If you've been jogging, playing tennis, biking or working for long stretches in the yard in this abominable combination of heat and humidity, you've been pushing your luck.

Heat prostration is not something that strikes you down as though you've been hit with a bolt of lightning. Instead, it starts as that slight nausea, dizziness, headache and shortness of breath you may have been experiencing after the jogging session to improve your health.

Odly enough the better shape you're in and the more you normally exercise, the more dangerous your exercising may be in this weather.

Dr. Jerry Meyer, assistant director of the coronary care unit at the George Washington University Hospital, explained that "if you're working hard you can lose as much as four liters (almost four quarts) of fluid an hour. If you're in better shape you sweat better."

The person who is "good at sweating," has to be especially careful to replace the fluids and salts lost through the perspiration process in order to maintain normal body function and avoid heat prostration or, even worse, heat stroke.

Heat prostration is "a fairly common reaction one sees in the normal, or debilitated, person with hrat exposure" explained Meyer, an associate professor of medicine at GW Medical School. "One experiences rapid heart beat, dizziness, fainting, nausea, headache. It's an extension of the normal hot weather complaints."

The symptoms are caused "by a number of reactions by the body to heat," said Meyer. "The dilation of the blood vessels causes a lowering in blood pressure and the heat causes a loss of body fluids.

"If you feel weakness, dizziness, headache or muscle cramps, it probably has something to do with loss of sodium chloride, or salt. It may be a warning that prostration is coming on."

The loss of salt can eventually throw the body's electrical system out of balance and could lead to heart failure if ignored.

The best treatment for the early stages of heart prostration is the most obvious! Try to stay cool.

"A lot of people go to the swimming pool and just laze around in the sun," said Meyer. "They should drink water and go into the pool to cool off. It's a misconception that people have that if they're hot they shouldn't put cold water on themselves."

The name of the game is lowering the body's temperature, said Meyer, and nothing does that more effectively than a quick dip in the pool.

"A hat certainly is helpful, particularly if it has a wide brim," he said. "What you want to do is keep the sun off the major part of your body. If the head is kept cool it's more likely to regulate your temperature better."

Covering the head, and therefore, the body's thermostat, is much akin to locating a home thermostat away from direct sunlight. If the thermostat is in the sunlight the heart on the thermostat forces the home air conditioning to work harder than necessary to cool off the house.

Heat or sun stroke is a more serious condition than heart prostration, and is far more rare. "It's usually seen in somewhat debilitated people or old people in a house that isn't air conditioned," said Meyer.

"The body just quits trying to bring the temperature down," he contrinued. "The skin is no longer sweaty - it's dry. The body temperature can go up to 108 or even 112. You have the headache, dizziness and nausea, like heat prostration, but then the effects" of the overheating are such that one looses consciousness, can become comatose and die.

Although one can still see die-hard joggers and others exercising outside, Meyer and other physicians believe most Washington area residents, particularly those with respiratory and heart conditions, have been taking seriously all the warnings to avoid the heart.

"It seems like most of the people in Washington have been able to stay in the cool enough and not get involved in this heart," said Meyer, commenting on the fact that there does not appear to have been any appreciable increase in local hospital admissions due to the heat.