Health A-Z

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What Is It?

The human body usually can regulate its temperature. When the body gets too hot, it uses several strategies to cool down, including sweating. But if a person spends too much time in the heat without taking in enough fluids, the body's cooling processes can't work properly. When the body becomes dehydrated, it can no longer cool itself by sweating. When this happens, body temperature can rise high enough to make the person sick.

The first symptoms of heat illness occur as the body temperature climbs above normal, and can include headache, nausea, vomiting, muscle cramps and fatigue. These early symptoms sometimes are called heat exhaustion. If steps are not taken to reduce body temperature, heat exhaustion can worsen and become heat stroke.

Heat stroke is a serious, potentially life-threatening form of heat illness. The body temperature rises to 105 degrees Fahrenheit or higher and you develop neurological changes, such as mental confusion or unconsciousness. At these high temperatures, body proteins and the membranes around the cells in the body, especially in the brain, begin to be destroyed or malfunction. The extreme heat can affect internal organs, causing breakdown of the heart muscle cells and blood vessels, damage to internal organs, and death. There are two main causes of heat stroke:

  • Exertional heat stroke occurs when someone is vigorously active in a hot environment, such as playing sports on a hot summer day or participating in military training activities. It typically strikes young, otherwise healthy people, those least likely to be concerned about the effects of heat on their health. Because of the lack of concern, early symptoms may be dismissed or ignored.

  • Nonexertional heat stroke tends to occur in people who have a diminished ability to regulate body temperature, such as older people, very young children or people with chronic illnesses. High heat in the surrounding environment, without vigorous activity, can be enough to cause heat stroke in these people.

Factors that can contribute to heat stroke include:

  • Dehydration from not drinking enough water

  • Wearing bulky or heavy clothing, such as firefighting gear, in the heat

  • Being overweight, which causes the body to generate more heat and reduces the body's ability to cool down

  • Sleep deprivation, which can decrease the rate of sweating

  • Being unaccustomed to the heat, such as moving from a cooler climate to a warmer climate

  • Some medications, most commonly antihistamines (taken for allergies), diuretics (taken for high blood pressure or leg swelling), laxatives (taken to relieve constipation), calcium channel blockers (one type of blood pressure or heart medicine), medicines for Parkinson's disease, some diarrhea treatments and tricyclic antidepressants

  • Being confined to a poorly ventilated or non-air-conditioned living space

  • Having had heat stroke in the past

  • Use of illicit drugs, including cocaine, heroin, amphetamines and ecstasy (MDMA)

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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