What makes a heatwave dangerous?
The combination of heat, humidity and duration — and humidity is the part people underestimate, because it determines whether the body's only real cooling mechanism still works.
How the body cools. Almost entirely by sweating and evaporation. Evaporation removes a great deal of heat, and it depends on the surrounding air being able to accept moisture. In high humidity, sweat does not evaporate and cooling fails, regardless of how much you sweat — which is why a humid 35°C is far more dangerous than a dry 40°C.
The wet-bulb temperature captures this in one number: the lowest temperature achievable by evaporative cooling in those conditions. Above roughly 35°C wet-bulb, a healthy person cannot shed metabolic heat even at rest in shade with unlimited water. Such conditions have been recorded briefly in a few places and are expected to become more frequent.
Why duration matters. Buildings and ground absorb heat during the day and release it at night. Warm nights are the strongest predictor of heat mortality, because the body never gets a recovery period — which is why urban areas, retaining heat overnight, suffer disproportionately.
Who is most at risk: older people, whose thirst response and sweating are reduced; people with cardiovascular, respiratory, kidney or psychiatric conditions; those on certain medications including diuretics and some antipsychotics; infants; outdoor and manual workers; and people living alone, which repeatedly emerges as a major risk factor in heatwave death investigations.
Why the death toll is invisible. Most heat deaths are recorded as cardiac or respiratory, so the toll is estimated as excess mortality — comparing deaths to the expected baseline — and is far higher than the count of people who obviously died of heat.
What actually works: cooling the body directly with water and airflow; keeping buildings cool by shading and shutting windows during the day then ventilating at night; checking on isolated neighbours, which is the intervention that saves most lives; and heat-health warning systems.
A fan alone can be counterproductive at very high temperatures with low humidity in some circumstances.