The Medical Minute: Heat stress

July 08, 2008

By John Messmer

Even without the threat of global warming, summer can pose risks from the heat. Part of the problem with living in a temperate zone is we forget how to deal with the heat until it’s upon us in full force.

The basic problem in heat related injury is dehydration. The main way we stay cool is through perspiration. As sweat evaporates, it carries heat away from our skin. If we lose more water than we replace, we cannot produce enough sweat to cool off. Lose too much water and our blood pressure drops and the sweat glands stop working at all, leading to increased body temperature and potentially to death.

High humidity and tightly fitted clothing slow the evaporation of sweat. Direct sunlight warms us further. Muscular activity such as sports or yard work produces even more heat. Combine physical activity with a sunny, humid day and the risk of heat injury is very high.

The most important part of preventing heat stress injury is drinking adequate liquids. Two to four cups of liquid per hour is a good place to start; more for larger people or hotter, more humid days. As long as a person continues to make dilute urine about every three to four hours, hydration is adequate.

Water is the best fluid. Sports drinks do not offer the average person any advantage. Other beverages, such as, soda or iced tea are fine. Although caffeine slightly increases fluid loss, the effect is negligible for most beverages as long as the volume consumed is adequate. Alcoholic beverages are not recommended because the volumes needed are more than should be consumed in one setting and too much alcohol can affect one’s ability to judge the safety of the environmental heat. The average person does not need extra salt, just a balanced diet.

Some medications can affect heat loss. A rule of thumb is if it causes dry mouth or dry eyes, it also may cut down on sweating. Those on that kind of medication should check with their physicians about their fitness to work or play in the heat.

Light colored, loosely fitting clothing reduces the amount of heat that sunlight contributes and allows air to circulate, improving evaporation of sweat. Bare skin may improve evaporation, but it increases the risk of sunburn and sun damage. It’s also a good idea to avoid the mid-day hours when it’s hottest and the sun is strongest.

Early detection and treatment are keys in battling heat-related injury. The first signs of trouble are usually leg and abdominal muscle cramps, often accompanied by weakness. If this happens, get to a cool place – inside or in shade – and drink cool liquids. Do not return to the heat until fully recovered, Better yet, consider waiting until the next day.

Heat exhaustion is the second stage. Low fever develops, and the person is dehydrated, weak and sometimes confused, nauseous and anxious. When this situation occurs, it's vital to get the person to a cool area, preferably indoors in air-conditioning. Keep the skin moist and run a fan to aid evaporation or apply cold towels and offer cold liquids. If recovery does not begin in an hour or if the person is confused or lethargic, call 911.

Heat stroke is the final and potentially fatal stage. Body temperature is high, as much as 104 degrees, and typically the skin is hot and dry. Although sweating usually stops at this point, the person may have slightly damp skin. The victim often is delirious or even unconscious because he or she is in shock and requires emergency treatment. Call 911, then apply ice packs to the victim's armpits and groin, because cooling the body is critical. Offer fluids only if the person is awake. Intravenous fluid usually is needed.

Children do not lose heat as effectively as adults and are more susceptible to heat injury at temperatures when healthy adults may be fine. Older people may not feel the heat as well and do not sweat as efficiently as younger people. This combined with medical problems and medications that can affect sweating are the reason more than half of all heat-related deaths occur in this age group. Alcohol abuse, sunburn, eating disorders, fatigue, obesity, thyroid disease, uncontrolled diabetes and upper respiratory infections all increase the risk of developing a heat-related illness.

Summer can be fun with picnics, sports and the beach. Whether it’s fun or work that puts us outside in the heat, remember to dress for the weather and drink fluids often.

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John Messmer is associate professor of family and community medicine at Penn State's College of Medicine and a staff physician at Penn State Hershey Medical Center.

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Last Updated July 22, 2015