Guide

The stages of heat illness

What are the stages of heat illness?

Broadly four, in increasing severity: heat cramps, heat syncope or fainting, heat exhaustion, and heat stroke. They are not a fixed sequence — heat stroke can arrive without the earlier stages — but the ordering is useful because the response changes at the last step, which is an emergency.

Four stages, and one line that matters

Increasing severity. The line between the third and fourth rows is the one that changes what you do.
Stage	What is happening	Response
Heat cramps	Painful muscle spasms, usually legs or abdomen, after heavy sweating. Fluid and sodium loss.	Stop, cool, rehydrate with something containing salt, do not resume that task.
Heat syncope	Fainting or dizziness on standing, from blood pooling in dilated vessels. Common when unacclimatised.	Lie down in shade, elevate legs, cool, rehydrate. Rule out other causes of collapse.
Heat exhaustion	The body still regulating but struggling: weakness, nausea, headache, clammy skin, fast weak pulse.	Cool actively, rest, fluids, out for the day. Seek medical help if it does not improve in 30 minutes.
Heat stroke	Regulation has failed. Core temperature above ~40°C with confusion, slurring, seizure or unconsciousness.	Emergency services now. Cool aggressively while waiting. No fluids to anyone confused.

The progression is real but not obligatory. What makes it useful is that it puts a decision point in the middle of what otherwise reads as a list of symptoms: everything above the last row is something you manage on site; the last row is something you hand to an ambulance.

Why fit people skip stages

The earlier stages are warnings, and warnings only work if someone stops. Two groups reliably do not:

Athletes, whose training is largely the practice of continuing when the body says stop.
Workers on piece rates or under time pressure, and anyone in their first days on a job who does not want to be the person who asked for a break.

This is why exertional heat stroke turns up in the fittest people on the field, and why NIOSH's guidance pairs its acclimatisation schedule with supervision rather than leaving it to self-report. See heat acclimatization for the schedule and work/rest schedules for the structure.

Where the numbers come in

None of these stages is caused by a number, but all of them become more likely as conditions worsen — and conditions are knowable in advance.

The heat index gives the four published bands, whose descriptions map almost exactly onto this progression.
Estimated WBGT is the better input when sun and exertion are involved, which is the exertional case.
Vulnerability shifts the thresholds down: see older adults and infants.

That is where this site's contribution ends and medical guidance begins. Heat exhaustion vs heat stroke is the other half of this, and there is deliberately no third medical page.

Common questions

Do the stages always happen in order?

No, and treating them as a reliable sequence is a mistake. Exertional heat stroke can present with no preceding cramps or exhaustion, particularly in fit young athletes and workers whose motivation carries them past the earlier warnings. The ordering describes severity, not a timetable.

Are heat cramps really a warning sign?

They are worth taking seriously. Cramps typically reflect sodium and fluid loss from heavy sweating, which means the person has been sweating hard for a while — and the same conditions that produced the cramps are still there. Continuing to work through them is how a manageable situation becomes an unmanageable one.

What is heat syncope?

Fainting or near-fainting caused by blood pooling in dilated skin vessels and in the legs, usually after standing still for a long time in heat, or standing up suddenly. It is more common in people who are not yet acclimatised, because plasma volume expansion has not happened. It resolves quickly when the person lies down and cools.

What is heat rash and where does it fit?

Heat rash is a skin condition from blocked sweat ducts in persistently sweaty, unventilated skin. It is uncomfortable rather than dangerous, but it reduces the skin's ability to sweat over the affected area, which matters if it is extensive. It is not part of the severity progression.

Is exertional heat stroke different from classic heat stroke?

Yes, in who it affects and how it looks. Classic heat stroke affects mostly older and chronically ill people during heatwaves, often without exertion, and often with hot dry skin. Exertional heat stroke affects athletes, soldiers and workers, arrives faster, and frequently presents with the person still sweating heavily.

Sources

Published 30 July 2026