Exercising in Washington's hot weather can be hazardous.The combination of summer heat, polluted air and heavy exercise puts extreme demands on the body's cooling system and unless it's ready for the thermal load the body, like New York City's power system, can collapse.

Here, gatered from an interview with a specialist exercise and physiology, from articles on marathon racing and from publications on heat stress research financed by The National Institutes of Health, are tips for hot-weather workouts.

The best defense against plain heat is water. The most effective tactic against polluted air is evasion.

If the temperature is 90 and the air is clean, a jogger should increase his fluid intake and go about business as usual. But if the air is thick with pollution, a jogger should run only when the air is at its cleanest - usually in the morning before rush hours. For night owls, pollution starts dropping at about 7 when the sunlight grows less intense and the traffic eases, but the change is gradual and the wise jogger waits until after 9.

Put simply, the advice of researchers to exercisers fighting the heat is four-fold: drink, sweat, sponge off and strip.

"In hot weather you want to rehydrate, to drink as much fluid as you lose," said Bryant Stamford, an associate professor at the University of Louisville. Stamford studied the relationship of heat and exercise at the University of Pittsburgh, where he received his doctorate, and at Louisville, where he is director of the university's exercise physiology laboratory. Perspiring uses up the body's supply of fluids.

Drinking large amounts of water replenishes the supply.

"The trouble is, most people don't drink enough water," Stamford said. "They quench their thirst before they quench their body's need for fluids."

In the heat of exercise, such as playing tennis on a humid afternoon, the body loses at least a quart of fluid per hour, but the exerciser has trouble absorbing that much water in an hour. It is a good idea, then, to sip water while you exercise, as well as before and after the exercise session.

Stamford said that during the recent wave of 90-degree days he drank 16 ounces of water before running his daily five miles. During the rest of the day he deliberately drank more.

Football coaches who restrict the amount of water their players are allowed to drink are "medieval," according to Stamford.

There are several ways to figure out how much water you should drink. One is to accept the challenge of drinking a quart per hour. Another is weight yourself before and after exercise. You shouldn't lose more than three per cent of your body weight. An afternoon of cycling shouldn't take more than 4 1/2 pounds off a 150-pound body.

Understanding the secrets of perspiration - the body's cooling system - requires a brief look at the body's responses to temperature. One function of blood is to carry heat from the core of the body to the skin. Here air currents sweep some of the heat away and colder objects nearby attract the rest.

But when the temperature gets near 90 and the humidity is high, the heat transfer is reversed. Skin picks up heat from hotter objects nearby.

When the body isn't perspiring properly, blood fails to circulate well and blood-starved organs - brain, kidney, liver and muscles - suffer. In mild cases this condition is called heat exhaustion; in more extreme cases, heat stroke.

Even on days when the temperature doesn't reach 90, Washington's humidity can cause problems. When humidity is high, perspiration won't evaporate and a layer of it sits on the body, slowing the body's cooling process.Sponging the body with water removes the layer of perspiration, cools the skin and gets the body perspiring efficiently again.

Clothing can hinder effective perspiration. A wet tee shirt, for example, captures a layer of air next to the body and acts as insulation, trapping the heat. To remedy that situation marathon runners wear fishnet shirts. Stamford suggested stripping to "as little clothing as possible."

Taking salt tablets, a practice once widely recommended for heat stress, has lost popularity.

The tablets replace the body's salt supply. Perspiration draws salt from the body and without salt the kidneys malfunction. But unless salt tablets are taken with the correct amount of water (approximately a quart of water for three grams of salt) the salt will remove water from the body.

"I won't recommend taking salt tablets unless you lose about eight pints of sweat. That would mean you'd lose about eight pounds, Stamford said. "Americans have so much salt in their diets their bodies rarely run out of it."

Finally, researchers said that exercising in the shade makes a difference. In hot weather the body has to cope with its own metabolic heat, the heat of environment and the direct heat of sun. When you exercise in the shade, obviously, you avoid one heat source.

Dr. Alfred Munzer, respiratory specialist at Washington Adventist Hospital and member of the board of the Metropolitan Washington Coalition for Clean Air, Inc., suggested two precautions for exercisers when air is bad.

Avoid rush hours when ozone levels are high. When ozone gets into the lungs it irritates air passages, causing them to constrict. Ozone also inhibits two of the body's defenses against disease - the cilia or hairs that line the lungs, and the macrophages, scavenger cells that destroy foreign bacteria.

Avoid roadways and inhaling exhaust fumes. When carbon monoxide gets into the lungs less oxygen gets into the blood and to the muscles.

"I have very, very mixed feelings about building bikeways next to roadways," said Munzer, a cyclist. He said masks worn by many cyclists may strain particles from the air, but he doubts they help against carbon monoxide.

If you can't do your exercising when air polution is at its lower levels, Munzer said, do less - one set of tennis instead of two.