Heat illness is one of the most serious conditions facing people who exercise in hot weather--and in extreme cases it can be fatal. Even spectators at athletic events aren't immune. So if you're going to be out in the heat, remember to seek SHADE, an acronym for these important heat-safety strategies:

* Sun protection. The sun is typically strongest at midday, so try to schedule exercise for the early morning or late afternoon. But even at those times, be sure to wear sunscreen and, if your sport permits, don sunglasses and a light-colored hat with a wide brim.

* Hydration. Have water readily available and drink small amounts at frequent intervals. Guidelines from the American College of Sports Medicine advise exercisers to drink about 17 ounces of fluid two hours before exercise, five to 12 ounces every 15 to 20 minutes during activity and more after finishing the workout.

* Acclimation. Most people don't even notice acclimation, the subtle process by which the body changes its physiological responses to adapt to heat. But for a dramatic example, taste your sweat early in the spring and see how salty it is. Then taste it again in mid-August and you'll find that it's much more diluted. The difference is due to changes in the sweating process that occurs during acclimation, which happens naturally when you're active in the heat and takes about 10 days to two weeks to complete. If your body isn't acclimated to hot weather, you're at higher risk for heat injury. This is why most heat illness occurs in the first days of training. To acclimate, exercise in the heat for shorter periods and/or at a lower intensity, gradually increasing time and intensity as you begin to feel comfortable.

* Dress properly. Wear clothing that is lightweight, light-colored and porous--such as comfortable shorts and a thin, loose cotton top--to allow evaporation. Avoid taping, padding and nonessential equipment such as bandannas and gloves. If a helmet is necessary, remove it during breaks.

* Environmental check. You've heard that "it's not just the heat, it's the humidity." This is especially important when it comes to exercise, since high humidity can hamper the body's ability to cool itself by evaporation. That's why a very humid but mildly warm day can be more dangerous for exercisers than a very hot, dry one. As a rule of thumb, "any time the humidity added to the temperature in degrees Fahrenheit equals or exceeds 150, racing and vigorous outdoor physical activity should be avoided by people of any age, but particularly children," advises Kenneth H. Cooper in his book "Kid Fitness" (Bantam, 1991).

Children and the elderly are at greater risk of heat injury as are people with certain conditions, such as obesity and diabetes, and those taking certain medications.

Heat illness ranges from the mild condition of heat cramps to the life-threatening emergency of heatstroke. Early symptoms of heat injury include clumsiness, stumbling, headache, nausea, dizziness, excessive sweating or cessation of sweating. If you suspect that someone is suffering from heat illness, have him stop activity and lie down in a cool, shaded place and drink water. If this doesn't relieve the symptoms or if you suspect heatstroke--which is characterized by mental confusion and hot, dry skin--seek medical help immediately.