Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link



The Manual's Editorial Staff

Reviewed/Revised Apr 2022 | Modified Sep 2022
Get the full details
Topic Resources

What is heatstroke?

Heatstroke is a medical emergency caused by your body temperature going too high. If you don't cool down quickly enough, you can die or have brain or organ damage.

  • Heatstroke happens when you're very hot and your body doesn't cool itself properly

  • Your temperature is usually over 104° F (40° C)

  • Heatstroke can affect older people or young children who live without air conditioning

  • Athletes and people working hard in high heat are also at risk for heatstroke

  • If not treated, heatstroke can damage your organs, such as your brain, heart, and lungs

  • Doctors cool your body and give you fluids directly into your vein (IV fluids)

  • Without treatment, about 80% of people with heatstroke die

What Is Heatstroke?

Call an ambulance right away if someone is having symptoms of heatstroke. While waiting for the ambulance, get the person out of the sun or heat. Cool the person down by soaking in cold water, such as in a lake, stream, or bathtub. If getting into cold water isn't possible, wet the person's skin with slightly warm water and blow air across the skin.

What causes heatstroke?

You can get heatstroke from:

  • Working or exercising hard when it's hot

  • Being locked in a hot car

  • Being inside a hot room for a few days

It can take only a few hours of work or exercise in the heat to get heatstroke, especially if your body hasn't gotten used to the heat.

Children who are locked in a car (or who are too young to open the door) in the hot sun can get heatstroke and die in less than an hour. Cars heat up very quickly inside, especially in the sun.

Older people who are shut in a hot room in the summer for a few days can get heatstroke even if the room doesn't seem really hot. Their body gradually gets overloaded by the heat.

What are the symptoms of heatstroke?

If you're getting sick from the heat, you may not realize that your body temperature is too high. You may have these warning signs of heatstroke:

  • Weakness, dizziness, and light-headedness

  • Headache

  • Feeling sick to your stomach or throwing up

If you have heatstroke, you may or may not sweat and your:

  • Body temperature goes above 104° F (40° C)

  • Skin becomes hot, flushed, and sometimes dry

  • Brain becomes affected and you become very confused, may have seizures, or may go into a coma

How can doctors tell if I am having heatstroke?

  • Doctors can tell based on what happened to you, your symptoms, and your body temperature

  • They'll do blood and urine tests to see if your organs are working right

How do doctors treat heatstroke?

Doctors will put you in the hospital and may:

  • Take off your clothes and cover your skin with water or ice

  • Use a fan to blow air on your body

  • Give you cool fluids through a vein

How can I prevent heatstroke?

To prevent heatstroke when it's hot outside, it's important to:

  • Wear lightweight clothes that aren't too tight

  • Stay out of the sun as much as you can

  • Drink plenty of fluids even if you don't feel thirsty

  • Avoid heavy exercise in the hottest part of the day

  • Ask your doctor if any of your health problems or medicines may raise your risk of heatstroke

  • Check on older people, especially those who don't have air conditioning

  • Never leave a young child in a parked car

If you know you're going to have to work or exercise in the heat, you should get your body used to the heat gradually. Don't do a full day's work or a heavy workout right away.

  • Start with about 15 minutes of moderate activity (enough to make you sweat) during a hot time of day

  • Over about 10 days, gradually increase how hard and how long you work in the heat

  • At the end of that time, you should be doing 90 minutes of vigorous activity in the heat

After doing this training, your body will be better able to handle exertion in the heat.

Being physically fit is different than being used to the heat. Even if you're in great shape, you still should follow these steps to get used to the heat.

quiz link

Test your knowledge

Take a Quiz!