Health risks during exercise in the heat

Despite the body’s defenses against overheating, excessive heat production by active muscles, heat gained from the environment, and conditions that prevent the dissipation of excess body heat may elevate the internal body temperature to levels that impair normal cellular functions. Under such conditions, excessive heat gains pose a risk to one’s health. Air temperature alone is not an accurate index of the total psychological stress imposed on the body in a hot environment. At least four variables must be taken into account:

• Air temperature
• Humidity
• Air velocity
• Amount of thermal radiation.

All these factors influence the degree of heat stress that a person experiences. The contribution of each factor to the total heat stress under various environmental conditions can actually be predicted mathematically using advanced heat balance equations.

An individual exercising on a bright, sunny day with an air temperature of 23°C(73.4°F) and no measurable wind experiences considerably more heat stress than someone exercising in the same air temperature but under cloud cover and with a slight breeze. At temperatures above skin temperature, which is normally 32 to 33°C(90-92°F), radiation, conduction, and convection substantially add to the body’s heat load rather than acting as avenues for heat loss. How, then, we can judge the amount of heat stress to which an individual may be exposed?

Measuring heat stress

It has become common to hear about the “heat index” on local weather channels. The heat index, a complex equation involving air temperature and relative humidity, is a measure of how hot it feels, that is, how we perceive the heat. However, it does not do a good job of reflecting the physiological stress on humans, so its use is limited in exercise physiology. Through the years, efforts have been made to quantify atmospheric variables into a single index that would reflect the physiological stress on an individual. In the 1970s, wet-bulb globe temperature(WGBT) was devised to simultaneously account for conduction, convection, evaporation, and radiation. It is based on three different thermometer readings and provides a single temperature to estimate the cooling capacity of the surrounding environment.

The dry-bulb temperature(T_db) is the actual air temperature one would measure with a typical thermometer. A second thermometer has a wet bulb that is kept moist. As water evaporates from this bulb, its temperature(T_wb) will be lower than the dry bulb’s, reflecting the effect of sweat evaporating from the skin. The difference between the wet- and dry-bulb temperatures indicates the environment’s capacity for cooling by evaporation. In still air with 100% relative humidity, these two bulb temperatures are the same because evaporation is impossible. Lower humidity and moving air promote evaporation, increasing the difference between these two bulb temperatures. The third thermometer, placed inside a black globe, typically shows a temperature higher than T_db as the globe absorbs radiant heat. Thus, its temperature(T_g) is a good indicator of the environment’s radiant heat load.

The temperatures from these three thermometers can be combined into the following equation to estimate the overall atmospheric challenge to body temperature in outdoor environments:

WGBT = 0.1 T_db + 0.7 T_wb + 0.2 T_g.

The fact that the coefficient for T_wb  is the largest reflects the importance of sweat evaporation in the physiology of heat exchange. Also note that WGBT reflects only the environment’s impact on heat stress and is most effectively used along with a measure or estimate of metabolic heat production.

This measurement of thermal stress is now routinely used by coaches, medical directors of distance races and triathlons, and athletic trainers to anticipate the health risks associated with athletic competitions in thermally stressful environment.

Heat-related disorders

Exposure to the combination of external heat stress and metabolically generated heat can lead to three heat-related disorders: heat cramps, heat exhaustion, and heatstroke.

Heat cramps

Heat cramps, the least serious of the three heat disorders, are characterized by severe and painful cramping or large skeletal muscles. They involve primarily the muscles that are most heavily used during exercise, and such instances of athletes “locking up” are different from cramps everyone has experienced in small muscles. Heat cramps are brought on by sodium losses and dehydration that accompany high rates of sweating, and thus are most common in heavy sweaters who lose a lot of sodium in their sweat. (A common misconception is that potassium-rich foods like bananas will prevent heat cramps.) Heat cramps can be prevented or minimized in susceptible athletes by proper hydration practices involving liberal salt intake with foods and in beverages consumed during exercise. Treatment for these cramps involves moving the stricken individual to a cooler location and administering a saline solution, either orally or intravenously if necessary.

Heat exhaustion

Heat exhaustion typically is accompanied by such symptoms as extreme fatique, dizziness, nausea, vomiting, fainting, and a weak, rapid pulse. It is caused by the cardiovascular system’s inability to adequately meet the body’s needs as it becomes severely dehydrated. During exercise in heat, active muscles and skin compete for a share of a limited, and decreasing, blood volume. Heat exhaustion may result when these simultaneous demands cannot be met, and it typically occurs when blood volume decreases as a result of excessive fluid loss from profuse sweating. A second form of heat exhaustion, from sodium depletion, is rare in athletes. Therefore, heat exhaustion can be thought of as a syndrome of dehydration and should be treated as such.

With heat exhaustion, the thermoregulatory mechanisms are functioning but cannot dissipate heat quickly enough because insufficient blood volume is available to allow adequate blood flow to the skin. Although the condition often occurs during mild to moderate exercise in the heat, it is not necessarily accompanied by extremely high core temperatures. Some people who collapse from heat exhaustion have core temperatures well below 39°C(102.2°F). People who are unfit or not acclimated to the heat are more susceptible than others to heat exhaustion.

Treatment for victims of heat exhaustion involves rest in a cooler environment with their feet elevated to facilitate return of blood to the heart. If the person is conscious, administration of salt water is usually recommended. If the person is unconscious, medically supervised intravenous administration of saline solution is recommended.

Heatstroke

Heatstroke is a life-threatening heat disorder that requires immediate medical attention. Heatstroke is caused by failure of the body’s thermoregulatory mechanisms. It is characterized by:

• An increase in internal body temperature to a value exceeding 40°C(104°F);
• Cessation of active sweating(although sweat may remain on the body);
• Rapid pulse and respiration;
• Confusion, disorientation, or unconsciousness.

The final element – altered mental status – is the key to recognizing impending heatstroke because neural tissues in the brain are particularly sensitive to extreme heat.

If heatstroke is left untreated, core temperature will continue to rise, progressing to coma and ultimately death. Treatment involves cooling the person’s body as rapidly as possible. In the field, this can best be accomplished by immersing the victim in a bath of ice water. In cases where immersing the victim is not possible or prudent, wrapping the body in cold, wet sheets and fanning vigorously is also an option.

For the athlete, heatstroke is not just a problem associated with extreme conditions. Studies have reported rectal temperatures above 40.5°C(104.9°F) in marathon runners who successfully completed races conducted under relatively moderate thermal conditions.

Preventing hyperthermia

We can do little about environmental conditions. Thus, in threatening conditions, athletes must decrease their effort(and thus their metabolic heat production) in order to reduce their risk of overheating. Athletes, coaches, and sport organizers should be able to recognize the symptoms of heat illness.

To prevent heat disorders, several simple precautions should be taken. Competition and practice should not be held outdoors when the WGBT is more than 28°C(82.4°F) unless special precautions are taken. Scheduling practices and events either in the early morning or in the late evening avoids the severe heat stress of midday. Fluids should be readily available, and drink breaks should be scheduled every 15 to 30min, with a goal of matching fluid intake to sweat loss. This is best accomplished by having athletes weigh themselves before and after exercise sessions and learn to estimate their fluid needs.

Clothing is another important consideration. Obviously, the more clothing that is worn, the less body area exposed to the environment to allow for direct heat loss. The foolish practice of exercising in a rubberized suit to promote weight loss from the body. This can rapidly lead to heat exhaustion or heatstroke. Football uniforms are another example of clothing that impedes heat loss. Coaches and athletic trainers should avoid practice sessions in full uniforms whenever possible, especially early in the season when temperatures tend to be hottest and players tend to be less fit and not well acclimated.

Distance athletes should wear as little clothing as possible when heat stress is a potential limitation to thermoregulation. They should tend toward underdressing rather than overdressing, because the metabolic heat generated will soon make extra clothing an unnecessary burden. Clothing should be loosely woven to allow sweat to be absorbed and wicked away from the skin and light colored to reflect heat back to the environment. Hats should be worn during exercise in bright sunlight or when cloud cover is limited.

It is also important to maintain hydration of the body, since the body loses considerable water through sweating. Briefly, drinking fluid both before and during exercise can greatly reduce the negative effects of exercising in the heat. Adequate fluid intake will attenuate the increase in core body temperature and heart rate normally seen when a person exercises in the heat and will allow exercise to be continued longer.