Merck Manual

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Heat Exhaustion


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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Heat exhaustion is excessive loss of salts (electrolytes) and fluids due to heat, leading to decreased blood volume that causes many symptoms, sometimes including fainting or collapse.

Heat exhaustion is one of several types of heat disorders.

Heat exhaustion is more severe than heat cramps. Fluids and salts are more depleted, and symptoms are more severe. Heat exhaustion may progress to heatstroke if people continue to be exposed to excessive heat.


Symptoms tend to be vague and similar to the symptoms of many other illnesses. People may not realize that their symptoms are related to the heat. Symptoms include

  • Dizziness

  • Light-headedness

  • Weakness

  • Fatigue

  • Headache

  • Blurred vision

  • Muscle aches

  • Nausea

  • Vomiting

Muscle cramps may occur but often do not. People may feel faint or even lose consciousness when standing. Drenching sweats are common. The heart rate and breathing rate may become rapid. Blood pressure may become low.

Unlike in heatstroke, confusion and incoordination do not occur in heat exhaustion. Also, body temperature is usually normal and if it is high, it is generally not higher than 104° F (40° C).


  • Symptoms and a history of exposure to heat

Heat exhaustion usually is diagnosed on the basis of the symptoms and occurrence after exposure to heat. Laboratory tests may be needed if doctors suspect a diagnosis other than heat exhaustion, or sometimes to measure the levels of sodium in the blood of people who may have drunk too much plain water.


  • Rest in a cool environment

  • Replace fluids and salts

Treatment involves rest (stopping activity), removing people from the hot environment, and replacing fluids and salts, either by mouth (with a sports drink or a solution of about 1 to 2 quarts of water containing 2 teaspoons of salt) or intravenously. Removing or loosening clothing and wetting the skin or applying wet cloths can also aid cooling.

After receiving fluids, people usually recover rapidly and fully.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
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