Sweating cools the body but also causes dehydration. Sweat that evaporates from the skin is more effective at cooling than sweat that drips off the body.

Drinking while you exercise does not replace all of the water lost in sweat quickly enough. It is important to increase your fluid intake before you exercise and continue to drink after you finish exercising in hot weather. It is easy to become dehydrated during exercise, which increases the risk for a heat-related illness.

You can help prevent dehydration and a heat-related illness. Drink plenty of water before, during, and after you are active. This is very important when it’s hot out and when you do intense exercise.

• Drink on schedule. Two hours before exercising, drink 24 fl oz (750 mL) of fluid. Drink 16 fl oz (500 mL) of fluid 15 minutes before exercising. Continue drinking 8 fl oz (250 mL) of fluid every 15 minutes while exercising.
• Drink rehydration drinks or sports drinks which are absorbed as quickly as water but also replace sugar, sodium, and other nutrients. Eat fruits and vegetables to replace nutrients.
• Watch your weight while exercising. Drink 16 fl oz (500 mL) for every 1 lb (0.5 kg) lost.
• Check your urine. Urine should be clear to pale yellow, and there should be a large amount if you are drinking adequately. You should urinate every 2 to 4 hours during an activity when you are staying properly hydrated. If your urine output decreases, drink more fluids.
• Do not spend much time in the sun. If possible, exercise or work outside during the cooler times of the day. Wear lightweight, light-colored, loose-fitting clothing in hot weather, so your skin can cool through evaporation. Wear a wide-brimmed hat or use an umbrella for shade.
• Stay cool when possible. Take frequent breaks in the shade, by a fan, or in air-conditioning. Cool your skin by spraying water over your body. Take a cool bath or shower 1 to 2 times a day in hot weather.
• Do not drink caffeine or alcohol—they increase blood flow to the skin and increase your risk of dehydration.

Do not use salt tablets, which are absorbed slowly and can cause irritation of the stomach. Salt tablets do not replace water loss.