For many people — especially those who suffer through cold winters — the onset of summer and hot weather leads to a rush outdoors for activity and exercise. Get active by all means, but be aware of the dangers that come with hot weather exercise.

You should begin taking hot weather precautions whenever the temperature rises above 80° Fahrenheit. High humidity (above 75% to 80%) can also contribute to overheating, so be especially careful when it is hot and humid. Precautions are especially important for the very young and the very old, as well as for anyone with high blood pressure, coronary artery disease or any other kind of cardiovascular disorder.

The most general dangers from heat are dehydration and overheating. Water lost through perspiring comes directly out of your bloodstream. If the water isn't replaced, you can develop heat exhaustion — a condition marked by fatigue, dizziness, pale skin and shortness of breath. Heat exhaustion can occur in as little as 30 minutes if you're sweating heavily.

The solution: Stay well hydrated at all times. Drink at least a pint of water before your workout, and continue drinking water during and after your exercise session. On hot days, drink a little even if you're not feeling thirsty.

Heat stroke is a less common but even more serious danger. Heat stroke occurs when you generate so much heat that your body can no longer release it. Your body's overloaded cooling mechanism shuts down, causing body temperature to skyrocket. Heat stroke can develop in just a few minutes if you're exercising intensely in hot weather, even when you're well hydrated. The usual treatment for heat stroke is putting the person in a tub of cool water. If not treated immediately, heat stroke can be fatal. Anyone who becomes disoriented or faints while exercising should be taken to a hospital.

The solution: Be extremely careful when doing any intense exercise in weather over 80° Fahrenheit or 90% humidity. In a hot, humid climate, even a three-mile running race can boost body temperatures to dangerous levels. Keep your efforts comfortable and steady.

• To tell whether you're dehydrated, check the color of your urine. It should be pale yellow. If it's darker, you need to drink at least a pint of water.
• Work out in the early morning or late evening when temperatures have dropped.
• Don't judge your hydration level by how much you appear to be perspiring. In a dry climate, you'll actually sweat up to 10% more than in a humid climate, even though you seem to be sweating less. (Reason: the sweat evaporates much faster in dry weather.)
• In hot weather, wear light-colored, porous clothing that allows sweat to evaporate freely. Full evaporation requires wet clothing, so avoid changing into a dry shirt.
• Drink often. When exercising or competing for more than an hour, use sports drinks in addition to water. If you prefer a fruit juice instead, dilute it with water and a pinch of salt.
• If you move from a cool environment to a hotter one, allow your body several weeks to adapt.
• If the temperature and humidity are too high, move the workout inside.

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