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Altitude where water boils at body temperature

The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. above which atmospheric air pressure drops below 0.0618 atm (6.3 kPa, 47 mmHg, or about 1 psi). The U. S. Standard Atmospheric model sets the Armstrong limit at an altitude of 63000 feet (19202 m). The term is named after United States Air Force General Harry George Armstrong, who was the first to recognize this phenomenon.

Well below the Armstrong limit, humans typically require supplemental oxygen in order to avoid hypoxia. For most people, this is typically needed at altitudes above 4,500 m (15,000 ft). Commercial jetliners are required to maintain cabin pressurization at a cabin altitude of not greater than U. S. regulations on general aviation aircraft (non airline, non government flights) require that the minimum required flight crew, but not the passengers, be on supplemental oxygen if the plane spends more than half an hour at a cabin altitude above The minimum required flight crew must be on supplemental oxygen if the plane spends any time above a cabin altitude of , and even the passengers must be provided with supplemental oxygen above a cabin altitude of Skydivers, who are at altitude only briefly before jumping, do not normally exceed .

The Armstrong limit describes the altitude associated with an objective, precisely defined natural phenomenon: the vapor pressure of body temperature water. In the late 1940s, it represented a new fundamental, hard limit to altitude that went beyond the somewhat subjective observations of human physiology and the timedependent effects of hypoxia experienced at lower altitudes. Pressure suits had long been worn at altitudes well below the Armstrong limit to avoid hypoxia. In 1936, Francis Swain of the Royal Air Force reached flying a Bristol Type 138 while wearing a pressure suit. Two years later Italian military officer Mario Pezzi set an altitude record of , wearing a pressure suit in his Caproni Ca.161bis biplane even though he was well below the altitude at which body temperature water boils. A pressure suit is normally required at around for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins. In an unpressurized cockpit at altitudes greater than above sea level, the physiological reaction, even when breathing pure oxygen, is hypoxia—inadequate oxygen level causing confusion and eventual loss of consciousness. Air contains 20.95% oxygen. At , breathing pure oxygen through an unsealed face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at around above sea level. At higher altitudes, oxygen must be delivered through a sealed mask with increased pressure, to maintain a physiologically adequate partial pressure of oxygen. If the user does not wear a pressure suit or a counter pressure garment that restricts the movement of their chest, the high pressure air can cause damage to the lungs. For modern military aircraft such as the United States' F22 and F35, both of which have operational altitudes of or more, the pilot wears a "counter pressure garment", which is a gsuit with high altitude capabilities. In the event the cockpit loses pressure, the oxygen system switches to a positive pressure mode to deliver above ambient pressure oxygen to a specially sealing mask as well as to proportionally inflate the counter pressure garment. The garment counters the outward expansion of the pilot's chest to prevent pulmonary barotrauma until the pilot can descend to a safe altitude.