Armstrong limit
the altitude above which water boils at human body temperature, making it absolutely impossible for humans to survive unpressurized; approx. 18–19 km above sea level
The Armstrong Limit is a measure of atmospheric pressure that is low enough to cause the boiling points of blood and water to be at or below the human core body temperature (around 37°C\98°F). Therefore it necessitates pressurized cabins and full pressure suits (such as spacesuits) to survive. The Armstrong limit corresponds to an altitude of about 19 kilometres or 12 statute miles above Earth.