Highest survivable altitude by a human in an unpressurised environment

The Armstrong limit was first realized by Harry Armstrong (USA), a pioneer in aviation medicine. It represents the altitude at which the pressure is just 0.0618 atmospheres, where water boils at normal human body temperature (37 C). Occurring at an altitude between 18,900 m and 19,350 m it represents the absolute highest altitude at which a person could theoretically survive without a pressure suit or a pressurised capsule. At or above the Armstrong limit, your lung fluids, saliva and tears would spontaneously boil away.

Armstrong discovered this effect while at the US Air Force’s Department of Space Medicine, which he founded in 1947.