Heat illness is a spectrum with a single decisive line across it, and the line is not a temperature — it is whether the brain is working.
1. The spectrum
| What it looks like | What to do | |
|---|---|---|
| Heat cramps | Painful muscle cramps during or after effort in the heat, otherwise well | Stop, shade, fluid with salt in it, gentle stretching |
| Heat exhaustion | Headache, nausea, dizziness, weakness, heavy sweating, cramps, feeling awful — but oriented and making sense | Shade, lie down, legs up, cool them actively, oral rehydration with salt and sugar. Improvement should be obvious within 30 minutes |
| Heat stroke | Any alteration in mental state with heat exposure — confusion, odd behaviour, aggression, ataxia, seizure, collapse. Core temperature at or above 40 °C | 999 and cool immediately. Section 3 |
Do not wait for someone to stop sweating. The textbook line about hot dry skin describes late classic heat stroke in an elderly person during a heatwave. In exertional heat stroke — a runner, a soldier, a builder, a child at sport — the skin is usually still wet, and may not even feel especially hot to your hand. In one published case a collapsed, confused runner felt cool and moist to touch; his rectal temperature was 41.1 °C.
2. Measuring it, and why most thermometers are useless here
The single most useful thing on this page
- Only a rectal temperature is accurate in heat illness. Tympanic, temporal, oral, axillary and infrared forehead readings are all unreliable at the extremes, and they fail in the direction that gets people killed — they read low.
- And the error is systematic, not random. This is not measurement scatter that averages out if you take the reading twice, and it is not a faulty device. Every one of those sites measures something other than core temperature — skin, an ear canal, a mouth — and in a hot, sweating or vasoconstricted patient those sites consistently sit below the core. So repeating the reading, or reaching for a second thermometer of the same kind, does not help. A low reading from any of them, in a person who fits the picture, is evidence about the thermometer rather than about the patient.
- In the case above, a tympanic thermometer read 36.7 °C in a man whose true core temperature was 41.1 °C. A normal-looking reading from an ear or forehead thermometer in a confused, collapsed, overheated person excludes nothing at all.
- So do not use a temperature to rule heat stroke out. Mental state rules it in; a thermometer you happen to have will not rule it out. Treat on the confusion.
- Sports events and military settings that manage this well carry rectal probes for exactly this reason, and that is not squeamishness on their part — it is the only measurement that works.
3. Cooling — and the principle that changed
Cold water immersion, neck down, is first choice
Whole-body immersion in cold water — anywhere from 1 to 26 °C, so a paddling pool, a wheelie bin, a cattle trough, a bath, a stream all count — continued until they are below 39 °C or clearly improving. Colder is faster and the fear of cooling too hard is misplaced at this end: ice water at 1–5 °C gives the fastest rate of all. Do not delay immersion to find a thermometer.
Three guards on this, because putting a confused person in cold water is not a small act. One: this is for heat stroke — altered mental state — not for heat exhaustion, where shade, lying down and fluids are the answer and immersion is unnecessary. Two: someone holds them, hands on, for the whole time. Head and airway clear of the water, and a named person whose only job is that. A confused or seizing patient can drown in a paddling pool, and having cooled them is no defence. Three: stop at 39 °C or at the point their mental state clears, whichever comes first — and then keep watching, because overshoot into hypothermia is a real complication and is easier to cause than people expect.
If immersion is impossible, use everything at once
Strip to underwear. Cold wet sheets replaced as they warm, ice packs to neck, armpits and groin, continuous fanning, cold shower, cold water poured over them repeatedly, hands and forearms in cold water. Combinations beat any single measure, and a fan plus wet skin beats a fan alone by a wide margin.
Keep cooling while help is on the way, and tell them you are doing it
Ambulance crews will continue it. Say “heat stroke, we are cooling them now” — it changes the response and the destination.
Stop at improvement, not at cold
Stop when they are below about 39 °C or their mental state clears. Overshooting into hypothermia is a real complication, particularly in children.
4. What not to do
- Do not give paracetamol or ibuprofen for it. Heat stroke is not fever — the thermostat is not the problem, heat load is — so antipyretics do not lower the temperature and both add risk to a liver and kidneys already under strain.
- Do not give fluids by mouth to anyone confused or drowsy. Aspiration is a real hazard and they need intravenous fluid, not a bottle.
- Do not assume a wet, cool-feeling skin means they are fine. Section 2.
- Do not stop at the first sign of improvement and send them home. Heat stroke can declare organ injury — kidney, liver, clotting, muscle breakdown — hours later, so everyone with it goes to hospital even if they look transformed by the water.
- Do not let them go back out. Anyone with heat exhaustion is finished for the day and, after heat stroke, for considerably longer.
5. Who is at risk when nobody is exercising
Classic heat stroke kills more people than the exertional kind, quietly, indoors, during heatwaves — and the people at risk are identifiable in advance.
| Group | Why |
|---|---|
| Older people living alone | Reduced thirst, reduced sweating, often no fan or no willingness to use one, and nobody checking |
| Babies and young children | Cannot remove their own layers, cannot get their own drink, and heat up faster. Never leave a child in a parked car, for any length of time — interior temperature rises far faster than people expect and cracking a window does not fix it |
| People on certain medicines | Anticholinergics and antihistamines reduce sweating; diuretics reduce volume; beta blockers blunt the cardiac response; antipsychotics impair temperature regulation; SSRIs and stimulants add heat production. This is the most under-recognised risk factor of the lot |
| Heart, kidney or lung disease, diabetes, obesity | Less physiological reserve to shed heat |
| Anyone using stimulants, cocaine, MDMA or amphetamines | Heat production up, judgement down, often in a hot crowded place |
| Outdoor and indoor manual workers | Kitchens, foundries, roofing, warehouses. Acclimatisation takes one to two weeks and the first days of a heatwave are the dangerous ones |
The prevention that actually works is dull: fluid before thirst, shade in the middle of the day, gradual acclimatisation over one to two weeks rather than a single hard session, checking on the people in the list above, and cooling one room rather than a whole house.