Human metabolism

There is a certain temperature and humidity at which a human can't cool down, and we will die from the heat of our own metabolism. A sustained wet-bulb temperature (i.e. temperature at 100% relative humidity) exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature our bodies switch from shedding heat to the environment, to gaining heat from it. Thus 35 °C is the threshold beyond which the body is no longer able to adequately cool itself.