To exercise safely in warm environments, understanding how our bodies regulate internal temperature is very important. Humans are homeotherms, which basically means that we must maintain our internal, or core, temperature independent of the environment. People produce constant internal heat and maintain a core temperature of approximately 98.6 o F or 37o C. This internal heat must be dissipated to the environment. Without effective heat dissipation, heat accumulation can lead to illness and death.

At rest, this internal heat production is rather minimal (about one calorie/min). Exercise, however, generates a large amount of body heat. During heavy exercise, heat production may increase to 10 to 20 calories/min, which necessitates a greater need for heat dissipation to prevent becoming overheated.

If both the ambient temperature and the humidity are high, it becomes difficult for the body to dissipate heat - and body temperature may rise.

If body temperature becomes too high, hyperthermia may result, along with concomitant heat related injuries, such as heat exhaustion and heat stroke. Heat exhaustion is characterized by dizziness, fainting, rapid pulse and cool skin. Heat stroke is a life-threatening medical emergency, symptomized by a high body temperature (above 106o F or 40 o C) and dry skin because sweating has stopped. In some cases of heat stroke, delirium, convulsions and loss of consciousness can occur.

To prevent complications from exercising in the heat, one must understand heat dissipation though radiation and evaporation of perspiration and take precautions during exertion in a hot environment.

At rest, radiation is the primary method of heat loss. Heat loss by radiation occurs by electromagnetic waves when someone is in an environment that is cooler than the body temperature. Therefore, if the ambient temperature is 60o F, and the body temperature is 98.6 o F, the body radiates heat to the environment, including buildings, trees, etc.

Factors that affect heat loss by radiation include body size, mass, body composition, peripheral blood flow, clothing and color of the skin and clothing. A person who is tall and slender has a greater ratio of surface area to mass and thereby can dissipate more heat than a person who is short and stout (even if they are muscular).

For outdoor exercise, particularly in bright sunshine, light colored skin or light clothing fosters radiation because some light waves from the sun are reflected away from light surfaces. Darker colors absorb heat, so wearing dark colored clothing is not recommended for exercise outdoors on a warm day in bright sunlight.

Evaporation of perspiration is the major method of heat loss for humans, especially during exercise. People possess millions of sweat glands located just beneath the skin. When body temperature exceeds its "set-point," sweat begins to form in these glands and is secreted to the surface. As the sweat evaporates into the air, heat is dissipated to the environment. One liter of sweat that evaporates can dissipate about 580 calories of heat, producing a cooling effect.

For cooling to occur, however, sweat must vaporize, or evaporate. This is hindered by the relative humidity (RH) of the ambient air. If the RH is greater than 80 percent, heat loss by evaporation will be minimal because the great amount of moisture in the air does not allow for additional evaporation of sweat from the skin.

When the ambient temperature and humidity approach 90o F and 90 percent relative humidity, exercise sessions should be shortened or include frequent rest periods. Any signs of hyperthermia, such as headache, disorientation, visual distortions and flushed skin, should result in immediate cessation of exercise.

Clothing also plays a role in evaporation. If a person wears plastic, rubber or other manmade fibers, sweating is encouraged. But this type of clothing does not allow moisture to penetrate, so evaporation of perspiration is inhibited - making heat dissipation minimal and potentially leading to overheating. Thus, when exercising outdoors on a hot day, wearing light colored, loose-fitting clothing, such as cotton that will allow moisture to penetrate, is the best choice.

Fluid ingestion is also critical during exercise in the heat, as adequate hydration before and during exercise is one of the best ways to help prevent heat illness. For an average person, sweat loss during exercise may average about one to two liters per hour during moderately heavy exercise on a warm day.

To help prevent dehydration, consuming 400 to 600 ml (13 to 20 ounces) of cold water before exercising in the heat is recommended. During exercise, a person also must drink fluid continuously, especially because voluntary thirst does not keep up with fluid loss. By the time a person feels thirsty, he/she may have a two percent reduction in body weight, which can impair performance and lead to heat illness.

The American College of Sports Medicine recommends approximately 250 ml of fluid (eight and one-half ounces) for every 10 to 15 minutes of activity. The best fluid to offset dehydration is plain water, but any cold fluid (other than diuretics such as alcohol and caffeinated beverages) is beneficial.

Taking salt tablets or substitutes generally is not advised, as most people will replenish salt through consumption of a normal diet.

Exercise can be safely performed on warm - and even hot - days. Certain precautions should be taken, however, including wearing proper attire, performing a sufficient warm-up and cooldown, ensuring adequate fluid intake and reducing the intensity or duration of workouts on extremely hot and humid days.

Robert Girandola, Ph.D., is a professor in the Department of Kinesiology at the University of Southern California in Los Angeles, Calif.