Air temperature alone is a poor guide to how dangerous heat is. The wet bulb reading captures what the body actually depends on, and it explains why humid heat becomes hazardous at numbers that look moderate.
What the measurement is
A wet bulb temperature is taken by wrapping a thermometer bulb in wet cloth and ventilating it, then reading the value once evaporation has cooled it as far as it will go.
In dry air, evaporation is rapid and the reading falls well below the ordinary air temperature. In saturated air, nothing evaporates and the two readings converge.
The wet bulb value is therefore the lowest temperature any surface can reach through evaporation under the conditions present.
Why the human body cares
Metabolism generates heat continuously, and at rest the body sheds it by conduction, convection and radiation to cooler surroundings.
Once air temperature approaches skin temperature those routes close, and sweating becomes the only remaining path for heat to leave.
Sweat cools only when it evaporates, so the wet bulb reading defines the ceiling on how much cooling is physically available.
Why humidity changes everything
Dry heat allows sweat to evaporate quickly, which is why very high air temperatures are survivable in arid regions with shade and water.
Humid air is already carrying moisture, so evaporation slows sharply and sweat runs off the skin without cooling it.
The same air temperature can therefore be routine in one climate and dangerous in another, and only a measure combining both distinguishes them.
How the limit works in practice
Above a wet bulb value close to normal skin temperature, a person at rest in shade cannot shed metabolic heat at all, and core temperature rises regardless of fitness or acclimatisation.
Well below that theoretical ceiling, exertion, age, medication and illness all reduce tolerance, so harm occurs at readings far short of the physical limit.
Anyone experiencing heat illness needs medical attention rather than a threshold table, since individual tolerance varies widely.
Why forecasts report indices instead
Heat index and similar measures translate temperature and humidity into an apparent temperature, which communicates risk in units the public already understands.
Those indices assume shade, light wind and a typical adult, so direct sun and physical work push actual strain above the posted value.
The wet bulb reading underlies them, and it is the quantity that determines whether cooling is available at all rather than merely uncomfortable.