Injury: Heat Exhaustion

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Injury: Heat Exhaustion

Causes and Symptoms
The Future
For more information


Heat exhaustion is a condition in which the body is overwhelmed by exercising or working in a hot environment; it produces more heat than it can get rid of through evaporation of sweat or moving into cooler surroundings. Heat exhaustion is the intermediate form of heat-related illness, heat cramps being the mildest and heat stroke the most serious.


Heat exhaustion is characterized by thirst, headaches, muscle cramps, shortness of breath, and nausea. Most patients with heat exhaustion have a normal level of alertness, although some people become slightly confused or feel anxious.


Heat exhaustion is the most common form of heat-related illness seen by physicians, although the exact number of people affected every year is not known because people can be treated for heat exhaustion outside a hospital or doctor's office. According to the Centers for Disease Control and Prevention (CDC), Arizona has the highest rate of cases of heat exhaustion in the United States.

Heat exhaustion affects people from all races and ethnic groups. It affects males and females equally.

Some groups of people have a greater risk of heat exhaustion:

  • Newborn infants. A baby cannot adjust to changes in temperature as efficiently as an adult can. In addition, babies have only a limited ability to get out of a hot environment.
  • Elderly people. As with infants, the bodies of elderly people do not regulate internal temperature as effectively as those of younger adults. In addition, elderly people may have underlying illnesses that make them more vulnerable to heat stress.
  • Workers whose jobs require working outdoors in hot weather or near ovens, blast furnaces, or other sources of heat.
  • People who are not physically fit or have not undergone conditioning to get their bodies used to work or exercise in the heat.
  • People who take certain types of medications, including diuretics, drugs that regulate blood pressure, tranquilizers, antihistamines, and drugs given to treat people with schizophrenia.
  • Homeless people.
  • Obese people.

Causes and Symptoms

Like heat cramps, heat exhaustion is caused by the loss of water and salt from the body due to sweating during exposure to heat or vigorous physical exercise in hot conditions. High humidity makes it harder for the

body to regulate its internal temperature through sweating, which is its normal way to get rid of heat when the outside temperature is 95°F (35°C) or higher. As sweat evaporates, it carries body heat with it. In addition to losing water through sweating, however, the body also loses electrolytes, which are minerals that are necessary to proper body functioning.

Other factors that can affect the body's ability to regulate its temperature in hot, humid weather include drinking alcohol, which leads to losing more water through the urine, and wearing tight clothes or clothes made of fabrics that do not allow sweat to evaporate easily.

The symptoms of heat exhaustion are more severe than those of heat cramps; they may come on either gradually or suddenly. People suffering from heat exhaustion may feel dizzy and faint as a result of the loss of body fluids and minerals.

  • Skin is hot and moist; the person may develop goose bumps.
  • Body temperature may be normal or a few degrees above normal.
  • Nausea and vomiting.
  • Rapid heartbeat and weak pulse.
  • Blood pressure is low or drops lower if the person tries to stand up.
  • Patient's legs may be swollen.
  • Urine is darker than normal.


In most cases the diagnosis is obvious from the weather conditions and the person's level of activity before feeling ill. People can take care of heat exhaustion themselves by moving into a cooler location; by drinking cool (not cold) water or sports drinks; and by lying down with the legs propped on a pillow or cushion to raise them above heart level.

If the person does not feel better in about half an hour; if they start to lose consciousness; or if their temperature goes above 104°F (40°C), they should be taken to an emergency room as soon as possible.


The most important aspect of treating heat exhaustion is to keep it from getting worse. Untreated heat exhaustion can develop into heat stroke, which is a much more serious condition. In some cases a doctor may give

the patient intravenous fluids if he or she appears to be severely dehydrated and is vomiting or otherwise unable to take fluids by mouth.


Most people recover from heat exhaustion within two to three hours with no long-term effects.


Preventing heat exhaustion is largely a matter of taking time to adjust to hot weather or visiting a hot climate and dressing sensibly for local weather conditions:

  • Most people in temperate climates need time to acclimate, or adjust, to seasonal temperature changes. People should work up gradually to outdoor activities during the first few warm days of summer rather than overdoing. The same is true of visiting a country with a tropical or hot climate; it is best to keep one's activity level moderate for a few days rather than crowding in too many activities. It can take people between seven and fourteen days to adjust to a hot climate; marathon runners generally take two weeks to acclimate to training in the heat.
  • Wear loose-fitting and light-colored clothing; choose fabrics that absorb sweat, such as cotton; wear a hat outdoors.
  • Drink some fluids before exercising or working outside in hot weather. The American College of Sports Medicine recommends drinking about 20 ounces (0.6 liter) of water or sports drink two to three hours prior to exercise, and 10 ounces (0.3 liter) of water or a sports drink ten to twenty minutes before exercise. It is important to not use thirst as a guide to fluid intake; a person can become dehydrated before feeling thirsty enough to want a drink.
  • Use sunscreen generously, as sunburn reduces the body's ability to get rid of excess heat.
  • Avoid caffeinated beverages and alcohol; they cause the body to lose additional fluid through the urine.
  • People who must take prescription medications for allergies, high blood pressure, heart conditions, or certain types of mental disorders should ask their doctor whether any of their medications affect their response to heat.
  • Exercise during the early morning or late evening, when the temperature is cooler and the humidity lower. Workers in occupations that require them to work in hot environments are often encouraged to take rest breaks during periods of hot weather. Some companies also provide rest areas where workers can cool off.
  • Consult a heat stress index like the one printed in the American Council on Exercise fact sheet listed below or the National Weather Service's heat index to help decide whether it is safe to exercise outdoors. There are times when the heat and humidity are so high that exercise should be avoided. Heat exhaustion is likely to occur when the heat stress index (the apparent temperature) is over 105°F (40.5°C).
  • People with elderly friends or relatives should check on them during summer heat waves to make sure that they are in good health. Heat waves that last longer than two days put the elderly at risk of heat exhaustion.

The Future

Heat exhaustion is a common hot-weather health problem or a consequence of exercising or working outdoors without proper conditioning or precautions. It is dangerous only if it progresses to heat stroke, however. Heat exhaustion can be prevented by dressing appropriately for hot weather, drinking enough fluids, consulting the local heat index before outdoor activity, and knowing when to slow down and cool off.


Acclimation: The process of adjusting to seasonal climate changes or to a new climate.

Conditioning: The process of becoming physically fit through a program of diet, exercise, and rest.

Electrolytes: Minerals that are essential for proper body functioning. They include potassium, sodium, calcium, and magnesium.

Heat illness: A general term for heat-related disorders, ranging from heat cramps (the mildest) to heat stroke (the most serious).

SEE ALSO Heat cramps; Heat stroke

For more information


Dvorchak, George. The Pocket First-Aid Field Guide: Treatment and Prevention of Outdoor Emergencies. Accokeek, MD: Stoeger Publishing Company, 2007.

Isaac, Jeff. Outward Bound Wilderness First-Aid Handbook, revised and updated. Guilford, CT: Falcon Guides, 2008.


Comeau, Matthew. “A Hot Issue for Summer Exercisers.” American College of Sports Medicine Fitness Page, Summer 2001, p. 4. Available online in PDF format at (accessed May 2, 2008).


American Council on Exercise (ACE). Fit Facts: Beat the Heat before It Beats You. Available in PDF format at (accessed May 2, 2008). This is a one-page fact sheet on heat stress that contains a useful temperature/humidity index to evaluate the risk of heat exhaustion.

Mayo Clinic. Heat Exhaustion. Available online at (updated February 8, 2008; accessed May 7, 2008).

National Institute for Occupational Safety and Health (NIOSH). Working in Hot Environments. Available online at (updated 1992; accessed May 7, 2008).

National Oceanic and Atmospheric Administration (NOAA), National Weather Service. Heat Safety Heat Index. Available online at (accessed December 9, 2008).

National Weather Service and the American Red Cross. Heat Wave: A Major Summer Killer. Available online at (updated June 8, 2007; accessed May 8, 2008). This is a guide to prevention of and basic first aid for heat-related illness.

Nemours Foundation. Heat Illness. (updated March 2007; accessed May 2, 2008).