What is the minimum air pressure the human body can tolerate if oxygen supply is not an issue?
Disregarding hypoxia, the lowest atmospheric pressure the human body can withstand is around 6 percent sea level pressure, or 61.8 millibars, below that pressure the water and blood in your body starts to boil. Harry George Armstrong, a physician, and an airman, was the first to recognise this limit, which on Earth occurs at an altitude of roughly 63,000 feet, beyond which humans absolutely cannot survive in an unpressurised environment. The limit was named in his honour and so is called the Armstrong Limit. The lowest atmospheric pressure humans can breathe in, with a pure oxygen supply on hand, is roughly around 12.2 percent sea level air pressure or 121.7 millibars, the pressure found at 49,000 feet. Or, as a slightly madder alternative example, in a terraforming Mars situation, which might arise one day; a fit person could in theory walk around outside without a spacesuit on, but breathing from an oxygen tank, only when the atmospheric pressure got above about 120 millibars, and hope to survive for long.