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Technology &
Development Center
Wildland Firefighter Health and Safety
Recommendations of the April 1999 Conference

Heat Stress

Brian J. Sharkey, Ph.D.
USDA Forest Service
Missoula Technology and Development Center

Wildland firefighting is arduous work. Shifts are long, often in steep terrain and at higher elevations. The weather is usually hot and dry, and the fire increases exposure to heat. This report focuses on the risks of heat stress and what firefighters should do to minimize those risks.

When hard work is performed in a hot environment, blood is sent to the skin to cool the body, primarily through evaporation of sweat. As sweating continues, often at a rate of more than 1 liter per hour, the body loses a considerable quantity of fluid. Loss of fluid can compromise heart and circulatory function and the ability to work. If fluids are not replaced, the temperature-regulating process begins to fail, work becomes impossible, and the possibility of life threatening heat stroke increases dramatically.

Heat Disorders

Heat stress disorders include heat cramps, heat exhaustion, and heat stroke.

Measuring Heat Stress

Figure 1 illustrates how temperature and humidity combine to create moderate or high heat stress conditions.

Image of a heat stress chart.
Figure 1—Heat stress chart.

The risk of heat stress increases when radiant heat from the sun or nearby flames is high, the air is still, or when someone is doing hard work and creating lots of metabolic heat.

Some organizations use the WBGT Heat Stress Index. The index uses dry bulb, wet bulb, and globe temperatures that are weighted to indicate the impact of each measure on the worker:

The WBGT heat stress index does not take into account the cumulative effects of long hours of hard work or the impact of personal protective clothing and equipment. Studies of wildland firefighting indicate that while WBGT values occasionally rise to dangerous levels, the low humidity and high air movement characteristic of fire weather combine to improve evaporative cooling (Ramberg 1974).


Personal protective clothing strikes a balance between protection and worker comfort. Australian researchers have concluded that:

The purpose of firefighter’s clothing is not to keep heat out but to let it out! (Budd, Brotherhood et al. 1996)

About 70% of the heat load comes from within, from metabolic heat generated by muscles during hard work. Only 30% comes from the environment and the fire. Studies recommend the use of loose-fitting garments to enhance air movement, cotton T-shirts and underwear to help sweat evaporate, and avoidance of extra layers of clothing that insulate, restrict air movement, and contribute to heat stress.

Individual Differences

There are individual differences in fitness, heat acclimatization, and heat tolerance. Some workers are at greater risk for heat disorders. The reasons include inherited differences in heat tolerance and sweat rate. Excess body weight raises metabolic heat production. Illness, drugs, and medications can also influence the body’s response to work in a hot environment. After an illness, workers need time to regain acclimatization to the heat.

Because a number of drugs increase the risk of heat stroke, workers should check with a physician or pharmacist if they are using prescription or over-the-counter medications, or if they have a medical condition. Large doses of the over-the-counter antiinflammatory drug ibuprofen can cause kidney damage when ibuprofen is used by persons who are dehydrated.


The consequences of heat stress can be avoided by improving the level of fitness and becoming acclimated to the heat.


Maintaining a high level of aerobic fitness is one of the best ways to avoid heat stress. The fit worker has a well-developed circulatory system, and increased blood volume; both are important for regulating body temperature. Fit workers start to sweat sooner, so they work with a lower heart rate and body temperature. They adjust to the heat twice as fast as the unfit worker. They lose acclimatization more slowly and regain it quickly. In a heat chamber study conducted in the University of Montana Human Performance Laboratory (Cordes and Sharkey 1995), fitness was inversely related to the working heart rate. A subject with a high level of aerobic fitness (68 mL/kg-min) worked at a heart rate of 118 bpm (beats per minute), while a less fit subject (45 mL/kgmin) had a heart rate above 160 bpm while doing the same work (Figure 2). In this 2 hour treadmill test conducted at 90 °F, differences in fitness overshadowed the effects of variations in clothing systems.

Image of a fitness and HR graph.
Figure 2—Fitness and HR Graph.


Acclimatization is necessary to prepare a firefighter for work in heat stress conditions. Acclimatization is a process of adjustment that occurs in 5 to 10 days of heat exposure as the body:

Acclimatize by gradually increasing work time in the heat, taking care to replace fluids and to rest as needed. Acclimatization can be maintained with periodic exposure to work or exercise in a hot environment.

On the Job

When heat stress conditions exist, workers must modify the way they work or exercise. When possible, workers should:

Most important of all, workers must maintain hydration by replacing lost fluids.


Studies of wildland firefighters indicate that fire suppression activities generate approximately 7.5 kilocalories of heat each minute worked, or over 400 kilocalories for each hour. Additional heat (about 180 kilocalories per hour) comes from the environment and the fire.

400 + 180 = 580 kcal/hr

Complete evaporation of 1 liter of sweat removes 580 kilocalories of heat. That means that a firefighter needs to evaporate about 1 liter (1 L = 1.06 qt) of sweat for each hour of work. Body fluids must be replaced if sweating is to be maintained. That means drinking before, during, and after work.

Before Work—Drink 1 to 2 cups of juice or water before work. Avoid excess caffeine since it hastens fluid loss in the urine. Studies of glycerol-induced hyperhydration did not support its use for wildland firefighters (Swan 1997).

While Working—Workers should take several fluid breaks every hour, drinking at least 1 quart each hour. They should drink as much as possible during the lunch break. Water is the body’s greatest need during work in the heat. However, studies show that workers drink more when lightly flavored beverages are available. Providing a portion of fluid replacement with a carbohydrate/electrolyte beverage will help retain fluids and maintain energy and electrolyte levels. The carbohydrate also helps maintain immune function (Nieman 1998) and mental performance (Puchkoff et al. 1998). Caution workers to avoid sharing water bottles except in emergencies.

After Work—Continue drinking to replace fluid losses. Thirst always underestimates fluid needs, so workers should drink more than they think they need. Rehydration is enhanced when fluids contain sodium and potassium, or when foods with these electrolytes are consumed along with the fluid.

Sodium lost in sweat is easily replaced at mealtime with liberal use of the salt shaker. Unacclimatized workers lose more salt in the heat so they need to pay particular attention to salt replacement. But salt intake should not be overdone; too much impairs temperature regulation, and excessive salt can cause stomach distress, fatigue, and other problems. Make potassium-rich foods like bananas and citrus fruits a regular part of the diet, and drink lots of lemonade, orange juice, or tomato juice. In fire camp, limit caffeine drinks such as coffee and colas because caffeine increases fluid loss in the urine. Alcoholic drinks also cause dehydration.

Hydration can be assessed by observing the volume, color, and concentration of urine. Low volumes of dark, concentrated urine, or painful urination indicate a serious need for rehydration. Other signs of dehydration include a rapid heart rate, weakness, excessive fatigue, and dizziness. Rapid loss of several pounds of body weight is a certain sign of dehydration. Workers should rehydrate before returning to work. Continuing to work in a dehydrated state can lead to serious consequences, including heat stroke, muscle breakdown, and kidney failure.


The risk of heat stress and heat disorders can be reduced dramatically if workers comply with the following guidelines:

  • Improve or maintain aerobic fitness
  • Acclimate to the heat
On the Job
  • Be aware of conditions (temperature, humidity, air movement)
  • Take frequent rest breaks
  • Avoid extra layers of clothing
  • Pace yourself
  • Change tasks or tools
  • Before work—drink several cups of water or juice
  • During work—take frequent fluid breaks (1 qt/hr)
  • After work—keep drinking to ensure rehydration (carbohydrate/electrolyte drinks increase fluid intake, decrease loss, provide energy, and help maintain immune function and mental performance)
  • Always work or train with a partner.

It is dangerous to work or exercise alone in heat stress conditions. Firefighters should always train and work with a partner who can help in the event of a problem. Partners should remind each other to drink lots of fluids, keep an eye on each other, and start treatment immediately if their partner shows signs of a heat disorder.

Selected References

Budd, G.; Brotherhood, J.; and others. 1999. Safe and productive bushfire fighting with handtools. Australian Government Publishing Service.

Cordes, K.; Sharkey, B. 1995. Physiological comparison of protective clothing variations. Medicine and Science in Sports and Exercise.

The standard on protective clothing and equipment for wildland firefighters (NFPA #1977, 1993) addresses outer garments and some accessories. This study describes a comparison of proposed uniform variations. Four male and 4 female volunteers performed prolonged (2 hour) treadmill tests with 4 variations of the standard uniform: no T-shirt (NT), a short sleeve T-shirt (ST), a long sleeve T-shirt (LT), and a ST plus a shroud for face and neck protection (SH), with test order determined by a balanced Latin square design. Two hour tests, conducted with a 3 day rest interval, consisted of a treadmill walk at 5.65 km/hr (3.5 mph) and 4.5% grade, with a 10.9 kg (24 lb) pack, reflecting the energy expenditure of firefighting tasks (6.5 Mets). The test was conducted at 32.2 C (90 F) and 30% RH, with an airspeed of 5 km/hr (3.1 mph), and with radiant heat (0.1 watts/cm2) during the first half of each hour. Heart rates, skin and tympanic temperatures, and perceived exertion (RPE) were recorded every 10 min, and weight loss and evaporative loss were determined after each trial. Male and female values were not significantly different so the data were pooled for repeated measures ANOVA. Significant order effects were found for HR and RPE (p < .029 and .0001 respectively), indicating some acclimatization. Analysis of treatment effects did not reveal significant differences, although HR, RPE, weight and evaporative loss tended to be greater for the LT. Tympanic and mean body temperatures tended to be higher with the SH. Individual differences in fitness overshadowed the effects of clothing variations. Treatment differences were more pronounced at 2 vs. 1 hour, and the radiant heat influenced the skin temperatures.

Nieman, D. 1998. Influence of carbohydrate on the immune response to intensive, prolonged exercise. Exercise Immunological Review. 4: 64-76.

Puchkoff, J.; Curry, L.; Swan, J.; Sharkey, B.; Ruby, B. 1998. The effects of hydration status and blood glucose on mental performance during extended exercise in heat. Medicine and Science in Sports and Exercise.

This study examined the differential effects of three hydration methodologies (carbohydrate, glycerol, and placebo) on the mental performance of 10 subjects during 3 hours of treadmill walking and simulated line digging in a heated environment. Each subject completed one three-hour exercise trial for each hydration methodology. The Paced Auditory Serial Addition Task (PASAT) was used to assess mental performance; each subject was given 3 practice tests before the first actual trial. The test required subjects to add pairs of single-digit numbers heard via a tape recorder and respond verbally. A set of 61 numbers was given at 3 speeds for each PASAT test and subjects were given the test 3 times during each trial. All subjects completed a VO2 peak test and intensity for each trial was set at 50% of this value. Measures of blood glucose, plasma volume, body weight, rates of perceived exertion (RPE), heart rate, core and tympanic temperatures, and urine output were recorded at regular intervals throughout each trial. A statistically significant difference between final scores in the carbohydrate and placebo trials was found at the speed of one digit every 1.6 seconds (p = 0.017). At a speed of one digit every 1.2 seconds, scores after 90 minutes and at the end of 180 minutes of exercise were significantly higher than baseline scores (p = 0.001). The carbohydrate trial showed significantly higher values than the placebo trial. Females maintain more consistent body weights than males at the end of the exercise trial (p = 0.0001). Males gained more weight than females during the 90 min pre-hydration period (p = 0.0004). The glycerol trial resulted in significantly higher plasma volume values following the pre-hydration period (p = 0,04). Females exhibited a greater ability to maintain plasma volume (p = 0.046). Blood glucose values were higher at all data collection points, beginning with 60 minutes, during the carbohydrate trial (p = 0.0001). RPE scores were significantly higher than baseline measures (p = 0.0001) beginning at 90 minutes of exercise. The results of this study suggest that mental performance is facilitated after long-duration submaximal exercise in a heated environment, and is better maintained with carbohydrate than with glycerol or water. The increase in scores could be attributed to a narrowing of attentional focus and arousal of the central nervous system. The improvement with carbohydrate is most likely due to the increase in blood glucose which facilitated brain function.

Ramberg, R. 1974. Firefighters physiological study. Tech. Rep. 7551-2205-MTDC. Missoula, MT: U. S. Department of Agriculture, Forest Service, Missoula Technology and Development Center.

Sharkey, B. 1997. Fitness and work capacity. Order NFES No.1596 from NIFC c/o Great Basin Cache Supply, 3833 S Development Ave., Boise, ID 83705

Swan, J. 1997. Glycerol-induced hyperhydration during long-term exercise in a heated environment. Unpublished master’s thesis, University of Montana.

The ability to hyperhydrate has been shown to negate the effects of hypohydration due to long-term exercise in a heated environment. The purpose of this study was to examine the efficacy of two hyperhydration strategies during exercise heat stress and the resulting physiological strain. Ten trained subjects (5 M, 5 F) performed 2 three-hour exercise trials in a heat chamber (32 C). Exercise trials included 2 hydration regimens and were completed in a randomized double-blind fashion. The experimental solution contained 1 g glycerol/kg BW mixed with 21.4 ml water/kg BW. The control solution was the same as the experimental solution without the addition of glycerol. Solutions were ingested over a period of 90 minutes prior to the extended work bout. During the work bout subjects completed treadmill walking (50% VO2 max) and simulated fireline building. This design was used to simulate a typical wildland firefighting work protocol. Subjects were given water to drink during the exercise so that the total amount of liquid taken in was equal to 5 ml/kg per 30 minutes, accounting for the amount of saline needed to keep the venous catheter open. Following the extended work bout, a performance trial was done on the digging treadmill. No significant difference was found between the two hydration strategies for the variables of heart rate, plasma osmolality, core temperature, sweat rate, % body weight loss, plasma volume changes, and post-exercise performance. The data did show that the glycerol hyperhydration strategy resulted in a significant reduction in urine output over the length of the entire trial (p<0.05). These data suggest that glycerol is distributed throughout all fluid compartments and not just extravascularly as previously thought. Based on the data collected it was concluded that there was no difference between the two hyperhydration strategies when rehydration was maintained during the exercise heat stress. Further research should consider the mechanism of glycerol’s ability to increase TBW, and gender differences in response to an exercise heat stress.

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