The distinction that matters
Everything else on this page is detail. The distinction is this:
In heat exhaustion, the body is still in control. It is losing the argument — sweating hard, heart racing, blood pressure dropping — but the thermostat is working. Cool the person down and they recover.
In heat stroke, control has failed. Core temperature is climbing without brakes and the brain is affected. This causes organ damage and it kills people. It is an emergency in the same category as a stroke or a cardiac event.
The practical test is not the skin or the sweat. It is the mind. Confusion, disorientation, unusual behaviour, slurred speech, staggering, seizure or loss of consciousness in someone who has been in the heat means treat as heat stroke and call for help now.
The signs, side by side
| Heat exhaustion | Heat stroke | |
|---|---|---|
| Mental state | Normal, if miserable | Confused, agitated, slurring, drowsy or unconscious |
| Core temperature | Usually below 40°C | Usually above 40°C |
| Skin | Pale, clammy, sweating heavily | May be hot and dry — or still sweating, if exertional |
| Pulse | Fast, weak | Fast, often strong |
| Other | Headache, nausea, cramps, dizziness, weakness | Vomiting, seizure, collapse |
| What to do | Cool, rest, sip fluids, stop for the day | Call emergency services. Cool aggressively while waiting. |
The conditions each becomes likely in
This is the part that gets left out of symptom lists, and it is the part you can act on before anyone is ill.
The National Weather Service bands describe exactly this progression:
- Caution, heat index 80–90°F — fatigue possible with prolonged exposure and activity.
- Extreme Caution, 90–103°F — cramps and exhaustion possible with prolonged exposure and activity.
- Danger, 103–125°F — cramps and exhaustion likely; heat stroke possible with continued activity.
- Extreme Danger, above 125°F — heat stroke highly likely with continued exposure.
Two riders. First, those bands assume shade and light activity: working or running moves each line earlier, and so does being older, very young, unacclimatised or in protective clothing. Second, exertional heat stroke does not need the Danger band at all — a hard session in humid 27°C on day one of a season is a well-documented route to it.
Use the heat index calculator for the band, and heat acclimatization for why the first days are the dangerous ones.
Where this page stops
Deliberately here. The clinical detail — differential diagnosis, treatment protocols, cooling rates, return-to-play criteria — belongs to medical bodies, and the CDC, the NHS and Mayo Clinic all publish it well. There is nothing this site can add to that and no reason to compete with it.
What is often missing from those pages, and what this site is for, is the numbers on the other side: the thresholds, where they come from, and what they assume. The stages of heat illness covers the progression from cramps upward, and that is the last medical page here.