The heat survivability threshold refers to the point at which the human body can no longer cool itself effectively through sweating, making prolonged exposure deadly even for healthy, well-hydrated people in the shade.
This threshold is commonly associated with a wet-bulb temperature of 35°C (95°F). At this level of heat and humidity, sweat can no longer evaporate fast enough to shed excess body heat. Because evaporation is the body’s primary cooling mechanism, this represents a hard physiological limit: even resting in front of a fan, a person’s core temperature will continue to rise.
Scientists arrived at the 35°C wet-bulb threshold through human physiology experiments, thermodynamic modeling and environmental observations. Mostly studying athletes and soldiers, laboratory studies have measured how the human body responds to different combinations of heat and humidity.
These showed that once the air becomes too moist for sweat to evaporate, core temperatures rise uncontrollably. Researchers used models of heat exchange — including metabolic heat production, evaporation, convection and radiation — to calculate the exact humidity – temperature combinations where the body’s cooling capacity reaches zero. Real-world deadly heat waves have caused mass mortality at wet-bulb levels below 35°C, confirming that 35°C is an upper bound, beyond which survival becomes impossible even for short periods.