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Heatwaves: how heat actually kills, and the humidity trap

Heat is the deadliest weather hazard in most years, and it kills quietly — dehydration and cardiovascular strain rather than dramatic collapse. The number that matters is not the air temperature but the combination of heat and humidity, because sweat is the only cooling system the body has, and it stops working when the air is already saturated.

  • 12High risk
  • 2Medium
  • 0Low
  • 14Zones tracked

Exhaustion versus heatstroke

Heat exhaustion means heavy sweating, cold clammy skin, dizziness, nausea, headache and a fast weak pulse. It responds to shade, fluids and cooling. Heatstroke is different: body temperature above about 40°C, skin that may be hot and dry, confusion, slurred speech, and loss of consciousness. It is a medical emergency with a real mortality rate, and the transition between the two can be quick.

Why humid heat is worse

At high humidity, sweat cannot evaporate, so the body cannot shed heat at all. This is why 35°C in the Persian Gulf or Japan is far more dangerous than 40°C in a dry desert. Several zones tracked here — the Gulf, Japan and Korea, the North China Plain — are flagged specifically for humid heat rather than record temperature.

How to avoid it

  • Shift all activity to early morning and evening; treat 11:00–17:00 as indoor time.
  • Drink steadily through the day rather than in response to thirst, and include salts.
  • Go easy on alcohol and caffeine, both of which worsen dehydration.
  • Use shade, light loose clothing, a hat, and air conditioning where available.
  • Never leave anyone — especially a child, an older person or a dog — in a parked car.
  • Check on older travelling companions; they feel thirst less and dehydrate faster.

If it happens

  • Move the person into shade, remove excess clothing, and cool them with water, wet cloths and airflow.
  • Give cool fluids only if they are fully conscious and able to swallow.
  • Call emergency services for confusion, loss of consciousness, seizures or hot dry skin — that is heatstroke.
  • Keep cooling aggressively while waiting for help; cooling speed determines the outcome.

General guidance only — not a substitute for medical advice or local emergency services.

Where we track it

14 zones in the Dangers Here hazard database, by danger level.

High

  • Arabian Peninsula
  • Australian Interior
  • Death Valley — World Heat Record
  • Japan & Korea — Humid Heat
  • Mediterranean Basin
  • North China Plain
  • Northern India & Pakistan
  • Northwestern Europe
  • Persian Gulf — Humid Heat
  • Sahara Desert — North Africa
  • Sahel — Mali, Niger, Chad, Sudan
  • Southwestern USA & Northern Mexico

Medium

  • Afghanistan
  • Central America

Questions

Where are heatwaves a risk?

We track 14 zones for this hazard. 12 are rated high risk, including Arabian Peninsula, Australian Interior, Death Valley — World Heat Record, Japan & Korea — Humid Heat, Mediterranean Basin. 2 are rated medium and 0 low.

How do you avoid heatwaves?

Shift all activity to early morning and evening; treat 11:00–17:00 as indoor time. Drink steadily through the day rather than in response to thirst, and include salts. Go easy on alcohol and caffeine, both of which worsen dehydration.

What is the first aid for heatwaves?

Move the person into shade, remove excess clothing, and cool them with water, wet cloths and airflow. Give cool fluids only if they are fully conscious and able to swallow. Call emergency services for confusion, loss of consciousness, seizures or hot dry skin — that is heatstroke.

Exhaustion versus heatstroke

Heat exhaustion means heavy sweating, cold clammy skin, dizziness, nausea, headache and a fast weak pulse. It responds to shade, fluids and cooling. Heatstroke is different: body temperature above about 40°C, skin that may be hot and dry, confusion, slurred speech, and loss of consciousness. It is a medical emergency with a real mortality rate, and the transition between the two can be quick.

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