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Exercising in the heat

Stephen D. Ball, Ph.D., state specialist & associate professor, Nutrition and Exercise Physiology, College of Human Environmental Sciences, University of Missouri Extension


The Science Behind Overheating

In recent years, there have been several well-documented cases of highly conditioned athletes dying unexpectedly while exercising in the heat. Why? At rest the body is able to balance heat production and heat loss so that its temperature remains around 98o F. However, during exercise body temperature begins to rise as it converts the chemical energy from food into the mechanical energy necessary for movement. The degree of increase is directly proportional to the intensity of the exercise bout. In other words, the harder you are exercising, the higher your body temperature becomes.


When you exercise in average indoor or outdoor conditions, the body is usually able to adjust quite well with this increase in body temperature. However, during hot conditions the body loses its ability to radiate heat from its surface. In fact, when the air temperature exceeds body temperature (above 98o F), the body actually gains heat from the environment. In this case, evaporation, or the conversion of a liquid (sweat) to a gas, becomes the body's only legitimate defense against overheating. Millions of sweat glands on the surface of the skin secrete large amounts of liquid, which when evaporated, help to cool the skin, which in turn cools the blood and ultimately the body. Serious problems can arise because high air temperatures stimulate excessively large amounts of sweat production, which if not replaced, can lead to a dehydrated state. Dehydration leads to higher body temperature. Severely dehydrated individuals can suffer circulatory collapse and death can occur.

Although air temperature is a key factor in determining your risk for overheating, the humidity level, or the water content of air, is equally important. Several football deaths have occurred with air temperatures below 75o F at 95% humidity levels. High humidity levels reduce the evaporation of sweat from the skin. Even though it may seem you are sweating a lot when it is humid, the sweat is mostly rolling off the skin and is not really contributing much to the cooling process. Sweat alone does not cool the body. It is the evaporation of sweat that cools the body. Obviously, the combination of high air temperatures and high humidity levels increases the risk for heat illness to occur.

Heat Cramps, Heat Exhaustion, Heat Stroke

There are several incremental stages of heat illness, which if not dealt with immediately, can lead to medical complication and even death.

Heat cramps:
Heat cramps are involuntary muscle spasms that can occur during or following physical exertion, which generally result from an electrolyte imbalance due to excessive loss of salts. Direct pressure, massage, and simple stretching of the cramped muscle should help alleviate the cramp. This is the first sign of heat illness and is a signal to drink more water and/or reduce the amount of intensity of physical exertion.

Heat exhaustion:
Heat exhaustion is a more serious state of heat illness that usually occurs among a person who has not gotten used to exercising in the heat. Heat exhaustion also can occur in unacclimatized individuals especially if they have not consumed enough water prior to or during their workout. Heat exhaustion may or may not include collapse with or without consciousness. Symptoms include excessive sweating, cold clammy skin, normal or slightly elevated body temperature, paleness, dizziness, weak and rapid pulse, shallow breathing, nausea, and head ache. Anyone experiencing these symptoms should immediately stop exercising and get into the shade, or better yet, get inside an air-conditioned building. An abundant amount of fluid should be consumed. If the situation seems severe enough, elevate the feet and seek medical help.

Heat stroke:
The most advanced stage of heat illness is called heat stroke. In simple terms, heat stroke is a break down of the cooling mechanisms of the body induced by high body temperature. Someone who is experiencing heat stroke may stop sweating altogether and the skin will appear dry and hot. The pulse is usually strong but very rapid. The individual may have labored and difficult breathing. Body temperature can exceed 106o F. Heat stroke is an emergency that requires immediate medical attention! Transport the individual to the hospital as quickly as possible, ideally in wet sheets. In the meantime, cool the person by any means possible (hose down, ice packs, complete body submersion in cold or icy water). Remove as much clothing as possible and treat for shock by elevating the feet slightly.

Avoiding Heat Illness:
The most effective way to manage heat illness is to prevent it. The following guidelines offer simple steps for the prevention of heat related injuries:

  • Recognize that any form of physical exertion, not just exercise, can result in heat related problems.
  • Recognize that unfit individuals are more likely to suffer a heat related illness.
  • Recognize when temperature or humidity conditions might require you to decrease the intensity of your physical activity.
  • Before strenuous physical activity drink plenty of water and other fluids.
  • Drink more fluids during the activity than your thirst is dictating. It is very difficult to drink too much.
  • Drink plenty of fluids after the workout.
  • As outside temperatures and humidity increase gradually decrease your physical activity in the short term and then increase your activity back to the previous level over 7-10 days. In other words, take it easy the first several times you are exposed to a hot environment before increasing your intensity level.
  • Monitor your heart rate to make sure you are in your target heart rate zone.
  • Monitor your weight before and after physical activity. If you have lost more than 3% of total body weight prior to the next workout, skip the workout entirely. For example, 3% of 120 pounds is about 3.5 pounds and 3% of 175 pounds is about 5 pounds.
  • Consume adequate amounts of carbohydrates, especially fruits and vegetables, as they contain high amounts of water.
  • Expose as much skin as possible to increase cooling by evaporation of sweat. Don't forget to wear sun block!
  • Plan your physical activities for the cooler parts of the day.
  • Recognize the early warning signs of heat illness such as heat cramps, excessive sweating, cold clammy skin, normal or slightly elevated body temperature, paleness, dizziness, weak and rapid pulse, shallow breathing, nausea, and headache etc.


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Last update: Wednesday, April 25, 2018