Thermoregulation is something the body does unnoticed every day. Heat stroke begins when that function fails: the body loses its ability to cool down and temperature climbs sharply.

The signs of overheating and of heat stroke were set out by the Public Health Centre of Ukraine.

How to recognise overheating

After time in the heat, someone who has overheated may feel fatigue and irritability, dizziness, headache, a rise in body temperature, nausea or vomiting. Add to that heavy sweating, pale and clammy skin, cramps in the limbs or abdomen, and intense thirst.

The key detail in that list is that the skin stays damp. The body is still sweating, meaning the cooling mechanism works, even if only just.

Signs of heat stroke

Here the picture differs: confusion, loss of coordination, slurred speech. The skin turns hot and dry and may look flushed. Body temperature exceeds 40 degrees, the heartbeat is rapid, breathing fast or laboured.

In more severe cases come convulsions — involuntary muscle contractions the person cannot control — and loss of consciousness.

Why dry skin is more alarming than wet

The difference between the two states is measured precisely by this: sweat is the cooling. When the skin is dry and hot despite the heat, it means sweating has stopped, and with it the last line of defence.

So "they have stopped sweating" is not relief but a signal to call an ambulance. Until medics arrive, the person is moved somewhere cool and their body cooled down.