Merck Manual

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David Tanen

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Jul 2019| Content last modified Jul 2019
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Heatstroke is a life-threatening condition that results in very high body temperature and malfunction of many organ systems.

  • Heatstroke can develop after hours of exertion in young athletes or after days of hot weather in rooms without air-conditioning in very old people.

  • Body temperature is higher than 104° F (40° C), and the brain malfunctions.

  • People should be cooled immediately.

Heatstroke is the most severe form of heat-induced illness. People with heatstroke are much sicker than people with other heat disorders. The following features in particular distinguish heatstroke from other heat disorders:

  • Body temperature is usually higher than 104° F (40° C)

  • Symptoms of brain malfunction develop

Heatstroke may occur fairly rapidly when people exert themselves in extreme heat or are in a closed, hot environment. For example, heatstroke can develop in young, healthy athletes and soldiers, particularly those who are not acclimatized, after only hours of intense exertion in hot, humid weather. Workers in hot environments, particularly those, such as firefighters and foundry workers, who must wear heavy protective clothing are at similar risk. Heatstroke is a common cause of death in athletes.

Heatstroke can also develop over days of hot weather when people, particularly older sedentary people, stay in rooms that are poorly ventilated and not air-conditioned. Older people, people who have certain medical conditions (such as those that involve malfunction of the heart, lungs, kidneys, or liver), and young children are most vulnerable to heatstroke. It can occur rapidly in children left in a hot car.

Heatstroke occurs because the body cannot lose heat rapidly enough in conditions of extreme heat. Because the body cannot cool itself, body temperature continues to rise rapidly to dangerously high levels. Conditions that interfere with heat loss, including certain skin disorders and drugs that decrease sweating, increase the risk.

Heatstroke can temporarily or permanently damage vital organs, such as the heart, lungs, kidneys, liver, and brain. The higher the temperature, especially when higher than 106° F (41° C), the more rapidly problems develop. Death may occur.


Dizziness, light-headedness, weakness, clumsiness and poor coordination, fatigue, headache, blurred vision, muscle aches, nausea, and vomiting (which are also symptoms of heat exhaustion) are common warning symptoms. Affected people do not sense that body temperature is greatly elevated.

During heatstroke, the skin becomes hot, flushed, and sometimes dry. Sweating may or may not occur, despite the heat.

Because of brain malfunction, people may become confused and disoriented and may have seizures or go into a coma. The heart rate and breathing rate increase. The pulse rate is usually rapid. The blood pressure may be high or low.

Body temperature usually exceeds 104° F (40° C) and may be so high that it exceeds the markings on a typical thermometer.


  • Symptoms and a history of exposure to high heat and humidity

The diagnosis is usually evident. People have a high fever, symptoms of brain malfunction, and a history of being exposed to high heat and humidity.

If the diagnosis is not evident, tests are done for other disorders that can cause similar symptoms, such as infections, stroke, drug use, and an overactive thyroid gland (hyperthyroidism).


The risk of death from heatstroke depends on the following factors:

  • How old adults are

  • How young children are

  • How severe any medical conditions (such as heart, lung, kidney, or liver disorders) are

  • What the highest body temperature is

  • How long body temperature remains extremely high

Without prompt treatment, about 80% of people will die. In about 20% of people who survive, the brain may not fully recover, leaving a person with personality changes, clumsiness, or poor coordination. In some people, the kidneys do not fully recover.

After recovery, body temperature may fluctuate abnormally for weeks.


  • Cold water immersion

  • Evaporative cooling measures

  • Sometimes cooled fluids given intravenously

The body must be cooled immediately, and an ambulance should be called. While awaiting transportation to the hospital, people should be immersed in cold water such as a lake, stream, or bathtub. If immersion is not possible, people should be cooled by misting the body with water and then blowing air across the body with a fan (evaporative cooling). Slightly warm or tepid water is better than cold, for misting because people are less likely to shiver, which generates more heat.

Using drugs (aspirin or acetaminophen) designed to treat a fever due to an infection is useless and should be avoided.

Did You Know...

  • If a person exposed to high heat and humidity develops confusion or symptoms of brain malfunction, immediately call an ambulance and begin cooling measures.

At the hospital, the body is usually cooled rapidly by removing the clothes and covering the exposed skin with water or occasionally ice. To speed evaporation and body cooling, a fan may be used to blow air on the body. Body temperature is measured frequently, often continuously. Cooled fluids may be given intravenously. To avoid overcooling, cooling is stopped when the body temperature is reduced to about 102° F (about 39° C).

Seizures, coma, and malfunction of other organs may also need treatment. Heatstroke is best treated in an intensive care unit of a hospital.

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