Guides · Health
Extreme heat: how to tell ordinary hot from dangerous, and what to do about it
7 min read · Updated September 2026
Heat is the deadliest weather hazard in the United States most years, and it kills quietly, indoors, over several days, usually among people living alone. There is no roar and no damage photo, so it is easy to underestimate.
Two numbers tell you whether a hot stretch is dangerous: the heat index and the overnight low.
The short version
- Heat index plus overnight lows tell you the real danger.
- Confusion and hot dry skin mean heat stroke: call for help immediately.
- A couple of hours in real air conditioning resets your risk for the day.
Read the heat index, not the temperature
The heat index combines air temperature with humidity to estimate what your body experiences. Sweating cools you only when the sweat evaporates, and humid air slows evaporation. Ninety degrees at 70 percent humidity stresses the body more than 100 degrees in dry desert air.
Direct sun adds roughly 15 degrees to the heat index, and that adjustment is not included in the published number. If you are working outdoors, add it yourself.
Overnight lows are the hidden risk
The body recovers from heat at night. When the overnight low stays above roughly 75 degrees for several nights running, that recovery never happens, and the cumulative strain is what sends people to hospitals on day three or four of a heat wave, not day one.
Cities stay hotter overnight than the surrounding countryside because pavement and brick release stored heat for hours. A neighborhood with little tree cover can run several degrees warmer at 2 a.m. than one a mile away with mature shade.
Who is most at risk
- Adults over 65, especially those living alone or without working air conditioning.
- Infants and young children, who heat up faster and cannot move themselves to a cooler room.
- People taking diuretics, beta blockers, antihistamines, or antipsychotics, several of which impair sweating or fluid balance.
- Outdoor workers and anyone exercising during the afternoon peak.
- People experiencing homelessness, who have no indoor baseline at all.
Heat exhaustion vs. heat stroke
Heat exhaustion looks like heavy sweating, cool and clammy skin, weakness, nausea, headache, and a fast weak pulse. Move the person to shade or air conditioning, loosen clothing, apply cool wet cloths, and give sips of water. They should improve within the hour.
Heat stroke is a medical emergency. The signs are confusion or slurred speech, a body temperature at or above 103 degrees, skin that is hot and often dry because sweating has stopped, and possibly loss of consciousness. Call emergency services immediately, move the person somewhere cooler, and cool them aggressively with wet cloths, ice packs at the neck, armpits, and groin, or a cool bath. Do not give fluids to someone who is not fully alert.
Cooling a home without air conditioning
- Open windows overnight if it is cooler outside, then close windows and blinds by mid-morning to trap the cool air.
- Blackout curtains or even cardboard on sun-facing windows cuts a surprising amount of heat gain.
- A fan blowing across a bowl of ice adds evaporative cooling. A fan alone stops helping above roughly 95 degrees and can make things worse by blowing hot air over you.
- Cook nothing that uses the oven. Use a microwave, a grill outside, or eat cold.
- Spend the afternoon somewhere with air conditioning: a library, mall, community center, or an official cooling center. Two hours in real air conditioning meaningfully lowers your risk for the rest of the day.
Check on people
During a heat wave, call or knock on the door of anyone you know who is elderly, isolated, or without cooling, once a day. In the reviews written after major heat disasters, the strongest protective factor is not equipment. It is having someone who checks.