Health Hazard Evaluation Report 2012-0039-3242 Page 1

Evaluation of Heat Stress, Heat Strain, and 

Rhabdomyolysis during Structural Fire 

Fighter Training 

Report No. 2012-0039-3242

September 2015

Judith Eisenberg, MD, MS

Mark Methner, PhD, CIH

Chad H. Dowell, MS, CIH

Charles Mueller, MS

U.S. Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Page 2 Health Hazard Evaluation Report 2012-0039-3242

Contents

Highlights ...............................................i

Abbreviations ..................................... iii

Introduction ......................................... 1

Methods ............................................... 2

Results .................................................. 6

Discussion .......................................... 12

Conclusions ........................................ 13

Recommendations............................. 14

Appendix A ........................................ 15

Appendix B ........................................ 17

Appendix C ........................................ 18

Appendix D ........................................ 22

References .......................................... 23

Acknowledgements ........................... 27

The employer is required to post a copy of this report for 30 days at or near the 

workplace(s) of affected employees. The employer must take steps to ensure 

that the posted report is not altered, defaced, or covered by other material.

The cover photo is a close-up image of sorbent tubes, which are used by the HHE 

Program to measure airborne exposures. This photo is an artistic representation that may 

not be related to this Health Hazard Evaluation. Photo by NIOSH.
Health Hazard Evaluation Report 2012-0039-3242 Page i

We evaluated cadets and 

instructors for heat stress, heat 

strain, and muscle breakdown 

during fire fighter training. 

Environmental conditions, 

core body temperatures, and 

heart rates often exceeded 

guidelines during strenuous 

activities. We identified one 

individual with rhabdomyolysis. 

We recommend scheduling 

strenuous training during 

cooler parts of the day/year and 

educating fire fighters on 

heat-related illnesses 

and rhabdomyolysis.

Highlights of this Evaluation

The Health Hazard Evaluation Program received a request from a fire department to evaluate 

the risk of heat related illness and rhabdomyolysis to cadets and instructors during training. 

Rhabdomyolysis is the breakdown of muscle tissue. 

What We Did

● We conducted a 4-day evaluation in August 2012 during week 8 of a 10-week training course.

● We asked 32 participants to complete a questionnaire about their work history, medical 

history, and health symptoms.

● We measured core body temperature and heart 

rate in 22 participants.

● We analyzed participants’ blood for markers 

of muscle breakdown (creatine kinase) and 

dehydration during and after the 8th week of 

the training course.

● We asked participants at the end of each 

workday how much fluid they drank. We also 

asked if they had symptoms of heat-related 

illness and muscle breakdown.

● We measured body weight before and after 

each training day. 

● We measured environmental conditions each day. 

● We had group discussions with participants 

about their knowledge of rhabdomyolysis.

What We Found

● One participant had rhabdomyolysis.

● Sixteen participants had a creatine kinase level in their blood that was elevated but below 

the level we considered as rhabdomyolysis. This means that muscle breakdown was 

occurring but not enough to require medical treatment.

● Most participants met our criteria for excessive heat strain at some point during the 

testing week.

● Environmental conditions often exceeded heat stress limits.

● Educational materials about the signs, symptoms, and risk of rhabdomyolysis specific 

for structural fire fighters are needed.
Page ii Health Hazard Evaluation Report 2012-0039-3242

What the Employer Can Do

● Conduct physically demanding activities during cooler parts of the day.

● Schedule training courses during cooler months.

● Educate all fire fighters about the signs, symptoms, and dangers of heat-related illness 

and rhabdomyolysis. 

What Employees Can Do

● Learn the signs and symptoms of excessive heat strain and rhabdomyolysis.

● Tell your supervisor immediately if you have symptoms of heat-related illness or 

rhabdomyolysis or if you see these symptoms in other fire fighters.

● Drink plenty of fluids, and take rest breaks as needed.

● Talk to your healthcare provider about your job and your increased risk for rhabdomyolysis. 
Health Hazard Evaluation Report 2012-0039-3242 Page iii

Abbreviations

ACGIH® American Conference of Governmental Industrial Hygienists

bpm Beats per minute

BUN Blood urea nitrogen

CBT Core body temperature

CFR Code of Federal Regulations

CK Creatine kinase

F Fahrenheit

HRI Heat-related illness

IU/L International units per liter

mOsm/L Milliosmoles per liter

NIOSH National Institute for Occupational Safety and Health

OSHA Occupational Safety and Health Administration

TLV® Threshold limit value

WBGT Wet bulb globe temperature
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Health Hazard Evaluation Report 2012-0039-3242 Page 1

Introduction 

The Health Hazard Evaluation Program received a request from a fire department about 

the potential for rhabdomyolysis and heat-related illness (HRI) in cadets and instructors 

participating in training classes. At the time of the evaluation, the cadet training course 

consisted of classroom lectures in an air-conditioned environment and outdoor activities 

that included physical training and live fire suppression exercises in full protective gear. We 

conducted our on-site evaluation in August 2012. 

Training was conducted Monday through Thursday, with Friday through Sunday designated 

as rest days. In addition to classroom lectures, outdoor activities consisted of physical 

training (e.g., running, calisthenics) and a 1-day live fire suppression exercise inside a sixstory building. During the live fire exercise, two cadets were paired with an instructor while 

fighting a fire inside the building. A camera monitored personnel inside the building. Once 

an entry team (2 cadets and an instructor) left the live fire building, they recovered in a 

shaded area equipped with fan-driven, water misting coolers and cold drinking water. On 

days without live fire training, activities included set-up, inspection, and wearing of personal 

protective equipment; fire hose handling; climbing ladders; and maneuvering up and down 

high rise building stairs. Cadets had access to air conditioned trailers during their lunch 

break. We considered all participants acclimatized because this evaluation was conducted 

during the eighth week of the 10-week training course under consistent environmental 

conditions. 

Heat-Related Illness and Rhabdomyolysis

Heat exposure resulting in elevated core body temperatures and prolonged, intense exertion 

are risk factors for rhabdomyolysis, but they are also, unfortunately, a consequence of fire 

fighting. Heat stress is the sum of the heat generated in the body (metabolic heat) plus the 

heat gained from the environment (environmental heat) minus the heat lost from the body 

to the environment [NIOSH 2013]. Many bodily responses to heat stress are desirable and 

beneficial; however, at some stage of heat stress, the body cannot maintain an internal 

temperature required for normal functioning. As a result, the risk of HRI and accidents 

occurring from impaired mental status increases.

The body’s response to heat stress is called heat strain. Heat strain is dependent on a number 

of factors and cannot be predicted on the basis of environmental heat stress measurements 

alone. As a result of working in a hot environment, HRI may develop. HRI includes disorders 

such as: 

● Heat stroke – Caused by an excessive rise in body temperature and failure of 

the temperature regulating mechanism. It is characterized by a change in mental 

status, which can range from confusion or bizarre behavior to seizures and loss of 

consciousness. It is often preceded by signs and symptoms of heat exhaustion as 

described below. Body temperature may be above 106°F, but there is no specific 

temperature used to make the diagnosis. Heat stroke is a medical emergency and can be 

fatal if not immediately treated.
Page 2 Health Hazard Evaluation Report 2012-0039-3242

● Heat exhaustion – Characterized by muscular weakness, distress, nausea, vomiting, 

dizziness, pale clammy skin, and fainting. It is usually associated with lack of heat 

acclimatization and physical fitness, poor health status, and inadequate water intake.

● Heat rash – Skin irritation that occurs most often in hot environments and causes 

skin to become red and itchy. The rash usually appears in areas where clothing 

is restrictive.

● Heat cramps – Muscle pains or spasms that can happen during prolonged work or 

exercise in high temperatures.

Rhabdomyolysis, or muscle tissue breakdown, is the result of any process that causes injury to or 

death of muscle tissue. Muscle tissue can be damaged by overheating, overexertion, crush injury, 

some medications or supplements, or certain medical conditions. When muscle cells die, their 

contents of electrolytes and proteins are released into the bloodstream, which can result in potentially 

life-threatening conditions affecting the heart and kidneys [Khan 2009]. The association between 

heat stroke and rhabdomyolysis is well known [Department of the Army and Air Force 2003; 

Bontempo and Kaji 2010; O’Connor and Duester 2011; ACGIH 2015]. Rhabdomyolysis and acute 

kidney failure often occur together in people with exertional heat stroke [Bontempo and Kaji 2010]. 

Rhabdomyolysis is serious; up to 8% of documented rhabdomyolysis cases are fatal [Cervellin et al. 

2010]. Rhabdomyolysis can also result in permanent disability. More information about heat stress, 

heat strain, and rhabdomyolysis is in Appendix C.

Methods

The objectives of this evaluation were to determine if cadets participating in the training class had: 

● Excessive heat strain 

● Signs and symptoms of HRI, or rhabdomyolysis

● Personal and work-related risk factors for excessive heat strain, HRI, or rhabdomyolysis

● Evidence of dehydration

We evaluated 30 of 32 cadets and the 2 primary course instructors during week 8 of a 

10-week training course. On the first day of our evaluation, we obtained informed consent 

from the cadets and instructors, and ensured they had no medical reasons that would exclude 

them from participating, such as digestive problems, having a pacemaker, or pregnancy. Our 

evaluation methods included:

● Administering a one-time questionnaire about work and medical history 

● Questioning participants daily about symptoms

● Testing participants’ blood before and after each training day and twice a day on 

rest days for markers of rhabdomyolysis and dehydration until results showed two 

successively declining CK levels or the HHE ended

● Measuring participants’ core body temperature (CBT), exertion, and heart rate during 

training days 
Health Hazard Evaluation Report 2012-0039-3242 Page 3

● Measuring participants’ body weight before and after each training day

● Assessing environmental conditions on training days

● Observing training activities to estimate workload 

● Conducting a focus group about cadets’ knowledge of rhabdomyolysis

Questionnaire

On the first day of the evaluation we asked each participant to complete a questionnaire 

on work and medical history, including risk factors for and previous diagnoses of 

rhabdomyolysis or HRI. We asked about recent use (past 2 weeks) of medications, 

supplements, and beverages that could increase the risk for rhabdomyolysis or HRI. We 

also asked about possible rhabdomyolysis and HRI signs and symptoms experienced since 

starting the training course 8 weeks earlier.

Postshift Symptom Surveys

At the end of their shift during training days (Monday through Thursday) and on their first 

rest day (Friday) following training, we asked participants if they had symptoms that could 

be associated with rhabdomyolysis or HRI. We asked if they had unusual soreness in the 

arms, legs, or back after completing that day’s training, if they felt rested or recovered prior 

to starting training that day, and if they had noted darker than normal urine on that day. We 

also asked them to estimate their total fluid intake during training that day. 

Blood Analysis for Markers of Rhabdomyolysis and 

Dehydration

We collected blood samples before and after each training day and twice a day on rest days. 

Approximately three to six drops of whole blood were collected via fingerstick and analyzed 

using the Abaxis Piccolo® Metlyte 8 Reagent Disc in a Piccolo XpressTM Analyzer. To 

identify rhabdomyolysis, we measured the amount of creatine kinase (CK), an enzyme 

that serves as a marker for muscle breakdown. We used a standard definition of 

rhabdomyolysis as a CK level five times the upper limit of the reference range of the assay 

[O’Connor and Duester 2011]. We used the following values to identify rhabdomyolysis: 

> 1,900 international units/liter (IU/L) for males, and 950 IU/L for females. The normal 

reference CK ranges for this assay were 39–380 IU/L for males and 30–190 IU/L for females 

[Abaxis 2011]. We categorized participants as having an elevated CK but not rhabdomyolysis 

if their CK was above the upper limit of normal for their sex but below the rhabdomyolysis 

threshold level. We stopped testing participants when results showed two successively 

declining CK levels on rest days. If CK values met or exceeded our case definition levels, we 

instructed the participant to seek immediate medical attention. We advised participants whose 

CK values did not demonstrate two successively declining levels before the end of our testing 

to follow-up with their healthcare provider within 1 week.
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We measured blood urea nitrogen (BUN) to creatinine ratios and calculated serum osmolarity 

to determine the level of dehydration. The formula for calculated serum osmolarity [Kelen et 

al. 2011] is as follows: 

Serum Osmolarity (mOsm/L)= 2× [serum sodium ] + [serum glucose ]/18 + [serum BUN ]/2.8 

We defined dehydration as either a BUN to creatinine ratio higher than 20:1 or a calculated 

serum osmolarity higher than 290 milliosmoles/liter (mOsm/L) [Singer and Brenner 2008]. 

When collecting blood samples, we followed the standard precautions for prevention of 

exposure to bloodborne pathogens as specified by the Centers for Disease Control and 

Prevention and the Occupational Safety and Health Administration (OSHA) [CDC 1998; 29 

CFR 1910.1000]. All blood test results were available within 30 minutes of collection. Any 

participant with a clinically significant, abnormal test result was contacted immediately. At the 

end of the week, all participants received a letter containing the results of all their blood tests.

Heat Strain Assessment

We classified participants as having excessive heat strain if they had one or more of the following:

● A CBT higher than 101.3° Fahrenheit (F). This definition is in accordance with the 

American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit 

value (TLV) heat stress guidelines for acclimatized individuals [ACGIH 2015].

● A heart rate above 180 beats per minute (bpm) minus the age of the participant 

sustained for at least 3 minutes [ACGIH 2015].

● A body weight loss over a training day greater than 1.5% [ACGIH 2015].

It should be noted that the 1986 National Institute for Occupational Safety and Health 

(NIOSH) heat stress criteria document guidance is being updated [NIOSH 2013]. 

Core Body Temperature

Before conducting the evaluation, we screened all participants for medical conditions that 

would preclude them from safely swallowing a Philips Respironics CBT sensor. The single 

use CBT sensor is about the size of a multivitamin, is biologically inert, and exits the body 

with a bowel movement. It measures temperature as it passes through the digestive system 

and wirelessly transmits the data to an Equivital LifeMonitor WQ02 receiver worn outside 

the body. Using a CBT sensor is considered the most accurate way to measure internal body 

temperature [Sawka and Pandolf 2001; McKenzie and Osgood 2004; Byrne and Lim 2007].

Each CBT sensor was set to transmit data every 15 seconds. Participants swallowed a 

new CBT sensor before starting training each morning. Each employee then drank about 

16 ounces of water and ate five to 10 saltine crackers to help move the sensor from the 

stomach into the small intestine where the most accurate CBT is measured. At the end of 

each workday, we downloaded and stored the CBT data. Participants were not asked to 

take the CBT sensor on rest days. With the equipment available we were able to monitor 22 

participants for CBT across the 4 training days. The first 22 participants who consented and 

did not have any disqualifying medical condition were selected for this part of the evaluation.
Health Hazard Evaluation Report 2012-0039-3242 Page 5

Heart Rate

We used the Equivital LifeMonitor EQ02 physiological monitoring system to measure and store 

employee heart rate data for 22 participants as noted above. The system was set to transmit 

heart rate data every 15 seconds during each training day. This system consists of a chest 

strap containing a sensor that contacts the skin and determines the heart rate. The maximum 

recordable heart rate for this instrument is 240 bpm. All heart rate data were wirelessly 

transmitted and stored. Participants were not asked to wear the heart rate sensor on rest days.

Body Weight

We measured each participant’s body weight before and after each training day to assess fluid losses 

using a Seca TraveliteTM Model 803 digital scale. We did not weigh participants on rest days.

Heat Stress

To measure the environmental conditions that contribute to heat stress, we used a Quest 

Technologies QUESTemp°36 instrument to obtain the wet bulb globe temperature (WBGT) 

at each activity location and in cooling areas. The WBGT measurements were logged 

electronically at 1-minute intervals during the entire training day. WBGT measurements 

were also manually recorded by NIOSH staff on a daily activity log at the start of each 

training activity. We used the daily activity log to describe each activity, its duration, its 

estimated average metabolic load (exertion level), and rest breaks. We classified metabolic 

loads as rest, light, moderate, heavy, and very heavy on the basis of NIOSH and ACGIH heat 

stress criteria, which are the same [NIOSH 1986; ACGIH 2015]. We grouped activities into 

approximately 1-hour periods over each day. We then calculated the maximum WBGT during 

these activity periods. All participants intermittently wore protective equipment such as flame 

retardant turnout gear and supplied air respirators. However, because the amount of time 

each participant wore this protective equipment varied, it was unfeasible to make a WBGT 

clothing adjustment. Instead, we used the maximum WBGT value measured in the area 

where a specific activity occurred to estimate the heat stress associated with that activity.

Activity Assessment

Because prolonged, intense physical activity is a risk factor for rhabdomyolysis, we used 

a Philips Respironics ActicalTM accelerometer to record activity counts or how often a 

specific motion was repeated. This system uses software to convert activity counts into each 

participant’s total energy expenditure. 

Statistical Methods

We used SAS Version 9.3 (SAS Institute Inc.) for our statistical analyses. We calculated 

descriptive statistics including means, ranges, frequencies, and percentages. We used the 

Spearman correlation coefficient with its associated P value to evaluate relationships between 

the average number of caffeinated beverages consumed each day and the maximum CK for 

the subject. P values less than 0.05 were considered statistically significant.
Page 6 Health Hazard Evaluation Report 2012-0039-3242

Focus Group

We asked questions on the general consent form regarding each participant’s age and years as 

a fire fighter. We used the responses to these questions to select six participants with diverse 

backgrounds. A separate consent form for focus group participation was required because the 

session was recorded. A NIOSH health communication specialist served as the moderator to 

guide discussions on the participants’ knowledge, experiences, beliefs, and needs [Morgan 

1998] related to rhabdomyolysis. Focus group discussion topics included the following:

● How familiar participants were with rhabdomyolysis and its risk factors

● Barriers to seeking treatment for possible rhabdomyolysis symptoms

We also asked the focus group participants for suggestions on how they could alert healthcare 

providers if they sought medical care for signs and symptoms of rhabdomyolysis. We asked 

for comments on the usefulness of a wallet-sized identification card containing information 

on rhabdomyolysis. This card had been developed for wildland fire fighters (Figure 1) 

[NIOSH 2011].

Figure 1. Rhabdomyolysis wallet card developed for wildland fire fighters.

Results

Questionnaire and Postshift Symptom Survey 

The average age of the 32 participants who completed the questionnaire was 27 (range: 19–

41) years. All but two were male. Participants reported working as fire fighters for an average 

of 3 years (range: 0–16 years). At the time the questionnaire was administered, current 

medical issues included one report of a heart condition, one report of high cholesterol, two 

reports of depression and reports of gastritis, allergies, low testosterone, and back/joint pain. 

One participant answered “yes” to ever being told by a healthcare professional that he/she 
Health Hazard Evaluation Report 2012-0039-3242 Page 7

had rhabdomyolysis, decreased kidney function, and “heat stroke, heat strain, or heat stress.” 

Following a discussion with this participant, we learned the participant had a fainting episode 

due to heat stroke earlier in training and was diagnosed with rhabdomyolysis and decreased 

kidney function, which subsequently returned to normal.

We asked participants to rate their level of agreement with statements about withdrawing 

from the training due to illness. The rating scale was 1–5, with 1 being strongly disagree and 

5 being strongly agree. Of the 27 responding participants, 12 (44%) reported a 4 (agree) or 5 

(strongly agree) for the statement “I feel that I could have withdrawn from this training event 

at any time if I did feel well.” Six reported a 3 (neither agree nor disagree), and nine reported 

a 1 (strongly disagree). Fifty-six percent (15/27) reported a 4 (agree) or 5 (strongly agree) 

when asked to rate the statement “If I did become sick during this event I would have notified 

my trainer/supervisor,” while seven reported a 3 (neither agree nor disagree), and 5 reported a 

1 (strongly disagree) or 2 (disagree).

We found very limited reported use of substances that have been associated with an increased 

risk of rhabdomyolysis within the 2 weeks prior to our evaluation. Two participants reported 

taking creatine-containing substances; three reported taking over-the-counter cold, cough, or 

allergy medicines; and two reported taking ibuprofen for back or joint pain. One participant 

reported taking oral steroids, and four reported consuming an over-the-counter energy drink. 

In order to look at caffeine intake, we asked if they drank caffeinated soda, energy drinks, 

or coffee/tea. Twenty-eight participants (88%) reported drinking these caffeinated beverages 

in the past 2 weeks, with the highest average amount being reported by one participant as 

one can of soda, one energy drink, and two cups of coffee per day. We found no statistically 

significant correlation between the average quantity of these drinks consumed and maximum 

CK level (r = −0.13, P = 0.49). 

The questionnaire also asked about engaging in intense physical exertion activities for 

recreation such as hiking, biking, or climbing as these type of activities can also result in 

increases in CK. Nineteen participants (59%) reported that they did so. When asked how 

many days since they last engaged in those activities, the responses ranged from 1 to 10 days.

Table 1 summarizes the questionnaire responses regarding symptoms experienced during the 

first 7 weeks of the training course. Nine participants reported no symptoms, 15 reported four 

or fewer symptoms while eight participants reported between 6–9 symptoms. No participant 

reported 10 or more. The most commonly reported symptoms were unusual muscle aches or 

pain in arms, legs, or lower back (56%) and decreased urine output (50%). 
Page 8 Health Hazard Evaluation Report 2012-0039-3242

On the postshift symptom surveys, unusual soreness of the arms, legs, and back were 

infrequently reported, with only two of 32 participants reporting such symptoms on any of 

the 4 training days. We also asked on the postshift symptom surveys if participants felt rested 

or recovered in the morning. Between 4 and 15 participants responded “no,” with 15 on day 

4 (the day after the live burn exercise), and 4 on day 5. The postshift symptom surveys also 

asked participants to estimate their fluid intake that day. Responses ranged from 1–10 liters 

with daily averages ranging from 4–5 liters. 

Blood Analysis for Markers of Rhabdomyolysis and 

Dehydration

We looked at markers in blood for dehydration: plasma osmolarity and the blood urea 

nitrogen (BUN)-to-creatinine ratio. We defined dehydration to be an osmolarity greater 

than 290 mOsm/L or a BUN-to-creatinine ratio greater than 20:1. Although none of the 

participants had a plasma osmolarity level above 290 mOsm/L during the 4 days of training, 

78% had a BUN-to-creatinine ratio above 20:1 during our sampling period, which included 

rest days. Because these are screening tests, it is possible that the elevated BUN-tocreatinine ratios were indicative of medical conditions other than dehydration. We informed 

all participants of their individual results and the need to follow up with their health care 

providers if their BUN-to-creatinine ratio results were high.

We expected to see signs of dehydration among participants because of the increased 

temperatures and considerable perspiration among the participants performing firefighting 

activities. We hypothesized that levels of both markers of dehydration would be greater in the 

postshift sample than in the preshift sample. However, we found that levels of both markers 

decreased from preshift to postshift on each of the training days in most participants, which 

suggested that this group of firefighters remained appropriately hydrated each day. Despite 

Table 1. Prevalence of symptoms and signs among participants since the start of the 

training course (n = 32)

Symptom or sign Number of participants (%)

Unusual muscle aches or pains in the arms, legs, lower back 18 (56)

Decreased urine output 16 (50)

Numbness or “pins and needles” sensation in arms or legs 12 (38)

Diarrhea 12 (38)

Feverish feeling 12 (38)

Nausea or vomiting 11 (34)

Unusually dark urine 7 (22)

Abdominal pain 5 (16)

Muscle swelling in the arms or legs 5 (16)

Oral temperature > 100.4°F 3 (9)

Felt like had flu 3 (9)
Health Hazard Evaluation Report 2012-0039-3242 Page 9

the possible differing interpretations of the findings regarding dehydration markers, we 

believe it is prudent to routinely focus on adequate hydration of fire fighters.

Maximum individual CK levels for each participant across all days ranged from 189 IU/L

to 12,665 IU/L. Of the 32 participants, 15 participants had normal CK levels throughout the 

training. Sixteen participants had CK levels above the normal range for their sex but below 

the cutoff level for rhabdomyolysis, and one participant demonstrated a CK level that was 

high enough to meet our case definition for rhabdomyolysis. 

The participant who met our case definition exceeded the CK criterion on day 6 and 

as per protocol, was advised to go to the hospital for treatment. This participant had a 

maximum CBT of 102.1°F on day 4 (final training day) of our evaluation, did not report 

any personal risk factors for rhabdomyolysis but did describe symptoms consistent with 

HRI or rhabdomyolysis that occurred during the earlier weeks of the training event on 

the questionnaire. However, during days 1 through 5 of the evaluation, the participant 

denied having muscle soreness or dark urine. When contacted by phone on testing day 6 

for notification that the second CK test that day met our case definition of rhabdomyolysis, 

the participant reported still being asymptomatic. As per our evaluation protocol, we asked 

the participant to go to the hospital for further evaluation and treatment. Hospital records 

revealed a peak CK value of 12,665 IU/L on day 7. After being discharged from the hospital 

and being medically cleared, this participant returned to training.

Heat Strain and Heat Stress

During our evaluation, 28 of 32 participants met our criteria for excessive heat strain at some 

time during the testing week. Twenty participants lost more than 1.5% of their body weight 

during the training day at least once, with the largest number of participants’ weight loss 

occurring on day 1 (Table 2). According to this indicator of dehydration, substantial fluid loss 

may have been occurring. 

Table 2 summarizes the number of participants whose CBT exceeded the ACGIH 

recommended limit of 101.3°F at any time during the 4 days of training [ACGIH 2015]. 

Maximum CBT values for each participant on each training day ranged from 98.9°F to 

107.2°F (Appendix A, Tables A1–A4). Most participants’ CBT exceeded the limit every 

training day. Over the 4 training days, at least 67 maximum CBT measurements across 

all participants exceeded the ACGIH heat strain criterion (some CBT and heart rate data 

were either not transmitted or not stored). The highest maximum recorded CBT of 107.2°F 

occurred on day 3 during the live fire suppression exercise. During this activity, most 

participants’ CBTs also exceeded the criterion.
Page 10 Health Hazard Evaluation Report 2012-0039-3242

Table 2. Participants exceeding heat strain criteria for 

weight loss, CBT, or heart rate

Criteria Number and percent 

of participants who 

met criteria at any 

time during testing

> 1.5% body weight loss

Day 1 (n = 32) 14 (44)

Day 2 (n = 32) 5 (16)

Day 3 (n = 32) 6 (19)

Day 4 (n = 32) 5 (16)

CBT > 101.3°F

Day 1 (n = 22) 20 (91)

Day 2 (n = 22) 15 (68)

Day 3 (n = 21)† 17 (81)

Day 4 (n = 22) 15 (68)

Heart rate > 180 minus age in years*

Day 1 (n = 22) 13 (59)

Day 2 (n = 21)† 13 (62)

Day 3 (n = 21)† 14 (67)

Day 4 (n = 18)† 13 (72)

*Heart rate sustained for 3 or more minutes with 

measures a maximum of 15 seconds apart

†Equipment failed to transmit or store data for some 

participants

Applying the 1986 NIOSH recommended exposure limit for heat strain (CBT of 100.4°F) 

would add 11 more CBT measurements for a total of 78 CBTs that exceeded the guideline. 

Most of the additional exceedances occurred during high rise building training while wearing 

protective gear. Nonetheless, all of the exceedances were associated with strenuous activities 

(high metabolic load > 520 Watts) performed in a hot environment (WBGT > 77.9°F) 

(Appendix A, Tables A1–A4).

Between 18 and 22 participants wore a heart rate monitor during the 4 days of training. 

Thirteen participants’ heart rate exceeded the ACGIH maximum heart rate guideline at some 

point on each of the 4 training days (Table 2). Like the CBT exceedances, most heart rate 

exceedances occurred during strenuous activities such as exercising (physical training), high 

rise training where participants climbed six flights of stairs while wearing full protective 

gear, and live fire suppression exercise (Appendix A, Tables A1–A4). The total number of 

heart rate measurements per person per day ranged from zero to 2,613, indicating that some 

participants had missing heart rate data, likely due to intermittent loss of contact between the 

heart rate sensor and the skin.

Our estimates for the metabolic load of each fire training-related activity ranged from 

115 Watts to 520 Watts [ACGIH 2015]. We made WBGT measurements at each activity 
Health Hazard Evaluation Report 2012-0039-3242 Page 11

location with the exception of inside the building during the live fire suppression exercise. 

The WBGT measurements ranged from 65°F (inside the air-conditioned classroom) to 96°F 

(outdoors, next to the high-rise building used for multistory building search and rescue 

training). On multiple occasions high metabolic load activities were performed under 

environmental conditions that exceeded heat stress guidelines (Appendix A, Tables A1–A4; 

Appendix B, Figure B1).

Activity Assessment

The activity data we collected using an Actical accelerometer were highly variable and showed 

no clear pattern between participant total energy expenditure and specific activities. For 

example, when participants performed strenuous activity (e.g., carrying loads, climbing stairs 

and ladders, or working with their hands overhead), there was no corresponding increase in the 

total energy expenditure data recorded by the Actical. We concluded that these discrepancies 

in the results were likely due to limitations in the Actical software and that the total energy 

expenditure calculated by the Actical underestimated the activity intensity. Therefore, we did 

not use these data and relied on our metabolic load estimates for energy expended.

Focus Group

We provided the six participants with background information about a prior NIOSH evaluation 

of wildland fire fighters where we had identified cases of rhabdomyolysis. Participants reported 

that when they became fire fighters, they were given information regarding the risk for extreme 

heat exposure, HRI, and dehydration, but not rhabdomyolysis. Four participants stated that they 

had never heard of rhabdomyolysis until this NIOSH evaluation. 

The six probes used to guide the focus group discussion can be found in Appendix D. When 

asked how rhabdomyolysis information should be presented to fire fighters, participants told us 

that educational materials should include a picture of a structural fire fighter. The focus group 

participants also mentioned that if video clips were used, they should show fire fighters in full 

protective gear. Handouts or brochures would likely not be read, as they already receive an 

abundance of paper materials. A video or an in-person presentation by a fire fighter who had been 

diagnosed with rhabdomyolysis was believed to be the best way to deliver this information. 

Several participants believed that rhabdomyolysis only happened to those who were not 

physically fit, or who ate poorly. We explained that rhabdomyolysis was not due to poor diets 

or lack of physical fitness and often occurred in very fit individuals.

Participants noted that educational information should cover what rhabdomyolysis is, signs 

and symptoms, diagnostic criteria, and the necessity for immediate evaluation and medical 

treatment to prevent permanent damage. Participants mentioned that materials must clearly 

state that without proper medical treatment, rhabdomyolysis can cause permanent disability 

or death. The mention of removal from the fire service due to disability, dialysis, or loss of 

muscle function was reported to be a motivator to seek proper medical care. 

All participants thought separate materials were needed for structural fire fighters and 

wildland fire fighters. 
Page 12 Health Hazard Evaluation Report 2012-0039-3242

We received mixed opinions when we asked participants about the usefulness of a 

wallet-sized identification card that could alert healthcare providers about work-related 

rhabdomyolysis risks. Most informed us that they did not discuss their occupation and 

associated risk factors with their healthcare provider, although they knew they should. Focus 

group participants thought having more fire fighters, especially ranking officers, educated 

about rhabdomyolysis and present when seeking care for possible rhabdomyolysis symptoms 

would not only help ensure appropriate treatment but could help bridge communication 

barriers with healthcare providers. 

Discussion

Fire fighters are exposed to heat from several sources, including heat generated by the 

fire, ambient temperature, heat generated by muscles during physical exertion, and heat 

trapped by turnout gear (e.g., bunker coat and pants, self-contained breathing apparatus, 

and NomexTM hood). Heat loss capacity is also reduced in high humidity environments 

where evaporative heat loss from sweating is impeded. The weight of the turnout gear and 

firefighting equipment adds to the physical exertion, which, in turn, increases the overall heat 

load. Many of these sources of heat exposure cannot be avoided during training, but can be 

reduced by scheduling training events during cooler months or cooler parts of the day.

During the 4 days of training, we found that at least 28 of 32 participants met or exceeded 

our criteria for excessive heat strain. Additionally, most participants exceeded the ACGIHrecommended heat stress guidelines for CBT and heart rate [ACGIH 2015]. On the daily 

post-shift data download, we found that one participant had a peak CBT of 107.2°F during 

that training day. Although no participant required transport to a hospital for evaluation and 

treatment of HRI during our evaluation, this was an example of a “near-miss” for a HRI 

incident and shows the need for closer CBT monitoring during training events. 

We found that in spite of the hot working conditions and strenuous training regime, most 

participants remained adequately hydrated during the work shift. However, our analysis 

of results from day to day showed some evidence of dehydration present before starting 

the training day despite leaving training the previous day being adequately hydrated. This 

indicates that additional emphasis on keeping up with fluid intake after the training day ends 

needs to occur.

Half of the participants in this evaluation had maximum CK levels which indicated that 

muscle breakdown was occurring, but not to the degree that required medical evaluation. 

Only one participant had a CK high enough to meet our case definition despite having had no 

symptoms of muscle aches or pains, or darkened urine during the course of their elevated CK 

levels. This individual was diagnosed with rhabdomyolysis in the emergency department. It 

should be noted that asymptomatic rhabdomyolysis (i.e. diagnosis made solely on elevated 

CK without any subjective symptoms like muscle pain or outward signs like darkened urine) 

is not uncommon. One study of patients hospitalized for medical conditions that put them at 

risk for rhabdomyolysis showed that 50% of those whose CK levels rose high enough to be 

rhabdomyolysis cases denied any muscle pain and it took up to 4 days for muscle swelling to 

manifest [Gabow et al. 1982]. This emphasizes the need for prompt evaluation if and when 
Health Hazard Evaluation Report 2012-0039-3242 Page 13

symptoms do become evident as the underlying condition could have been underway for 

a considerable period by the time someone with rhabdomyolysis begins to feel anything is 

wrong. The risk for permanent injury from rhabdomyolysis increases the longer it takes to get 

diagnosed and treatment started.

The focus group participants reported that they had not received any information regarding 

rhabdomyolysis and its work-related risk factors. Participants noted that educational 

materials should be tailored to structural fire response fire fighters by using actual fire fighters 

in any graphics on the educational materials. The focus group consensus was that educational 

materials would be more likely to prompt fire fighters to report and seek immediate medical 

attention for rhabdomyolysis related symptoms if they clearly spelled out the potentially 

life-threatening and career ending consequences of severe rhabdomyolysis that is not quickly 

treated.

In addition to the instrumentation issues with the Actical accelerometers and heart rate 

measurement strap listed previously, this evaluation has the following limitations:

● The results from controlled fire training exercises may not be comparable to conditions 

experienced during actual structural firefighting activities. For example, participants 

were strictly timed during each live fire entry and were only involved in a single live 

fire training event. In an actual structural fire response fire fighters may have longer 

and/or multiple entries, and the structural fire conditions may differ with fuel sources 

(industrial chemicals, accelerants, etc.).

● Our focus group of six participants consisted of cadets who were undergoing training. 

The findings from this group may be limited in how they can be extrapolated to more 

experienced structural fire fighters as these individuals may have different opinions on 

the issues discussed by the group. Also, the findings from a structural fire fighter focus 

group may not represent opinions of wildland fire fighters of similar experience.

Conclusions 

All participants were exposed to heat stress conditions that often exceeded NIOSH and 

ACGIH criteria, and 28 of 32 participants met our criteria for excessive heat strain. Half of 

the participants had above normal CK levels, and one exhibited CK elevations high enough 

to meet our case definition of rhabdomyolysis. This individual was diagnosed with and 

treated for rhabdomyolysis in the hospital. 

Blood tests for hydration markers showed most participants were adequately hydrated during 

the training day, but some participants were not hydrating adequately between the end of one 

training day and the start of the next one. Discussions about firefighting as an occupation 

and work-related risk factors for rhabdomyolysis may not be occurring between fire fighters 

and their healthcare providers, especially when seeking care outside of traumatic injuries 

sustained during a fire response. Additionally, explicit and customized educational materials 

are needed to raise awareness about rhabdomyolysis within the fire service. 
Page 14 Health Hazard Evaluation Report 2012-0039-3242

Recommendations 

On the basis of our findings, we recommend the actions listed below. Additional 

recommendations can be found in the NIOSH document, “Preventing Heat-related Illness 

or Death of Outdoor Workers” at http://www.cdc.gov/niosh/docs/wp-solutions/2013-143/

pdfs/2013-143.pdf and on the OSHA website, which gives guidance on heat stress prevention 

in outdoor workers at https://www.osha.gov/SLTC/heatstress/.

Our recommendations are considered administrative controls, which refer to employerdictated work practices and policies to reduce or prevent hazardous exposures. Their 

effectiveness depends on employer commitment and employee acceptance. Regular 

monitoring and reinforcement are necessary to ensure that policies and procedures are 

followed consistently.

1. Schedule physically demanding training such as exercising or live fire training during 

cooler parts of the day or cooler months when temperatures and humidity are lower. 

Ensure incoming cadets are acclimatized before starting the training course.

2. Use a WBGT meter to monitor environmental conditions. Use the chart presented in 

Appendix B, Figure B1 as a general guideline to determine how long specific activities 

can be performed.

3. Train cadets and their instructors to recognize early signs and symptoms of 

rhabdomyolysis and HRI and educate them regarding the potential consequences of 

untreated rhabdomyolysis and HRI. Encourage cadets to inform their instructor (or 

supervisor once they are assigned a fire station after completion of cadet training) 

immediately if they develop any possible rhabdomyolysis or HRI symptoms. Inform 

cadets at the start of training that those who report symptoms will be allowed back into 

the training course once they have been medically cleared. 

4. Require onsite medical staff to evaluate each participant’s vital signs and mental status 

before and after each physically demanding training activity.

5. Encourage participants to increase fluid intake during time away from training (rest 

days or off-duty hours). Discourage participants from consuming drinks that contain 

large amounts of caffeine (totaling more than the equivalent of six cups of coffee per 

day), large amounts of sugar, alcohol, or creatine supplements. These may worsen 

dehydration and increase HRI and rhabdomyolysis risk.

6. Ensure that fire fighters have information about their risk for rhabdomyolysis and HRI 

that can be shared readily with medical care providers.
Health Hazard Evaluation Report 2012-0039-3242 Page 15

Appendix A: Tables

Table A1. Heat stress and strain measurements during training activities − Day 1

Activity (Participants) Time

period

Met

rate* 

(Watts)

Max

WBGT 

(°F)

Range of 

max CBTs 

exceeding 

101.3 (°F)

Number 

with 

max CBTs 

exceeding 

101.3 (°F)

Number

exceeding

max 

heart rate 

criterion†

Physical training (22) 0700–0745 520 82.0 105.0 2 4

Shower/classroom (22) 0800–0945 115 68.3 None None None

Prepare for high rise 

training (22)

1000–1115 300 90.0 None None None

High rise training (22) 1130–1145 520 86.4 102.3–103.0 2 1

Lunch (22) 1200–1245 115 66.8 None None None

High rise training (22) 1300–1415 520 88.5 103.0–103.7 7 2

Doff gear, prepare for 

high rise training (22) 

1415–1500 300 88.6 None None None

High rise training (22) 1500–1530 520 87.4 103.4–104.5 9 6

Put gear away, 

dismissed (22)

1530–1600 300 87.8 None None None

*Estimated average metabolic rate

†The maximum heart rate criterion is 180 minus the age of the participant, sustained over 3 minutes 

during activity. Note: some individual datasets had missing values due to loss of contact with the sensor.

Table A2. Heat stress and strain measurements during training activities – Day 2

Activity (Participants) Time

period

Met 

rate* 

(Watts)

Max

WBGT 

(°F)

Range of 

max CBTs 

exceeding 

101.3 (°F)

Number

with 

max CBTs 

exceeding 

101.3 (°F)

Number

exceeding 

max 

heart rate

criterion†

Physical training (22) 0645–0715 520 77.9 101.6–103.1 3 4

Shower/classroom (22) 0715–1000 115 68.8 None None None

Prepare for high rise 

training (22)

1000–1015 300 82.3 None None None

High rise training (22) 1030–1130 520 83.1 101.5–102.8 6 4

Lunch (22) 1145–1245 115 68.4 None None None

Prepare for high rise 

training (22)

1300–1330 300 85.4 None None None

High rise training (22) 1330–1445 520 85.6 101.4–103.7 6 5

Put gear away, 

dismissed (22)

1500–1530 180 85.8 None None None

*Estimated average metabolic rate

†The maximum heart rate criterion is 180 minus the age of the participant, sustained over 3 minutes 

during activity. Note: some individual datasets had missing values due to loss of contact with the sensor.
Page 16 Health Hazard Evaluation Report 2012-0039-3242

Table A3. Heat stress and strain measurements during training activities – Day 3

Activity (Participants) Time

period

Met 

rate* 

(Watts)

Max

WBGT 

(°F)

Range of 

max CBTs 

exceeding 

101.3 (°F)

Number

with 

max CBTs 

exceeding 

101.3 (°F)

Number 

exceeding 

max 

heart rate 

criterion†

Classroom (22) 0700–0730 115 68.2 None None None

Prepare for live burn 

exercise (22)

0745–0800 300 79.0 None None None

Live fire entry team (4) 0842–0915 520 80.7‡ 103.7–104.8 4 1

Live fire entry team (6) 0945–1026 520 83.2‡ 103.5–105.6 4 1

Live fire entry team (4) 1047–1111 520 82.9‡ 102.2–105.1 4 3

Live fire entry team (4) 1140–1215 520 84.9‡ 102.8–107.2 3 4

Lunch (22) 1230–1300 115 68.1 None None None

Live fire entry team (5) 1350–1418 520 84.8‡ 103.0–104.3 2 4

Put gear away, 

dismissed (22)

1445–1500 300 83.8 None None None

*Estimated average metabolic rate

†The maximum heart rate criterion is 180 minus the age of the participant, sustained over 3 minutes 

during activity. Note: some individual datasets had missing values due to loss of contact with the sensor.

‡WBGT measurements made in the recovery area outside the burn building

Table A4. Heat stress and strain measurements during training activities – Day 4

Activity (Participants) Time

period

Met 

rate* 

(Watts)

Max

WBGT 

(°F)

Range of 

max CBTs 

exceeding 

101.3 (°F)

Number

with 

max CBTs 

exceeding 

101.3 (°F)

Number 

exceeding 

max 

heart rate 

criterion†

Physical training (22) 0640–0730 520 81.4 101.3–101.9 3 7

Prepare for low rise 

training (22)

0745–0800 300 79.8 None None None

Low rise training (22) 0830–0930 520 82.7 None None None

High rise training (22) 1000–1130 520 87.7 101.3–104.4 8 3

Lunch (22) 1145–1215 115 68.2 None None None

Low rise training (22) 1300–1345 520 85.6 103.4–104.4 4 3

Classroom 1400–1500 115 69.8 None None None

Put gear away, 

dismissed (22)

1500–1530 180 71.0 None None None

*Estimated average metabolic rate

†The maximum heart rate criterion is 180 minus the age of the participant, sustained over 3 minutes 

during activity. Note: some individual datasets had missing values due to loss of contact with the sensor.
Health Hazard Evaluation Report 2012-0039-3242 Page 17

Appendix B: Figure

Figure B1. NIOSH-recommended heat stress exposure and ceiling limits for acclimatized workers
Page 18 Health Hazard Evaluation Report 2012-0039-3242

Appendix C: Occupational Exposure Limits and 

Health Effects

Heat Stress

Many heat stress guidelines have been developed to protect people against heat-related 

illnesses. The objective of any heat stress index is to prevent a person’s CBT from rising 

excessively. In 1969, the World Health Organization published a document that concluded, 

“it is inadvisable for CBT to exceed 100.4°F or for oral temperature to exceed 99.5°F in 

prolonged daily exposure to heavy work and/or heat” [WHO 1969]. Additionally, a CBT 

of 102.2°F should be considered reason to terminate exposure even when CBT is being 

monitored [NIOSH 1986]. This does not mean that an employee with a CBT exceeding those 

levels will necessarily experience adverse health effects. However, the number of unsafe acts 

increases as does the risk of developing heat stress illnesses [NIOSH 1986].

NIOSH recommends controlling total heat exposure so that unprotected healthy employees 

are not exposed to metabolic and environmental heat combinations that exceed the 

applicable NIOSH criteria. These criteria state that most healthy employees who work in 

hot environments and are exposed to combinations of environmental and metabolic heat 

below the NIOSH recommended action limit for unacclimatized employees or the NIOSH 

recommended exposure limit for acclimatized employees should be able to tolerate total heat 

stress without substantially increasing their risk of incurring acute adverse health effects. 

Also, no employee should be exposed to metabolic and environmental heat combinations that 

exceed applicable ceiling limits without being provided with and properly using appropriate 

and adequate heat-protective clothing and equipment [NIOSH 1986].

The 1986 NIOSH heat stress criteria document referenced above is being updated. A draft 

document released for public comment as part of a notice placed in the Federal Register in 

December 2013 can be viewed at http://www.gpo.gov/fdsys/pkg/FR-2013-12-27/pdf/2013-

31066.pdf. This draft document describes five basic preventive practices that should be 

followed to control heat stress among employees working in hot environments:

● Limiting or modifying the duration of exposure time 

● Reducing the metabolic component of the total heat load

● Enhancing the heat tolerance of the workers by heat acclimatization, physical 

conditioning, etc.

● Training the workers in safety and health procedures for work in hot environments 

● Providing initial and periodic medical examinations to determine whether an employee 

can meet the total demands and physical stresses of the job with reasonable assurances 

the health and safety of the worker and/or fellow employees will not be placed at risk 

[NIOSH 2013]

The ACGIH heat stress guidelines use a decision-making process that provides step-by-step 

situation-dependent instructions that factor in clothing insulation values and physiological 
Health Hazard Evaluation Report 2012-0039-3242 Page 19

evaluation of heat strain [ACGIH 2015]. ACGIH WBGT screening criteria factor in the 

ability of the body to cool itself (clothing insulation value, humidity, and wind) and, 

like the NIOSH criteria, can be used to develop work/rest regimens for acclimatized and 

unacclimatized employees. The ACGIH WBGT-based heat exposure assessment was 

developed for a traditional work uniform of long-sleeved shirt and pants, and represents 

conditions under which it is believed that nearly all adequately hydrated, unmedicated, 

healthy employees may be repeatedly exposed without adverse health effects. Clothing 

insulation values and the appropriate WBGT adjustments, as well as descriptors of the other 

decision-making process components can be found in the ACGIH document “Documentation 

of the Threshold Limit Values for Chemical Substances and Physical Agents and Biological 

Exposure Indices” [ACGIH 2001]. The ACGIH TLV for heat stress provides a framework for 

the control of heat-related illnesses only. Although accidents and injuries can increase with 

increasing levels of heat stress, it is important to note that the TLVs are not directed toward 

controlling these outcomes [ACGIH 2015].

NIOSH and ACGIH criteria can only be used when WBGT data for the immediate work area 

are available and must not be used when employees wear encapsulating suits or garments 

that are impermeable or highly resistant to water vapor or air movement. Further assumptions 

regarding work demands include an 8-hour workday, 5-day workweek, two 15-minute 

breaks, and a 30-minute lunch break, with rest area temperatures the same as, or less than, 

those in work areas, and at least some air movement. While NIOSH and ACGIH guidelines 

distinguish between safe and dangerous levels, professional judgment must be used in 

administering a heat stress management program to ensure adequate protection. OSHA does 

not have an exposure limit for heat stress. However, the OSHA technical manual’s section on 

heat stress refers to the ACGIH document for guidelines to evaluate employee heat stress and 

how to investigate the workplace [OSHA 1999].

Heat Strain

The body’s response to heat stress is called heat strain. Operations involving high air 

temperatures, radiant heat sources, high humidity, direct physical contact with hot objects, 

and strenuous physical activities have a high potential for inducing heat strain in employees. 

Heat strain is highly individual and cannot be predicted on the basis of environmental heat 

stress measurements alone. Physiological monitoring for heat strain becomes necessary when 

impermeable clothing is worn, when heat stress screening criteria are exceeded, or when data 

from a detailed analysis such as the International Standards Organization required sweat rate 

index show excess heat stress.

ACGIH considers one indicator of physiological strain, a sustained, elevated peak heart rate, 

to be a useful measure of acute exposure to high-level heat stress. Sustained peak heart rate, 

defined by ACGIH as 180 bpm minus an individual’s age over several minutes, is a leading 

indicator that thermal regulatory control may not be adequate and that increases in CBTs 

have occurred or will soon occur [ACGIH 2015]. According to ACGIH, an individual’s heat 

stress exposure should be discontinued when any of the following heat strain indicators 

occur:

● Sustained (over several minutes) heart rate exceeds 180 bpm minus the individual’s age 
Page 20 Health Hazard Evaluation Report 2012-0039-3242

in years for those with normal cardiac performance

● CBT is greater than 100.4°F for unselected, unacclimatized personnel and greater than 

101.3°F for medically fit, heat-acclimatized personnel

● Recovery heart rate at 1 minute after a peak work effort exceeds 110 bpm

● Presence of symptoms of sudden and severe fatigue, nausea, dizziness, lightheadedness

In addition, the ACGIH states than an individual may be at greater risk of heat strain if:

● Profuse sweating is sustained over several hours

● Weight loss over a shift is greater than 1.5% of body weight

● Twenty-four-hour urinary sodium excretion is less than 55 millimoles

Acclimatization

When employees are first exposed to a hot environment they may show signs of distress and 

discomfort, experience increased CBTs and heart rates, and may have headache or nausea. 

However, following repeated exposure, employees can adapt to the hot environment. This 

adaptation is called acclimatization.

Employees begin to lose acclimatization when they stop working in the heat stress 

conditions, and a noticeable loss occurs after 4 days. However, this loss is usually rapidly 

made up. Chronic illness, a short episode of mild illness (e.g., gastroenteritis), the use or 

misuse of pharmacologic agents, a sleep deficit, poor nutrition, or a disturbed water and 

electrolyte balance may reduce an employee’s capacity to acclimatize [ACGIH 2015].

Rhabdomyolysis

Rhabdomyolysis is a medical condition associated with heat stress and prolonged physical 

exertion, resulting in the rapid breakdown of muscle which can damage the kidneys. Classic 

symptoms of rhabdomyolysis are muscle pain, cramping, swelling, weakness, and decreased 

range-of-motion of joints. One of the signs of rhabdomyolysis is dark or tea-colored urine 

[Brudvig and Fitzgerald 2007; Khan 2009; Cervellin et al. 2010]. However, symptoms vary 

between individuals, and some might not have any symptoms at all [Huerta-Alardin et al. 

2005; Brudvig and Fitzgerald 2007]. 

Rhabdomyolysis is diagnosed by measurement of CK, also known as creatine phosphokinase, 

in the blood by a licensed healthcare provider. The severity of rhabdomyolysis depends upon 

damage to other organ systems and the peak CK level. Mild rhabdomyolysis can be treated 

by drinking lots of fluids [George et al. 2010]. Severe cases require hospitalization to provide 

fluids intravenously, monitor CK levels to guide treatment, and follow kidney function as 

emergent dialysis may be needed if the kidneys fail. [Bosch et al. 2009]. Inpatient monitoring 

would also include cardiac telemetry to look for abnormal heart rhythms induced by high 

potassium levels, neurological monitoring for seizures, and neurovascular monitoring for 

compartment syndrome.
Health Hazard Evaluation Report 2012-0039-3242 Page 21

It is not uncommon for individuals who engage in exertional activities higher than their 

baseline level of fitness to develop exertional rhabdomyolysis. However, it also occurs in 

highly conditioned individuals who may engage in supramaximal exercise (extreme short 

duration anaerobic exercise) or who have other risk factors along with an exertional activity 

[Walsh and Page 2006].

Dehydration, Volume Depletion, and Fluid Replacement

When working in hot environments, completely replacing lost fluids as the day’s work proceeds 

can be difficult. Sweat contains water and salt, and excessive sweating can cause dehydration, 

volume depletion, and electrolyte imbalances. Volume depletion is different from pure 

dehydration and occurs when loss of both water and salt/sodium results in a reduced circulatory 

blood volume [Mange et al. 1997]. Volume depletion also negates the advantage granted by 

high levels of aerobic fitness and heat acclimatization. Several studies have shown that volume 

depletion or dehydration increases CBT during exercise in temperate and hot environments. 

Therefore, maintaining enough water improves the body’s overall function. 

Drinking fluids is important to ensure adequate rehydration, and evidence shows that having 

drinks that taste good leads to increased consumption. Glucose-electrolyte solutions like 

Gatorade® can increase sodium and water absorption, and the glucose in these drinks 

provides energy for muscular activity [Rolls et al. 1990]. However, employees should avoid 

drinking large amounts of sugar-laden beverages in hot climates as this causes increased 

urine production that increases fluid loss through urination. Intake of caffeinated beverages 

and alcohol also increases urinary fluid loss and should be avoided. Because average 

Americans consume adequate, if not excessive, amounts of sodium in their diet, oftentimes 

only water replacement is needed. Oral electrolyte replacement formulas such as Gatorade 

can be used for moderate volume depletion or for situations involving prolonged sweating. 

Salt tablets are not recommended. More information on heat stress and strain is available at 

http://www.cdc.gov/niosh/topics/heatstress/.
Page 22 Health Hazard Evaluation Report 2012-0039-3242

Appendix D: Probes Used During Focus Groups

A. Probe: Are you familiar with rhabdomyolysis? If so, do you believe that you are at risk 

for this condition because you are a structural fire fighter?

B. Probe: Does your personal physician know you are a fire fighter? If so, do you talk 

about your risk factors for certain conditions such as rhabdo with him or her? When 

you are seen by a different physician (emergency room, urgent care, physicals, doctor 

is unavailable) do you always tell him/her that you work as a fire fighter? 

C. Probe: If so, how easy is it to talk with your healthcare provider about your job 

and how it can affect your health? If you don’t tell healthcare providers about your 

occupation, why don’t you?

D. Probe: What sorts of items do you think would aid you in talking to healthcare 

providers about your occupation, and what conditions they should watch for due to the 

risks associated with this profession?

E. Probe: We have drafted this wallet card that identifies you as a fire fighter. There is 

room on the back for you to fill in your personal information. Would you carry this 

with you? If not, why? If so, what types of information would be useful to include on 

this card to alert healthcare providers to the risk associated with your job, specifically 

that they should have a lower threshold to test you for rhabdo?
Health Hazard Evaluation Report 2012-0039-3242 Page 23

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Page 26 Health Hazard Evaluation Report 2012-0039-3242

Keywords: North American Industry Classification System 611519 (Fire fighter Training 

Schools), Texas, fire fighters, heat stress, heat strain, rhabdomyolysis, creatine kinase, wet 

bulb globe temperature, WBGT, core body temperature, heart rate, dehydration
Health Hazard Evaluation Report 2012-0039-3242 Page 27

The Health Hazard Evaluation Program investigates possible health hazards in the workplace 

under the authority of the Occupational Safety and Health Act of 1970 (29 U.S.C. § 669(a)

(6)). The Health Hazard Evaluation Program also provides, upon request, technical assistance 

to federal, state, and local agencies to investigate occupational health hazards and to prevent 

occupational disease or injury. Regulations guiding the Program can be found in Title 42, Code 

of Federal Regulations, Part 85; Requests for Health Hazard Evaluations (42 CFR Part 85).

Disclaimer

The recommendations in this report are made on the basis of the findings at the workplace 

evaluated and may not be applicable to other workplaces.

Mention of any company or product in this report does not constitute endorsement by NIOSH.

Citations to Web sites external to NIOSH do not constitute NIOSH endorsement of the 

sponsoring organizations or their programs or products. NIOSH is not responsible for the 

content of these Web sites. All Web addresses referenced in this document were accessible as of 

the publication date.

Acknowledgments

Desktop Publisher: Shawna Watts

Editor: Ellen Galloway 

Health Communications Specialist (Focus Group): Stefanie Brown

Industrial Hygiene Field Assistance: Kevin L. Dunn, Bradley King

Logistics: Donnie Booher 

Medical Field Assistance: Deborah Sammons, Shirley Robertson, Donnie Booher, and 

John Clark

Availability of Report

Copies of this report have been sent to the employer, employees, and union at the facility. The 

state and local health department and the Occupational Safety and Health Administration Regional 

Office have also received a copy. This report is not copyrighted and may be freely reproduced. 

This report is available at http://www.cdc.gov/niosh/hhe/reports/pdfs/2012-0039-3242.pdf.

Recommended citation for this report:

NIOSH [2015]. Evaluation of heat stress, heat strain, and rhabdomyolysis during 

structural fire fighter training. By Eisenberg J, Methner M, Dowell CH, Mueller C. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, NIOSH 

HHE Report No. 2012-0039-3242.
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