Armstrong limit

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The Armstrong limit, also called 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. Therefore, airplanes usually fly below the Armstrong limit. On Earth, the limit is around 18–19 km (11–12 mi; 59,000–62,000 ft) above sea level, above which atmospheric air pressure drops below 0.0618 atm (6.26 kPa; 47.0 mmHg; 0.908 psi). The U.S. Standard Atmosphere model sets the Armstrong limit at an altitude of 63,000 ft (19,202 m). The Armstrong limit is often used as the lower limit of near space. The term is named after United States Air Force general Harry George Armstrong, who was the first to recognize this phenomenon.