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Armstrong limit
The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere or ) that water boils at the normal temperature of the human body: 37 °C (98.6 °F). It is named after Harry George Armstrong, who founded the U.S. Air Force’s Department of Space Medicine in 1947 at Randolph Field, Texas.〔Along with Malcolm C. Grow, Armstrong became one of the first two surgeons general for the United States Air Force when the United States Air Force split from the Army Air Forces to become a separate branch of the U.S. military on 18 September 1947. Randolph Field was officially renamed Randolph Air Force Base shortly thereafter on 13 January 1948.〕 Armstrong was the first to recognize this phenomenon, which occurs at an altitude beyond which humans absolutely cannot survive in an unpressurized environment.〔(NAHF - Harry Armstrong )〕 On Earth, the altitude is variously reported as being between 18,900 and 19,350 meters (62,000 and 63,500 feet, or about ).〔(NASAexplores Glossary ) on web.archive.org〕
==Effect on bodily liquids==
At or above the Armstrong limit, exposed bodily liquids such as saliva, tears, and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away without a pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes. The NASA technical report ''Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects'', which discusses the brief accidental exposure of a human to near vacuum notes the likely result of exposure to pressure below that associated with the Armstrong limit: "The subject later reported that ... his last conscious memory was of the water on his tongue beginning to boil."〔(Ask an Astrophysicist: Human Body in a Vacuum )〕
At the nominal body temperature of , water has a vapor pressure of ; which is to say, at an ambient pressure of 63 hPa, the boiling point of water is 37.0 °C. A pressure of 63 hPa—the Armstrong limit—is about 1/16 of the standard sea-level atmospheric pressure of . Modern formulas for calculating the standard pressure at a given altitude vary—as do the precise pressures one will actually measure at a given altitude on a given day—but a common formula shows that 63 hPa is typically found at an altitude of .
Blood pressure is a ''gauge'' pressure, which means it is measured relative to ambient pressure. To calculate blood pressure, it has to be summed to ambient pressure for calculating when blood will boil. This is similar to a flat automobile tire: even with zero gauge pressure, a flat tire at altitude of the Armstrong limit would still have an absolute pressure (pressure relative to a perfect vacuum) of 63 hPa, that is, it will have the ambient pressure at 19,000 meters, both inside and out of it. If one inflates the tire to non-zero gauge pressure, this internal pressure is ''in addition'' to those 63 hPa of ambient pressure. This means that for an individual with a diastolic low blood pressure, typically , their blood pressure would be (the sum of the blood pressure and the ambient pressure). This pressure is more than twice the ambient pressure at the Armstrong limit. This extra pressure is more than sufficient to prevent blood from outright boiling at 19,000 meters while the heart is still beating.〔〔(Human Exposure to Vacuum )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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