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http://en.wikipedia.org/wiki/Armstrong_limit
The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere) 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. Armstrong was first to recognize that the phenomenon represented an absolute altitude beyond which humans could not survive in an unpressurized environment. The altitude is variously reported as being between 62,000–63,500 feet (18,900–19,350 meters or about 12 miles).
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."
At the nominal body temperature of 37 °C (98.6 °F), water has a vapor pressure of 47 millimetres of mercury (63 hPa); which is to say, at an ambient pressure of 47 mmHg, water's boiling point is 37 °C. A pressure of 47 mmHg—the Armstrong limit—is one‑sixteenth that of the standard sea level atmospheric pressure of 760 millimeters of mercury (1013 hPa). 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 47 mmHg is typically found at an altitude of 63,100 feet (19,200 m).
Blood pressure is a gage pressure, which means it is measured relative to ambient pressure and is therefore additive when determining the absolute pressure to be used in equations of state (gas laws and the formulas relating pressure-dependent phase changes between liquid and gas). This is similar to a flat automobile tire. Even with zero gage pressure, a flat tire at 63,100 feet would still have an absolute pressure (pressure relative to a perfect vacuum) of 47 mmHg surrounding it—both inside and out. If one inflates the tire to non-zero gage pressure, this internal pressure is in addition to what the tire started with. Even for an individual with a diastolic blood pressure on the low threshold of the normal range, 60 mmHg, blood pressure more than doubles the absolute pressure on the blood and is more than sufficient to prevent blood from outright boiling at 63,100 feet while the heart is still beating.
Hypoxia below the Armstrong limit
The Armstrong limit does not delineate the altitude at which it first becomes necessary to wear a pressure suit. A pressure suit is customarily required at 50,000 feet (15,240 m) for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins. The prompt physiological reaction when breathing pure oxygen through a face mask in an unpressurized cockpit at altitudes greater than 50,000 feet above sea level is hypoxia—inadequate oxygen causing confusion and eventual loss of consciousness. Air is 20.95% oxygen. At 50,000 feet breathing pure oxygen through a face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at 15,655 feet above sea level. This is a particularly high altitude insofar as hypoxia-related risks go; it is over 1100 feet greater than the tallest mountain in the continental U.S., Mount Whitney.
Commercial jetliners are required to pressurize their cabins to an equivalent altitude not greater than 8,000 feet (2,438 m). U.S. regulations on general aviation aircraft (private pilots in small planes) require that the pilot—but not the passengers—be on supplemental oxygen if the plane spends more than a half hour at an altitude above 12,500 feet. General aviation pilots must be on supplemental oxygen if the plane spends any time above 14,000 feet, and even the passengers must be provided with supplemental oxygen at 15,000 feet. Sky divers, who are at altitude only briefly before jumping, do not normally exceed 14,500 feet. Since 50,000 feet is the point at which breathing pure oxygen through an oxygen mask delivers the same oxygen partial pressure as is found with regular air at a hypoxia-inducing 15,655 feet, an altitude of 50,000 feet or higher requires increasing the pressure delivered into the lungs—as well as outside the lungs to avoid pulmonary barotrauma (lungs expanding like a balloon and tearing); thus, the requirement for a pressure suit.
For modern military aircraft such as the United States' F‑22 and F‑35, both of which have operational altitudes of 60,000 feet (18,200 m) or more, the pilot wears a "counter-pressure garment," which is a G‑suit 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.
Historical significance
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 time‑dependent effects of hypoxia experienced at lower altitudes. Pressure suits had long been worn at altitudes well below the Armstrong limit to avoid hypoxia. In 1938, an Italian military officer, Mario Pezzi, set an altitude record of 56,850 feet (17,330 m) and wore a pressure suit in his open-cockpit Caproni Ca.161 biplane even though he was well below the altitude at which body-temperature water boils. Two years earlier, Francis Swain of the Royal Air Force reached 49,967 feet (15,230 m) flying a Bristol Type 138—also while wearing a pressure suit.
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