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Heat Stress
Brian Sharkey, Ph.D.

Wildland firefighting is arduous work. Shifts are long, often in steep terrain, and at high 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 the steps firefighters can take to minimize those risks.

When firefighters perform hard work in a hot environment, the body sends blood to the skin where sweat glands produce sweat. The sweat evaporates, cooling the body.

As sweating continues, the body loses a considerable quantity of fluid, often at a rate of more than 1 L/h. Dehydration 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 disorders include heat cramps, heat exhaustion, and heat stroke.

Heat cramps are involuntary muscle contractions caused by failure to replace fluids or electrolytes, such as sodium and potassium. Cramps can be relieved by stretching and replacing fluids and electrolytes.

Heat exhaustion is characterized by weakness, extreme fatigue, nausea, headaches, and wet clammy skin. Heat exhaustion is caused by inadequate fluid intake. Symptoms can be relieved by resting in a cool environment and by replacing fluids and electrolytes.

Heat stroke is a medical emergency caused by failure of the body's heat regulating mechanisms. Sweating stops and the body temperature rises dramatically. Heat stroke is characterized by hot dry skin, a body temperature higher than 41 ºC (105.8 ºF), mental confusion, loss of consciousness, convulsions, and coma. If you are awaiting medical help while caring for a victim of heat stroke, begin cooling the victim with ice or cold water. Fan the victim to promote evaporation. Treat for shock if necessary. For rapid cooling, partially submerge the victim in cold water.

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

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 working hard and creating a lot of metabolic heat.

Some organizations, including the U.S. Marine Corps, use the wet bulb globe temperature (WBGT) heat stress index. The dry bulb, wet bulb, and globe temperatures are weighted to indicate each measure's impact on the individual:

• Wet bulb (humidity) accounts for 70 percent.
• Black globe (radiant heat and air movement) accounts for 20 percent.
• Dry bulb (air temperature) accounts for 10 percent.

When the sum of the weighted temperatures exceeds 80 ºF, instructors are advised to use discretion in training. When the index exceeds 85 ºF, recruits must avoid strenuous activity. When the index exceeds 88 ºF, they must cease physical activity. However, trained individuals who have been acclimated to the heat are allowed to continue limited activity.

The WBGT 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 heat stress during 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.

In military studies, the frequency of heat illness has been related to the temperature of the previous day. When the temperature was hot, the frequency of heat illness increased even though the recruits were forced to rehydrate. High temperatures on one day should be viewed as a warning flag for the following day.

Clothing— Personal protective clothing needs to provide for firefighter safety and comfort. Australian researchers Budd and Brotherhood (1996) concluded that:

Clearly, the task of firefighters' clothing is not to keep heat out but to let it out.

About 70 percent of the heat load comes from metabolic heat generated by muscles during hard work. Only 30 percent of the heat load comes from the environment and the fire. Studies recommend loose-fitting garments to enhance air movement and cotton underclothing to help sweat evaporate. Firefighters should avoid extra layers of clothing that insulate, restrict air movement, and contribute to heat stress.

Individual Differences— Some workers are at greater risk for heat disorders due to individual differences in fitness and heat acclimatization, or heat intolerance. Some workers sweat profusely, while a few have an inadequate number of sweat glands. Excess body weight increases metabolic heat production and interferes with heat loss. Illness, sleep deprivation, medications, drugs, and alcohol can also influence the body's response to work in a hot environment. After an illness, workers need time to reacclimate to the heat.

A number of prescription and over-the-counter medications, as well as recreational drugs and alcohol, increase the risk of heat stroke. Workers should check with a physician or pharmacist if they are using medications or drugs, or if they have a medical condition. Large doses of the over-the-counter, anti-inflammatory drug, ibuprofen, can cause kidney damage if it is used by someone who is dehydrated.

Prevention— The serious consequences of heat stress can be avoided by increasing fitness and by acclimating to the heat.

 

Fitness—Maintain a high level of aerobic fitness to avoid heat stress. The fit worker has a well-developed circulatory system and increased blood volume. Both are important for the regulation of 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 more quickly. In a heat chamber study conducted in the University of Montana Human Performance Laboratory, 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 beats per minute (bpm), while a less fit subject (45 mL/kg-min) had a heart rate higher than 160 bpm (figure 2). In this 2-h treadmill test conducted at 90 ºF, differences in fitness overshadowed the effects of variations in clothing systems.

Figure 2—Fitness and heart rate.

Acclimatization—With 5 to 10 days of heat exposure, the body:

• Begins sweating at a lower temperature.
• Increases sweat production.
• Improves blood distribution.
• Decreases the heart rate, and lowers skin and body temperatures.

Firefighters can prepare for work in heat-stress conditions by gradually increasing their work time in the heat, taking care to replace fluids, and resting as needed. They can maintain acclimatization 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 and exercise. When possible workers should:

• Pace themselves.
• Avoid working close to heat sources.
• Do harder work during cooler morning and evening hours.
• Change tools or tasks to minimize fatigue.
• Take frequent rest breaks during work.

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

Hydration— Studies of wildland firefighters indicate that fire suppression activities generate about 7.5 kcal of heat each minute worked, or over 400 kcal/h. Additional heat (about 180 kcal/h) comes from the environment and the fire.

400 + 180 = 580 kcal/h

Complete evaporation of 1 L of sweat removes 580 kcal of heat. Firefighters need to evaporate about 1 L (1 L = 1.0567 qt) of sweat for each hour of work. These body fluids must be replaced. This can be accomplished by drinking before, during, and after work.

Before Work—Drink 1 to 2 cups of water, juice, or sport drink before work. Avoid excess caffeine because it hastens fluid loss in the urine. Studies of glycerol-induced hyperhydration (temporary storage of extra fluids) do not support its use by wildland firefighters (Swan et al. 2000).

 

During Work—Workers should take several fluid breaks every hour, drinking at least 1 qt of fluid each hour and as much as possible during lunch (figure 3). Water is the body's greatest need while working in the heat. Studies show that workers drink more fluids when lightly flavored beverages are available. Providing a portion of fluid replacement with a carbohydrate/electrolyte (C/E) beverage helps firefighters retain fluids and maintain their energy and electrolyte levels. The carbohydrate also helps maintain immune function (Nieman 1998) and mental per-formance (Puchkoff et al. 1998). The sodium in the C/E beverage reduces urinary water loss.

Figure 3—Workers should avoid sharing water
bottles except in emergencies.

After Work—Continue drinking to replace fluid losses. Thirst always underestimates fluid needs. 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 saltshaker. Unacclimatized workers lose more salt in the heat, so they need to pay particular attention to salt replacement. Salt intake should not be overdone; too much salt impairs temperature regulation, causes stomach distress, fatigue, and other problems. Firefighters should eat potassium-rich foods like bananas and citrus fruits and drink plenty 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 or concentrated urine 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 signals dehydration. Workers should rehydrate before returning to work. Continuing to work while dehydrated can lead to serious consequences, including heat stroke, muscle breakdown, and kidney failure.

Summary

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

Prevention

• Improve/maintain aerobic fitness.
• Acclimate to the heat.

On the job

• Be aware of weather conditions (temperature, humidity, air movement).
• Take frequent rest breaks.
• Do not wear unnecessary layers of clothing.
• Pace yourself.
• Change tasks or tools.

Hydrate

• Before work—drink 1 to 2 cups of water, juice, or sport drink.
• During work—take frequent fluid breaks (1 qt/h).
• After work—keep drinking to ensure rehydration.

Partner

• 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 provide assistance in the event of a problem. They should remind each other to drink lots of fluids, keep an eye on each other, and call for help and start treatment immediately if their partner shows signs of a heat disorder.

This paper is an updated version of Dr. Sharkey's presentation at the Wildland Firefighter Health and Safety Conference hosted by MTDC in April 1999. Dr. Sharkey, a project leader at MTDC, is professor emeritus of the University of Montana Human Performance Lab.