledgements
We would like to acknowledge Sheila Kalenge ’s contribution to prostate cancer screening behaviours in firefighters.

Data availability statements
All data generated or analyzed during this study are included in this published article.

Funding
The corresponding author is supported by the Occupational Cancer Research Centre which is funded by the Canadian Cancer Society, Cancer Care Ontario and the Ontario Ministry of Health and Long-Term Care. Grant number 2014 –703257.

Authors’ contributions
The corresponding author, JS, is the main contributor in the design and conceptualization of the study, analysis and interpretation of the data, drafting the work and revising content critically, final approval of the work to be published. MP contributed to the design and conceptualization of the study, analysis and interpretation of the data, drafting and revising the work critically, and final approval of the version to be published. PD contributed to the conceptualizing and design of the study, acquisition and interpretation of the data, drafting the work and revising content critically, and final approval of the work to be published. SH and DC contributed to the design and interpretation of the work, drafting and revising content critically, and final approval of the version to be published. MEP contributed to the design and conceptualization of the study, interpretation of the data, drafting and revising the work critically, and final approval of the version to be published.

Ethics approval and consent to participate
Not Applicable.

Consent for publication
Not Applicable.

Competing interests
The authors declare that they have no competing interests.

Publisher’sN o t e
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Author details
1Occupational Cancer Research Centre, Cancer Care Ontario, 525 University Avenue, Toronto, ON M5G 2L3, Canada. 2Institute of Medical Science, University of Toronto, 525 University Avenue, Toronto, ON M5G 2L3, Canada. 3CAREX Canada, Simon Fraser University, Burnaby, Canada. 4Dalla Lana School of Public Health, University of Toronto, 525 University Avenue, Toronto, ON M5G 2L3, Canada. 5Population Health and Prevention, Cancer Care Ontario, 525 University Avenue, Toronto, ON M5G 2L3, Canada. 6INRS-Institut Armand-Frappier, University of Quebec, 531 Boulevard des Prairies, Laval, Quebec H7V 1B7, Canada.

Received: 20 September 2017 Accepted: 27 October 2017
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Risk of Cancer Among Firefighters in California, 1988–2007 Rebecca J. Tsai, PhD1, Sara E. Luckhaupt, MD, MPH1, Pam Schumacher1, Rosemary D.

Cress, PhD2,3, Dennis M. Deapen, DrPH, and Geoffrey M. Calvert, MD, MPH1 1Division of Surveillance, Hazard Evaluations and Field Studies, National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Cincinnati, Ohio 2Public Health Institute, Cancer Registry of Greater California, Sacramento, California 3Department of Public Health Sciences, UC Davis School of Medicine, Davis, California Abstract
Background—Most studies of firefighter cancer risks were conducted prior to 1990 and do not reflect risk from advances in building materials.

Methods—A case–control study using California Cancer Registry data (1988–2007) was conducted to evaluate the risk of cancer among firefighters, stratified by race.

Results—This study identified 3,996 male firefighters with cancer. Firefighters were found to have a significantly elevated risk for melanoma (odds ratio [OR]=1.8; 95% confidence interval [CI] 1.4–2.1), multiple myeloma (OR 1.4; 95%CI 1.0–1.8), acute myeloid leukemia (OR 1.4; 95%CI 1.0–2.0), and cancers of the esophagus (OR 1.6;95%CI 1.2–2.1), prostate (OR 1.5; 95%CI

1.3–1.7), brain (OR 1.5; 95%CI 1.2–2.0), and kidney (OR 1.3; 95%CI 1.0–1.6).

Conclusions—In addition to observing cancer findings consistent with previous research, this study generated novel findings for firefighters with race/ethnicity other than white. It provides additional evidence to support the association between firefighting and several specific cancers.

Keywords
cancer; firefighters; occupation; registry; risk
Correspondence to: Rebecca J. Tsai, PhD, National Institute for Occupational Safety and Health, 1090 Tusculum Avenue, R-17, Cincinnati, OH 45226. [email protected].

Author Contribution Statement: All authors contributed to the conception, acquisition, analysis, or interpretation of data. Additionally, all authors were involved in reviewing and revising this manuscript for intellectual content, approving the final manuscript, and agreeing to account for all aspects of this manuscript.

Disclaimers: The collection of cancer incidence data used in this study was supported by the California Department of Public Health as part of the statewide cancer reporting program mandated by California Health and Safety Code Section 103885; the National Cancer Institute’s Surveillance, Epidemiology and End Results Program under contract HHSN261201000140C awarded to the Cancer Prevention Institute of California, contract HHSN261201000035C awarded to the University of Southern California, and contract HHSN261201000034C awarded to the Public Health Institute; and the Centers for Disease Control and Prevention’s National Program of Cancer Registries, under agreement U58DP003862-01 awarded to the California Department of Public Health. The ideas and opinions expressed herein are those of the author(s) and endorsement by the National Institute for Occupational Safety and Health, the National Center for Health Statistics, the State of California, Department of Public Health, the National Cancer Institute, and the Centers for Disease Control and Prevention or their Contractors and Subcontractors is not intended nor should be inferred.

Conflict of interest: All authors are federal or state government employees and preparation of this manuscript was completely funded by the US Government. Otherwise, no additional financial disclosures were reported by the authors of this paper.

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Am J Ind Med. 2015 July ; 58(7): 715–729. doi:10.1002/ajim.22466.

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INTRODUCTION
Firefighting is considered one of the most hazardous occupations [Guidotti, 1993]. In 2013, there were about 354,600 (31%) career firefighters and 786,150 (69%) volunteer firefighters in the United States [
National Fire Protection Association, 2014]. Firefighters are expected to perform under stressful conditions and are at risk for physical, biological, and chemical- related injuries and illnesses, including cancer [Melius, 2001].

Firefighters are often exposed to known carcinogens in their line of work [Stefanidou et al., 2008]. Additionally, the introduction of newer building materials beginning approximately 40 years ago (e.g., engineered thermoplastics such as polyvinylidene fluoride, and laminated veneer lumber), may expose firefighters to potentially new carcinogenic combustion products [Beaumont et al., 1991; Grundahl, 1992]. The burning of plastics found in home appliances, furniture, and electronics may also introduce firefighters to new carcinogens [Korst, 2012]. While the use of a self-contained breathing apparatus (SCBA) can eliminate or significantly decrease respiratory exposure to toxic particles during firefighting [Bates, 2007
], SCBAs are not always worn, especially during overhaul. Overhaul is a late-stage in fire suppression when the burned area is inspected for flammable sources that can rekindle a fire. During overhaul, firefighters can be exposed to combustion products through disassembling walls or removing furniture [Bates, 2007]. Furthermore, even when SCBAs are worn, firefighters can absorb combustion products through the unprotected skin on their neck [
Fent et al., 2014]. Several combustion products are classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans (Group 1), probably carcinogenic to humans (Group 2A) or possibly carcinogenic to humans (Group 2B). These combustion products include benzene (Group 1), benzo[a]pyrene (Group 1), vinyl chloride (Group 1), formaldehyde (Group 1), 1, 3-butadiene (Group 1), and polychlorinated biphenyls (PCBs) (Group 1) [
Melius, 2001; Stefanidou et al., 2008; International Agency for Research on Cancer, 2015]. Firefighters may also be exposed to asbestos (Group 1) and lead (Group 2A) when present in burning buildings [International Agency for Research on Cancer, 2015]. Non-fire-related exposures may also increase cancer risk, as firefighters can be exposed to diesel exhaust from fire trucks, and diesel exhaust was found to be associated with increased cancer risks [International Agency for Research on Cancer, 2015].

Studies have found that firefighting is significantly associated with an increased risk for developing the following cancers: colorectal [Vena and Fiedler, 1987 ; Burnett et al., 1994
;
Baris et al., 2001; Daniels et al., 2014], lung [Pukkala et al., 2014; Daniels et al., 2015], melanoma [Howe and Burch, 1990; Sama et al., 1990], prostate [Grimes et al., 1991; Demers et al., 1994; LeMasters et al., 2006], testis [LeMasters et al., 2006; Ma et al., 2006], urinary bladder [Vena and Fiedler, 1987
; Sama et al., 1990; Ma et al., 2006], kidney
[Burnett et al., 1994; Delahunt et al., 1995; Daniels et al., 2014], brain [Grimes et al., 1991; Demers et al., 1992; Aronson et al., 1994; Ma et al., 2006], myeloma [LeMasters et al., 2006], non-Hodgkin lymphoma [Burnett et al., 1994; Figgs et al., 1995; Golden et al., 1995], and leukemia [Morton and Marjanovic, 1984; Golden et al., 1995; Baris et al., 2001; Daniels et al., 2015]. In recognition of these cancer risks, as of 2015, a total of 33 states cover firefighters for one or more cancers under workers’ compensation as a result of presumption legislation [
International Association of Fire Fighters, 2015]. In 20 of these states, the Tsai et al. Page 2
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language in the presumption legislation contains broad or nonspecific language that can be interpreted to cover any cancer experienced by a firefighter. In the other 13 states, only certain specific cancers are covered, most commonly leukemia (12 states), brain cancer (10 states), bladder cancer (9 states), non-Hodgkin lymphoma (9 states), and gastrointestinal cancer (8 states).

Ongoing assessment of cancer risks among firefighters is needed because of inconsistent findings across previous studies [International Agency for Research on Cancer, 2010], and because most studies were conducted prior to 1990 which may limit their ability to detect new risks arising from advances in building materials. In addition, few studies have examined the cancer risks among firefighters of other race/ethnicity. This study aims to update and expand a previous study by Bates [Bates, 2007], assessing cancer risks among firefighters using data from the California Cancer Registry (CCR). In addition, this is one of the first studies to include an examination of firefighter risk for subtypes of leukemia, esophageal cancer and lung cancer, and cancer risks among firefighters of other race/ ethnicity (e.g., blacks and Hispanics).

MATERIALS AND METHODS
California Cancer Registry (CCR)
CCR is a population-based cancer surveillance system that collects data on all cancers (excluding non-melanoma skin cancers and in-situ cervical carcinoma) among California residents. It is estimated that at least 95% of cancer cases are ascertained by CCR [North American Association of Central Cancer Registries, 2015]. Cancer reporting has been mandated by California law since 1985 and CCR has collected statewide cancer data from doctors, hospitals, and other medical facilities since January 1, 1988. Data collected by CCR include demographics, cancer characteristics, and cancer treatments. Information on the industry and occupation (I&O) of the job held longest by each case are also collected in narrative form. CCR provided the National Institute for Occupational Safety and Health (NIOSH) with de-identified cancer data collected from 1988 to 2007. Because this is a public health surveillance study with analyses conducted on anonymous data without links to personal identifiers, it was exempted from review by the NIOSH Institutional Review Board.

Firefighter Definition
To identify all cancer cases among firefighters, the I&O narrative fields were extensively searched for key words consistent with firefighting. The identified I&Os were coded using 1990 Bureau of Census (BOC) codes. Occupation codes used to indicate a career in firefighting are 413, 416, and 417. All firefighters with these codes were selected regardless of industry. The BOC manual states that the 417 code includes firefighters, fire chief’s aides, smoke jumpers, forest-fire fighters, and crash-crew men [U.S. Department of Commerce, 2000]. Their main duties are to control and extinguish fires that threaten life, property or environment, fire prevention, emergency medical service, hazardous material response, search and rescue, and disaster management [U.S. Department of Commerce, 2000]. A total Tsai et al. Page 3
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of 29 search terms (e.g., firefighter, fire crew worker) were used to identify and code individuals as BOC code 417.

Individuals who work at a fire department but do not usually carry out firefighting duties have a BOC occupation code of 413 or 416. BOC code 413 indicates positions that supervise and coordinate the firefighter’s activities, as well as participate in fire prevention and control [U.S. Department of Commerce, 2000]. A total of 11 search terms (e.g., fire captain, fire chief, and fire marshal) were used to identify and code individuals as BOC code 413. BOC code 416 refers to positions that inspect buildings and firefighting equipment for fire hazards, enforce state and local fire-related ordinances, determine cause of fires or explosions, and recommend fire prevention measures [
U.S. Department of Commerce,
2000]. A total of 23 search terms (e.g., fire inspector, arson investigator, forest fire control officers, fire ranger, fire warden, and fire lookout) were used to identify and code individuals as BOC 416. Many of these individuals (i.e., those with BOC codes 413 or 416) likely started their career as firefighters (BOC code 417) and were labeled as firefighters in this study. Because our findings were very similar whether firefighters were defined as BOC code 417 only versus combining 413, 416, and 417, to maximize sample size we report only the findings for the combined definition.

Cancer cases can have multiple cancer records at CCR if they were diagnosed with the same primary cancer multiple times or diagnosed with two or more primary cancers at different times. A separate cancer record is created for each cancer diagnosis or recurrence. The I&O assigned to these cases with multiple cancer records were the I&O present at the initial diagnosis because I&O information from the earliest record is thought to provide the best indication for longest-held job. In addition, some individuals were found to have more than one primary cancer at initial diagnosis. CCR assigned the cancer with the worse prognosis as the “first” primary.

Exclusion Criteria
This study included only adult male subjects (18–97 years of age) who had I&O information available. Excluded were females, homemakers, those with insufficient I&O narratives (e.g., narratives that mentioned only unemployed, disabled, or retired, or were blank), those who never worked, and those in the military.

Selection of Cases and Control Cancers
In situ and benign tumors were excluded. Cancers that spread into surrounding tissues (malignant/invasive), and were identified as the first “malignant” primary were included in this study. The cancer risk among firefighters was assessed for all cancers that included at least 10 firefighters. Control cancers were selected after reviewing the literature for cancers that appeared to have little or no association with firefighting and its related exposures.

These control cancers were cancers of the pharynx, stomach, liver, and pancreas. Cancers and histological subtypes were defined using Surveillance, Epidemiology, and End Results Program (SEER) recodes.

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Data Analysis
SAS ® 9.3 (Research Triangle Institute, Research Triangle Park, NC) was used. For each type of cancer analyzed, the proportion of cases who were firefighters was compared to the proportion of control cancer cases who were firefighters. Unconditional logistic regression was used to calculate odds ratios (OR) and ORs were adjusted for age at diagnosis (5-year intervals), year of diagnosis (5-year intervals), and race. The Wald test was used to test the level of statistical significance and was defined by a P < 0.05.

The risk of cancer among firefighters was examined in three ways: (i) all firefighters combined, (ii) firefighters of other race/ethnicity (i.e., blacks, Hispanics, Asians/Pacific islanders, Indian/Alaskan natives, other/unknown) and, (iii) white firefighters. Analyses of case and control groups were restricted by race category. That is, when other races/ ethnicities were assessed, only other races/ethnicities were included in the case and control groups. The analyses involving only whites were handled similarly. All cancers examined and reported in the “all firefighters combined” group, were also examined and reported in the race-stratified groups.

RESULTS
A total of 2,470,496 cancer reports did not meet the eligibility requirements and were excluded from analysis (Table I). The study sample was selected from 678,132 cancer subjects diagnosed in California who met all eligibility requirements. A total of 48,725 of those in the study sample had a control cancer. Among the control cancers, 31% were diagnosed with pancreatic cancer, 29% with stomach cancer, 23% with liver cancer, and 18% with pharyngeal cancer.

The study sample included 3,996 firefighters. Compared to non-firefighters, firefighters in the study sample were slightly but significantly older (aged 63.3 years vs. 62.6 years) and more likely to be white (90.2% vs. 74%).

Among the 32 examined cancers, three were significantly elevated among all firefighters combined and among firefighters in both race groups (Tables II–IV). These three cancers were melanoma, prostate cancer, and brain cancer.

Three cancers were significantly elevated among all firefighters combined and among white firefighters: adenocarcinoma of the esophagus; non-specific, non-small cell lung cancer; and, acute myeloid leukemia (AML). Three cancers were significantly elevated among all firefighters combined and firefighters of other race/ethnicity: kidney cancer, multiple myeloma, and overall leukemia.

There were six cancers that were significantly elevated among firefighters of other race/ ethnicity only: tongue cancer, testicular cancer, bladder cancer, non-Hodgkin lymphoma, chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML). Neither of the two other groups (i.e., all firefighters combined and white firefighters) had a significantly elevated cancer risk that was unique (i.e., that was not observed in at least one of the other two groups).

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There were 18 cancers for which a significantly elevated risk was not found among any firefighter group. These were: cancer of the lip; cancer of the salivary gland; gum and other mouth cancer; pharyngeal cancer; esophageal squamous carcinoma; stomach cancer; colorectal cancer; liver cancer; pancreatic cancer; laryngeal cancer; four lung cancer subtypes (i.e., adenocarcinoma, squamous cell carcinoma, small cell carcinoma, and large cell carcinoma); soft tissue sarcoma; mesothelioma; thyroid cancer; and Hodgkin lymphoma.

DISCUSSION
To our knowledge, this study included more firefighters with cancer than any previous study. This allowed us to assess the association between firefighters and the development of 32 cancers in all firefighters combined, white firefighters, and firefighters of other race/ ethnicity. Of the 32 cancers assessed in this analysis of CCR data from 1988 to 2007, the risk for 14 cancers was significantly elevated in one or more firefighter groups. Firefighters of other race/ethnicity had significantly increased risk for more cancers than white firefighters. These findings warrant the need for further investigation of cancer risks among firefighters of other race/ethnicity.

Comparison With a Pooled Cohort of Firefighters from San Francisco, Chicago, and Philadelphia
Daniels et al. [2014] reported mortality and cancer registry findings for firefighters who were employed for at least one day between 1950 and 2009 in fire departments that served San Francisco, Chicago, or Philadelphia. Their findings were similar to ours. Both Daniels et al. [2014]
and our study found elevated risks for esophageal and kidney cancer among all firefighters combined and prostate cancer among firefighters of other race/ethnicity. In addition,
Daniels et al. [2014] also found significantly elevated risks for melanoma and brain cancer incidence among San Francisco firefighters only, which were also elevated in our study of firefighters in the entire state of California. However, differences were identified when we compared the
Daniels et al. [2014] overall findings (all three cities combined) with our study results. Daniels et al. [2014], unlike our study, found a significantly increased risk for mesothelioma and cancers of the pharyngeal/buccal cavity (including lip, tongue, other buccal, and pharynx), colon, larynx, and lung. Our study, unlike Daniels et al. [2014] , found
an increased risk for non-Hodgkin lymphoma and overall leukemia among all firefighters combined, and an increased risk for testicular cancer, bladder cancer, and multiple myeloma among firefighters of other race/ethnicity. Study design dissimilarities that may explain some of the differences in findings include: (i) the types of firefighters included (structural firefighters in Daniels et al. [2014] vs. all firefighters in ours); (ii) location of fire departments (Daniels et al. [2014] studied three major US cities, whereas we studied California); (iii)
Daniels et al. [2014] used a retrospective cohort study design whereas we used a case– control study approach involving cancer registry data only; (iv) Daniels et al. [2014] had a smaller sample size of firefighters of other race/ethnicity with cancer; (v) inclusion of study participants for Daniels et al. [2014] was based on year employed (i.e., between 1950 and 2009), whereas ours was based on year of cancer diagnosis (i.e., between Tsai et al. Page 6
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1988 and 2007); and (vi) inclusion of female firefighters in Daniels et al. [2014] but not in our study.

Comparison With Another Large Firefighter Study and a Meta-Analysis Two other reports are notable for including large numbers of firefighters: Pukkala et al. [2014] and a meta analysis by LeMasters et al. [2006]. Like our study, Pukkala et al. [2014] and
LeMasters et al. [2006] found firefighters to be at significantly increased risk for melanoma and prostate cancer. In addition, our study and LeMasters et al. [2014] found a significantly increased risk for brain cancer and non-Hodgkin lymphoma. Additionally, our study (i.e., only in firefighters of other race/ethnicity) and LeMasters et al. [2006] found an increased risk for multiple myeloma and testicular cancer. Unlike our study, neither Pukkala et al. [2014] or LeMasters et al. [2006] found increased risks for leukemia, or cancers of the esophagus, bladder, or kidney. Differences in findings between Pukkala et al. [2014] and our study may be attributed to sample size (2,536 firefighters with cancer in Pukkala et al. [2014]
vs. 3,996 in ours), and differences in geographic region studied (Nordic countries in Pukkala et al. [2014] vs. the state of California).

Commonly Observed Increased Cancer Risks Among Firefighters The only two cancers consistently found significantly elevated in three large studies [
Daniels et al., 2014, Pukkala et al., 2014, and ours] and a meta-analysis [LeMasters et al., 2006] were melanoma and prostate cancer. Significant elevations in two of the studies/meta- analysis, were observed for non-Hodgkin lymphoma, multiple myeloma, and cancers of the tongue, esophagus, colon, testis, kidney, and brain. Significantly increased risks identified in at least one large study/meta-analysis but not ours included: larynx [Daniels et al., 2014]; lung and bronchus [Daniels et al., 2014; Pukkala et al., 2014]; mesothelioma [Daniels et al., 2014]; colon cancer [LeMasters et al., 2006; Daniels et al., 2014] and stomach cancer [LeMasters et al., 2006
].

Comparison With Bates [2007]
A previous study by Bates also assessed cancer risks among firefighters using CCR data; however, Bates did not conduct analyses by race and did not examine as many cancers [Bates, 2007]. Both Bates and our study found a significantly increased risk for esophageal, melanoma, prostate, and brain cancers among all firefighters combined. Cancers significantly elevated in one or more of the firefighter groups in our study, but not in Bates were bladder cancer, kidney cancer, non-Hodgkin lymphoma, multiple myeloma, and leukemia. No cancers were significantly elevated in Bates and not in our study.

In addition to conducting analyses by race, there were other differences in study design that likely explain at least some of the differences observed between Bates and our study. These include: (i) Our study used a more exhaustive keyword search for firefighters; (ii) Our study used four more years of data, as our study collected data from 1988 to 2007, while Bates only included data from 1988 to 2003; (iii) Differences in the cancers selected to serve as controls. For each examined cancer, Bates used all other cancers as the controls. In contrast, our study used cancers not thought to be associated with firefighting (i.e., cancers of the pharynx, stomach, liver, and pancreas); (iv) Our study examined more cancers. Only one Tsai et al. Page 7
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cancer examined in our study but not by Bates had a significantly elevated risk: cancer of the tongue.

Firefighters of Other Race/Ethnicity
Firefighters of other race/ethnicity had significantly elevated risks for 12 cancers (tongue, melanoma, prostate, testicular, bladder, kidney, brain, non-Hodgkin, multiple myeloma, leukemia [overall], CLL, and CML). In contrast, only six cancers were significantly elevated among white firefighters.

Most of the 365 firefighters of other race/ethnicity in our study were Hispanic (62.2%) or black (27.7%). Since Hispanics and blacks generally have higher incidence rates for cancers than Asians [
United States Cancer Statistics Working Group, 2014] and less than 10% of firefighters of other race/ethnicity were Asians, cancer risks observed among firefighters of other race/ethnicity were most likely driven by the Hispanic and black firefighters.

The reasons for the race/ethnicity-associated differences in our findings are unclear. Race/ ethnicity is related to general constraints that can lead to differential access to opportunities in society [
Jones, 2001]. In addition, those of other race/ethnicity have historically been subjected to prejudice and discrimination [Jones, 2001], including those seeking employment or promotion in fire departments [Ricucci and Saldivar, 2014]. Furthermore, those of other race/ethnicity may have selectively been assigned to busier fire stations. This is supported by studies of other industries that demonstrated that workers of other race/ ethnicity may be more frequently exposed to occupational hazards than white workers [Birdsey et al., 2007]. These societally imposed conditions, as experienced by firefighters of other race/ethnicity, may lead to differential exposure to carcinogens or may heighten susceptibility to the effects of carcinogenic exposures.

To our knowledge, only two other studies reported cancer risks among firefighters of other race/ethnicity: Daniels et al. [2014] and Ma et al. [1998]. Ma et al. [1998] looked only at black firefighters, whereas Daniels et al. [2014] did not report the race/ethnicity distribution of firefighters of other race/ethnicity. Our study included more firefighters of other race/ ethnicity with cancer (n=365) than Daniels et al. [2014] or Ma et al. [1998] (n=240 and
n=66, respectively). Daniels et al. [2014] found only prostate cancer at significantly increased risk among firefighters of other race/ethnicity.

Ma et al. [1998] found significantly
increased risks for cancers of the prostate and brain among firefighters of other race/ ethnicity, as in our study; they also found significant increases in nasopharyngeal and colon cancers, unlike our study. Differences between
Ma et al. [1998] and our study may be due to
our inclusion of nonwhite race/ethnicity other than blacks in our study, and different study periods. In addition, our larger study population of other race/ethnicity may have allowed us to detect more differences. Moreover, the evolving mix of carcinogenic exposures among firefighters may have also affected the types of cancer that were observed.

Prostate Cancer
The prostate is a hormone-regulated gland (i.e., testosterone). Chemicals, such as pesticides, cadmium, Bisphenol A (BPA), or PCB, have been shown to be endocrine disruptors [
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elevates the bioavailability of androgen which can initiate prostate cancer. The increased prostate cancer risk could also be due to an increased frequency of prostate cancer screening among firefighters as compared to the general population. Such a screening effect is supported by a recent study showing a lack of a positive dose-response relationship between fire-fighting exposure and prostate cancer incidence and mortality [Daniels et al., 2015].

Melanoma
Although exposure to ultraviolet radiation (i.e., sunlight, tanning beds) is commonly associated with melanoma, melanoma has also been found on the unexposed skin of petrochemical refinery workers [Mehlman, 2006]. Researchers have found a significant positive association between melanoma and exposure to benzene, PAH, PCB, aromatic hydrocarbons, and heavy oil [Mehlman, 2006].

Esophageal Cancer
The inhalation of smoke and dust during fire suppression activities and overhaul may have contributed to an increased risk of esophageal cancer. It is possible that mucociliary clearance of combustion products in the trachea led to esophageal irritation and inflammation. The inflammatory response, such as the infiltration of reactive oxygen species and inflammatory mediators, may further damage esophageal tissue [Kavanagh et al., 2014].

A study following a group of firefighters who responded to the 9/11 attack found an increased prevalence of gastroesophageal reflux disease (GERD) symptoms (5.8% prevalence pre-9/11 to a prevalence of 40% 4 years post 9/11) [Webber et al., 2009 ]. GERD
is a strong predictor for esophageal adenocarcinoma, the most common type of esophageal cancer today [Lagergren and Lagergren, 2013]. Our study found that 68% of esophageal cancer in firefighters was adenocarcinoma, and that only esophageal adenocarcinoma, and not squamous carcinoma, was significantly elevated among firefighters.

Limitations
This study has several limitations. First, using other cancer cases as controls may bias our findings towards the null if the selected control cancers are related to firefighting exposures.

To minimize this bias, we did a comprehensive literature review of cancers found to be elevated among firefighters. Cancers that consistently showed very weak or no association with firefighters were selected as control cancers. Second, I&O data were missing for approximately 50% of cancer cases in CCR. Individuals in the CCR dataset who had unknown I&O were more likely to be older and of Hispanic descent compared to the CCR cases meeting study eligibility. Ascertainment bias would be present if the proportion of firefighters who were ascertained by the CCR varied across the different cancers. However, the California cancer presumption law enacted in 1982 and amended periodically can award compensation and benefits to any firefighter diagnosed with any cancer within 10 years of their last day worked [William Dallas Jones Cancer Presumption Act, 2010]. It should be noted that the employer can challenge the firefighter’s compensation filing on the basis of an insufficient latency period (i.e., 10 years or less), or if the employer can prove that the association between the cancer and firefighting has been examined scientifically and shown not to exist [
Heald, 2005]. Since firefighters can presume that any cancer was caused by work, it is unlikely that ascertainment bias was introduced. Third, because I&O recording is Tsai et al. Page 9
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not standardized, I&O data obtained may not be the longest-held job, but rather the current job. It is reassuring that findings based on large representative samples of U.S. workers found moderate to high correlation between current and longest-held job [Gomez-Marin et al., 2005
; Luckhaupt et al., 2013], indicating that current job may be an acceptable surrogate for longest-held job. Fourth, those who worked as volunteer firefighters may have been classified as non-firefighters in our analyses because it is unlikely that volunteer firefighting was captured as the longest-held job. Conversely, some individuals with the designation of firefighter in this study may not have been involved in firefighting. The former misclassification of fire-fighting exposure may have biased the estimates towards the null, and it’s not clear how the latter misclassification would bias our findings. Fifth, CCR does not have information on smoking, alcohol consumption, obesity, workplace exposures, length of employment, and actual job duties. Although many cancers of interest are influenced by lifestyle factors, this study was not able to adjust for these potential confounders. Recent studies found that firefighters working in the central region of the United States were less likely to smoke [Haddock et al., 2011], but have high rates of heavy and binge drinking on their off-duty days [Haddock et al., 2012]. Any differences in lifestyle factors could have biased our estimates in either direction. Sixth, due to small sample sizes, we were unable to examine blacks, Hispanics, and Asians separately. Finally, this study did not adjust for multiple comparisons and some findings may have arisen due to chance.

Nevertheless, it is reassuring that many of the findings from this study are similar to those of previous studies.

Strengths
CCR is one of the most comprehensive and complete cancer registries with a case ascertainment rate of at least 95% [North American Association of Central Cancer Registries, 2008,2015]. In addition, our study is among the largest cancer studies of firefighters. It is also one of the few studies that reported firefighter risk for cancer subtypes, including for leukemia, esophageal cancer, and lung cancer. Finally, ours is one of the few studies that reported findings for firefighters of other race/ethnicity.

CONCLUSION
This study found that firefighters had a significantly elevated risk for melanoma, multiple myeloma, leukemia (i.e., AML), and cancers of the esophagus, prostate, kidney, and brain.

Moreover, firefighters of other race/ethnicity, in addition to being at significantly increased risk for the same cancers identified for all firefighters combined, were found to have a significantly increased risk for non-Hodgkin lymphoma, leukemia (i.e., CLL, CML) and cancers of the tongue, testis, and bladder. The consistency of many of these findings with prior large studies (i.e., melanoma, non-Hodgkin lymphoma, multiple myeloma, and cancers of the prostate, esophagus, testis, bladder, kidney, and brain) strengthens the evidence supporting the association between firefighting exposures and these cancers.

Acknowledgments
Contract grant sponsor: National Cancer Institute’s Surveillance, Epidemiology and End Results Program; Contract grant number: HHSN261201000140C; Contract grant sponsor: National Cancer Institute’s Surveillance, Tsai et al. Page 10
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Epidemiology and End Results Program; Contract grant number: HHSN261201000035C; Contract grant sponsor:

National Cancer Institute’s Surveillance, Epidemiology and End Results Program; Contract grant number:

HHSN261201000034C; Contract grant sponsor: Centers for Disease Control and Prevention’s National Program of Cancer Registries; Contract grant number: U58DP003862-01; Contract grant sponsor: Centers for Disease Control and Prevention’s National Program of Cancer Registries; Contract grant number: 1U58DP000807-3.

The authors thank the following individuals for their review of earlier versions of this manuscript: Dr. Doug (Robert) Daniels and Dr. David Lee. The collection of data used in this publication was supported by the California Department of Health Services as part of the statewide cancer reporting program mandated by California Health and Safety Code Section 103885; by the National Cancer Institute, National Institutes of Health, Department of Health and Human Services under Contract No. N01-PC-2010-00035; and grant number 1U58DP000807-3 from the Centers for Disease Control and Prevention.

The study was conducted at the National Institute for Occupational Safety and Health.

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