There is a temperature at which the human body simply stops working. It is called the wet-bulb temperature—a single figure that folds heat and humidity together, measured by wrapping a thermometer in a wet cloth so that evaporation cools it, exactly as sweat cools skin. Below a certain point, a person in the shade can shed metabolic heat and survive. Above it, they cannot. The body’s core temperature climbs regardless of how still they sit or how much they drink, and after a few hours, organs fail.

For years, the accepted ceiling was a wet-bulb temperature of 35°C (95°F). At that threshold, sweat can no longer evaporate fast enough to matter, and the body becomes, in effect, a sealed system. Any…