Human tolerance of heat depends on as much as on temperature.
The body cools by sweat, which requires air that can still absorb water.
temperature combines both variables into a single measure.
Above a wet-bulb reading of roughly thirty-five degrees, a healthy adult cannot cool down at all.
Shade and water do not help; only cooler air does.
Such readings were once theoretical and have now been recorded briefly in the Gulf and South Asia.
The practical is lower than the one.
Older people, outdoor workers and anyone with heart disease are in danger far earlier.
Policy responses split into the immediate and the structural.
Immediate measures are heat warnings, cooling centres and changed working hours.
These are cheap, and evaluations show they work when they reach the people who need them.
Reaching them is the hard part, since the highest-risk groups are often and offline.
Structural measures are slower: insulation, shade, water and reliable electricity.
A cooling centre without power during a peak is worse than useless, because people travelled to reach it.
Heat is the deadliest weather in most countries and the least likely to appear in a photograph.