Hazard guide · Extreme heat
Heat illness: how to spot it and stay cool
Heat is the leading weather-related killer in the U.S., ahead of hurricanes, tornadoes and floods. It doesn't knock down houses, so people underestimate it. Here's how heat harms the body, who's most at risk, and what to do when it turns dangerous.
Last reviewed October 2026
Why heat is so dangerous
Your body cools itself mainly by sweating. When the air is hot and humid, sweat can't evaporate fast enough, and your core temperature starts to climb. That's why forecasters use the heat index, which combines temperature and humidity into what the air actually feels like.
The danger builds over time. Heat waves that last several days are far deadlier than a single hot afternoon, and warm nights are a big part of the reason. When the temperature never drops below the upper 70s, homes without air conditioning stay hot around the clock and the body never gets a chance to recover.
| Heat index | Category | What can happen |
|---|---|---|
| 80–90°F | Caution | Fatigue is possible with long exposure and activity |
| 90–103°F | Extreme caution | Heat cramps and heat exhaustion are possible |
| 103–124°F | Danger | Heat cramps and heat exhaustion are likely; heat stroke is possible |
| 125°F and up | Extreme danger | Heat stroke is highly likely |
Direct sun can make it feel up to 15°F hotter than the heat index shows.
Heat exhaustion vs. heat stroke
Heat exhaustion is the body's warning. Heat stroke is a medical emergency that can kill or cause permanent damage within hours. Knowing the difference tells you whether to cool someone down or call 911.
| Heat exhaustion | Heat stroke | |
|---|---|---|
| Skin | Cool, pale and clammy, with heavy sweating | Hot and red, dry or damp |
| Other signs | Fast, weak pulse; nausea; cramps; dizziness; headache; fainting | Body temperature of 103°F or higher; fast, strong pulse; confusion; passing out |
| What to do | Move somewhere cool, loosen clothing, apply cool wet cloths, sip water. Get medical help if they vomit, get worse or don't improve within an hour. | Call 911. Move them somewhere cooler and cool them with cool cloths or a cool bath. Don't give them anything to drink. |
Who's most at risk
- Adults over 65, whose bodies don't sense or adjust to heat as well, especially those who live alone.
- Babies and young children, who heat up faster and depend on adults to keep them cool.
- People with heart, lung or kidney disease, and people taking some medications for blood pressure, mental health or allergies. Ask your doctor or pharmacist whether yours affect heat tolerance.
- Outdoor workers and athletes, who build up heat through exertion.
- Anyone without air conditioning, including people whose AC fails during a power outage.
Staying cool
- Air conditioning is the best protection. If you don't have it, spend the hottest hours somewhere that does: a cooling center, library, mall or a friend's home. Many cities open cooling centers during heat alerts; call 211 to find one.
- Fans aren't enough in high heat. Once temperatures reach the high 90s, a fan can make you feel better but won't prevent heat illness.
- Drink water before you're thirsty. Limit alcohol and sugary drinks. If your doctor limits your fluids, ask how much to drink in hot weather.
- Block the sun. Close blinds and curtains on sunny windows during the day and open windows at night if it's cooler outside.
- Take cool showers or baths, and keep a wet cloth on your neck.
- Plan outdoor work for early morning or evening, and rest often in the shade.
Heat alerts, explained
The National Weather Service now calls its most serious heat alerts "extreme heat" alerts. They used to be called "excessive heat." Exact criteria vary by region, since 95°F means something different in Phoenix than in Seattle.
- Extreme heat watch: dangerous heat is possible in the next few days. Make your plan.
- Heat advisory: hot conditions are expected that can cause illness, especially for people at higher risk.
- Extreme heat warning: dangerous, possibly deadly heat is expected or happening. Take action now.
The Weather Service's HeatRisk forecast rates each day from 0 (little risk) to 4 (extreme) for your location, up to a week ahead. Search for "NWS HeatRisk" to find it.
If the power goes out
A summer outage can turn dangerous fast. Go somewhere with power if your home gets too hot, especially if anyone in your household is at higher risk. Keep refrigerated medicines cold, and throw out perishable food that's been above 40°F for two hours or more.
Get ready for the next heat wave with the extreme heat checklist.
Open the checklistSources: National Weather Service heat safety program and HeatRisk; Centers for Disease Control and Prevention (CDC); NHTSA; Ready.gov.