Guidelines for exercising in heat and humidity have been developed for road races, but these can be applied to any strenuous physical activity performed outdoors during warm weather. Ambient conditions are considered safe when the temperature is below 70° F and the humidity is below 60%. Caution should be used when the temperature is greater than 80° F or the humidity is over 60%. People who are sensitive to heat and humidity should reconsider exercising when the temperature is greater than 80° F or the humidity is more than 60%. People who are trained and heat acclimated can continue to exercise in these conditions, but they should be aware of the potential hazards and take precautions to prevent heat illness.

The keys to exercising without incident in hot weather are to acclimate to the heat and maintain the body’s normal fluid level. The major consequence of dehydration (excessive fluid loss) is a reduction in blood volume. This results in sluggish circulation that decreases the delivery of oxygen to the exercising muscles. second, lowered blood volume results in less blood that can be sent to the skin to remove the heat generated by exercise. If too much of the blood volume is lost, sweating will stop and the body temperature will rise, leading to heat-stress illness. Heat illness is a serious problem that can be avoided by following a few guidelines designed to preserve the body’s fluid level.

Preexercise fluid ingestion drink 12 to 20 ounces of a noncarbonated, lightly salted liquid 15 to 30 minutes before exercising. You can purchase commercially prepared sports drinks, such as Gatorade, 10-K, All Sport, and so on, or mix a teaspoon of salt in a gallon of lemonade. Water is not the best liquid source because it stimulates the production of urine, leaving less liquid for sweating.

Fluid replacement during exercise. The primary reasons for drinking during exercise are to maintain body water stores so that sweating can continue and to maintain blood volume.

Water is the preferred drink when exercise lasts less than 2 hours. Water exits the stomach rapidly and moves to the tissues that need it.

Urine production is not a problem during exercise because fluid is used to produce sweat.

A noncarbonated beverage containing salt and sugar is preferred if exercise lasts longer than 2 hours (for example, during marathons, long distance cycling, and ultra distance running events and triathalons).

You should drink 6 to 8 ounces every 15 minutes during exercise.

Postexercise fluid replacement

Plain water is a poor drink during recovery from exercise because it suppresses the thirst drive, so fluid intake stops before rehydration is complete.

Carbonated drinks are filling, prematurely blunt the thirst drive, and do not effectively contribute to recovery.

You should avoid alcoholic beverages and caffeinated beverages because these stimulate urine production. All ingested fluids need to be retained for the purpose of rehydration.

You should drink fluids that contain salt and sugar. Again commercial sports drinks are appropriate. In addition, they taste good, which encourages exercisers to drink more. This counteracts the tendency of most people to drink less than they need if water is the alternative.

Estimating water loss

Weigh yourself nude before and after exercise.

Towel off sweat completely after exercise and then weigh yourself.

Each pound of weight loss represents about 1 pint of fluid loss. Be sure to drink that and more after exercise.

Modify the exercise program by

Working out during cooler times of day.

Choosing shady routes where water is available.

Slowing the pace and/or shortening the duration of exercise on particularly oppressive days.

Wearing light, loose, porous clothing to facilitate the evaporation of sweat.

Other considerations

Never take salt tablets. They are stomach irritants, they attract fluid to the gut, they sometimes pass through the digestive system undissolved, and they may perforate the stomach lining.

Exercise must be prolonged, produce profuse sweating, and occur over a number of consecutive days to reduce potassium stores. For the average bout of exercise, you do not need to worry about depleting potassium or make a special effort to replace it. The daily consumption of fresh fruits and vegetables, as suggested by the new food pyramid, is all that is needed.

Remember to use a sunscreen lotion when the weather is sunny or hazy. Be sure that the sunscreen you select has a “sun protection factor” (SPF) of at least 15, and apply it liberally over exposed skin.