Heat exhaustion sets in when you lose too much water and salt through heavy sweating. Watch for a headache, nausea, dizziness, weakness, and thirst, with a fast, weak pulse. The reliable sign that it's still heat exhaustion, not heat stroke, is a clear head; confusion means heat stroke, even if the person is still sweating.
Heat exhaustion is your body struggling to keep up with heat. It sets in when you lose too much water and salt, usually through heavy sweating.
The signs tend to build gradually. You might get a headache, feel nauseated, dizzy, weak, or very thirsty, and you'll usually be sweating heavily. Your pulse can run fast and weak, and you may feel close to fainting.
What separates heat exhaustion from heat stroke is mental state: with heat exhaustion you're still thinking clearly, while confusion signals heat stroke. Sweating is not a reliable tell, because a heat-stroke victim's skin can be dry or still sweaty. That more dangerous stage, heat stroke, is a medical emergency that can cause permanent disability or death.
So treat these signs as your cue to act: get out of the heat, cool down, and sip water preferably accompanied with electrolytes or salty snacks. And if the person turns confused or hard to wake, get emergency help right away.
Sources
- [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] 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
- [3] US National Park Service. Hike Smart — Grand Canyon National Park. Accessed August 5, 2026. https://home.nps.gov/grca/planyourvisit/hike-smart.htm
- [4] CDC/NIOSH. Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (DHHS/NIOSH Publication 2016-106). February 2016. Accessed August 5, 2026. https://www.cdc.gov/niosh/docs/2016-106/pdfs/2016-106.pdf
- [5] Wilderness Medical Society / Wilderness & Environmental Medicine. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. 2024. Accessed August 5, 2026. https://doi.org/10.1177/10806032241227924
About the author
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