Is a wet-bulb temperature of 35 &LCIRC;C the correct threshold for human survivability?

A wet-bulb temperature of 35 C-& LCIRC; is widely used as the threshold for human survivability, but the wet-bulb temperature is not a particularly accurate metric for human heat stress. For a person in the shade, a more accurate metric is the heat index, which is based on a model of human thermoregulation that accounts for metabolic heat, radiation, respiratory ventilation, and finite wind speeds. The heat index has two critical values: the highest heat index for which a healthy core temperature can be maintained and the highest heat index that is survivable. It is shown here that a wet-bulb temperature of 35 C-& LCIRC; corresponds to conditions between these two critical values. For example, in a world warmer than pre-industrial by 10 C-& LCIRC;, about 30% of the world's population would be exposed once or more per year to a wet-bulb temperature above 35 C-& LCIRC;, but the heat index reveals that less than 2% would be exposed to fatal conditions while over 60% would be exposed to conditions that would cause hyperthermia.