No one who watched Gabriella Anderson-Schweiss stumble spasmodically across the finish line of the women's marathon in last year's Summer Olympics is likely to forget the disastrous effects of heat and exertion on the human body -- even a well-trained one.

And it doesn't take an Olympic marathon to bring the point home. With its infamous humidity -- June through September averages are 54 percent humidity at 7 a.m. and almost 78 percent at 1 p.m. -- Washington weather puts a serious damper on the body's attempt to cool itself.

Even the pros have a tough time keeping a warm-blooded body cool.

When the Redskins start training camp next Wednesday, even in Carlisle, Pa., where "there is a bit of an afternoon breeze," says trainer Keoki Kamau, "it's just a terrible time of year for us. It usually takes the players a good solid week to acclimate. They have 25 pounds of equipment and sometimes 100 to 106 degree temperatures."

Kamau still remembers his first summer camp in 1977, when "it was a very hot day, and Coach George Allen just kept them out there. He never really allowed water on the field. We had eight or nine guys" -- including old pros like Billy Kilmer -- who couldn't finish practice and needed hospitalization.

So it's no surprise that heatstroke is second to head and spine injuries as cause of death among young athletes in the United States.

Because at 98.6 degrees Fahrenheit, we are on the 'high side' of the body's temperature range -- body temperature can go as low as 85 degrees and as high as 105. "We are but a few degrees away from risking death from hyperthermia," says Dave Martin, a physiologist formerly with the U.S. Olympic Committee Sports Medicine Council.

Exercising during hot, humid weather can cause problems for almost any kind of aerobic activity such as running, tennis, cycling and soccer. The risk of heat problems increases with activities done for long periods of time or at high energy levels. The faster and longer your body exerts itself, the more heat it has to deal with, and the faster it has to try to cool it.

Heat tolerance varies from person to person. Some genetic factors result in certain individuals having lower heat tolerance, says Capt. Lawrence Armstrong, research physiologist at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. These include: age (some studies indicate that people under 15 and over 40 have lower tolerance), heart disease and inherited skin diseases. Another factor affecting heat tolerance is the amount of oxygen your muscles can burn in the production of energy -- called low maximal oxygen consumption. A well-conditioned muscle burns more oxygen than a poorly conditioned muscle.

Though no scientific studies confirm it, Armstrong says many anecdotal reports suggest that drinking too much alcohol the night before and having too little sleep may also decrease heat tolerance.

With more than 2.5 million sweat glands on the forehead, palms, soles and armpits, the body's well-designed sprinkler system works by pumping a solution of sodium chloride to the skin, where evaporation works to cool the body.

Untrained women sweat less than untrained men, but athletic women produce the same amount of sweat proportionate to body weight as athletic men, show studies by Christine Wells exercise physiologist at Arizona State University. But even athletes who are heat acclimated -- most take four to 10 days to adjust -- and who generally sweat more have to be cautious.

Armstrong found that marathon winner Alberto Salazar has one of the highest sweat rates ever recorded -- three quarts an hour. (The average fluid loss in a moderate workout is one to two pints an hour; four to five pints an hour is considered extreme.) During the Olympic marathon Salazar lost 12 pounds -- one pound of water every 11 minutes, for an 8 percent loss of body weight, considered dangerous for most less-conditioned bodies.

"Sweating is a benefit to a point," Armstrong says. But after that point, "what you're producing is ineffective sweat. When it drips off the body, it can't help you cool."

Most overheating problems can be prevented with attention to risk factors, acclimitization, monitoring the body's signals, and -- it can't be said enough -- water, water, water.

To acclimate to hot weather, Armstrong advises against wearing extra clothing -- cotton or nylon sweats -- a technique popular with some runners who are training in the spring for summer races. This practice does not allow the body to thermoregulate -- or cool off -- he says. To prepare for hot weather activity ahead of time, he suggests exercising in a warm room. During hot weather, slowly increase your exercise time to allow the body to adapt.

While the body does lose salt, most experts agree that it can easily be replenished with light salting of meals. Armstrong advises against salt tablets, which may "cause nausea and stomach distress in many runners."

Cool air may help the evaporation process. Trainer Dave Martin turns on a fan while doing treadmill stress testing. While cool water showers may be psychologically uplifting, research indicates that they do little to lower internal temperature.

While stopping for water in many activities used to seen as a "weakness," increased attention to the body's cooling mechanisms have focused on encouraging water even when you don't feel like it. Because of a delay in the thirst mechanism, by the time you feel thirsty, it may be too late.

"Ever since Coach [Joe] Gibbs has been here, he's been very intelligent about encouraging water," says Redskins trainer Kamau. "We just keep hydrating the players -- a lot of fluids while they're on the practice field -- and we also encourage a lot of fluids off the field. You want to keep that body cool at all times."