Your body holds a narrow internal temperature and sheds excess heat almost entirely by evaporating sweat. That system has one hard dependency: the air must be able to accept the moisture. In humid heat it can't, so sweat pours off without cooling you, and core temperature climbs.
This is why humid heat is far more dangerous than dry heat at the same temperature, and why the heat index exists at all.
What the numbers mean for a body
Remember these assume shade. In direct sun, add 10 to 15 degrees to what your body is actually experiencing.
Heat exhaustion: heavy sweating, cool clammy skin, weakness, nausea, headache, fast weak pulse. Move to shade or AC, loosen clothing, sip water, apply cool cloths. Improvement should come within an hour.
Heat stroke: body temperature above 103°, hot dry skin or heavy sweating, confusion, slurred speech, loss of consciousness. This is a medical emergency, call 911, move them somewhere cool, and actively cool with water or ice while waiting. Do not give fluids to someone who isn't fully alert.
The classic teaching that heat stroke always presents with dry skin is unreliable, exertional heat stroke often comes with soaking sweat. Confusion is the red flag. Altered mental state in the heat is an emergency until proven otherwise.
What raises risk
- Consecutive hot days: heat accumulates in bodies and buildings. Day three of a heat wave is more dangerous than day one.
- Warm nights: recovery happens overnight. When the low stays above about 75°, it doesn't.
- Age and medication: older adults sweat less, and common drugs including diuretics, beta blockers, and antihistamines impair heat regulation.
- No acclimatization: the first hot spell of a season is disproportionately deadly.
- Vehicles: an interior can gain 20°F in ten minutes and exceed 130°F. Never leave children or pets, not even briefly, not even with windows cracked.
The overnight number nobody checks
Dew point sets a floor on how far temperatures can fall overnight, because cooling stalls as air approaches saturation. A dew point of 76° means the night will not meaningfully cool, which means bodies and buildings never get their recovery window. That single measured number often predicts a dangerous night better than the daytime high does.