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What are the warning signs of heat stroke?

Dr. Colin Little

Dr. Colin Little

MD, FAWM, Chief Medical Officer, GOES Health

The short answer

The clearest warning sign of heat stroke is a change in mental state (confusion, slurred speech, or passing out) in someone who is overheated. Other signs are a very high body temperature, hot skin (dry or sweaty), a rapid strong pulse, and sometimes seizures. Heat stroke is a medical emergency: call 911 and start cooling the person right away.

Heat stroke is the emergency end of heat illness. It is the most serious heat-related illness, and without emergency treatment it can cause permanent disability or death.1

The warning sign that matters most is a change in how someone is thinking. Confusion, slurred speech, or passing out in a person who is overheated is the red flag for heat stroke.1 That mental change is exactly what separates heat stroke from heat exhaustion, so a confused or unconscious overheated person should be treated as heat stroke.12

Other signs include a very high body temperature, skin that is hot to the touch and may be dry or still sweaty, a rapid strong pulse, and sometimes seizures.13 Don't wait on a thermometer, since you can't reliably measure someone's core temperature in the moment.

Treat it as the emergency it is. Call 911, then start cooling the person right away.1 Use a cold-water or ice bath if you can, or soak their clothes and put cold wet cloths on the head, neck, armpits, and groin1 while help is on the way. Fanning wet skin while you wait helps12, but this person needs medical attention as soon as possible.

Sources

  1. [1] US Centers for Disease Control and Prevention / NIOSH. Heat-related Illnesses (NIOSH Heat Stress). Accessed August 6, 2026. https://www.cdc.gov/niosh/heat-stress/about/illnesses.html
  2. [2] US National Park Service. Hike Smart — Grand Canyon National Park. Accessed August 5, 2026. https://home.nps.gov/grca/planyourvisit/hike-smart.htm
  3. [3] Extreme Heat Emergency Brochure (CDC content). Accessed August 7, 2026. https://dds.dc.gov/sites/default/files/dc/sites/dds/release_content/attachments/CDC%20Heat%20Emergency%20Brochure.pdf

About the author

Dr. Colin Little

Dr. Colin Little

MD, FAWM, Chief Medical Officer, GOES Health

  • Board Certified Emergency Physician
  • Fellow of the Academy of Wilderness Medicine (FAWM)
  • Wilderness Medicine Fellowship, University of Utah
  • Emergency Medicine Residency and Chief Resident, Stanford / Kaiser
  • Doctor of Medicine, University of Arizona College of Medicine – Phoenix
Keep reading

Related answers

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When should you call 911 for heat illness?

Call 911 right away for heat stroke: anyone who is overheated and confused, slurring their speech, or not fully conscious. Heat stroke is a medical emergency, so make the call and start cooling them while you wait. For heat exhaustion, start cooling them and get medical help if symptoms are severe or don't improve, since heat exhaustion can lead quickly to heat stroke in the wrong environment.

Read Heat Exhaustion

How do you tell heat exhaustion from heat stroke?

What separates heat stroke from heat exhaustion is altered mental status, a change in how clearly the person thinks . Heat exhaustion leaves the nervous system uninjured, so the person still makes sense and can answer you . Heat stroke adds confusion, slurred speech, seizures or loss of consciousness . Treat that change as an emergency. Cool the person immediately and call for help .

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How much water should you drink when hiking in the heat?

On a hot hike, carry about a quart (roughly a liter) of water for every hour you'll be out, and let thirst tell you when to drink, since forcing down far more than you sweat out can be dangerous. On longer hikes, add salty snacks as well.

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