The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere or 6.3 kPa (47 mmHg)) 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.[Note 1] Armstrong was the first to recognize this phenomenon, which occurs at an altitude beyond which humans absolutely cannot survive in an unpressurized environment. Above Earth, this begins at an altitude of approximately 18 km (60,000 ft) to about 19 km (62,000 ft).
Effect on bodily liquids
|Olympus Mons summit||0.03 kPa (0.0044 psi)|
|Mars average||0.6 kPa (0.087 psi)|
|Hellas Planitia bottom||1.16 kPa (0.168 psi)|
|Armstrong limit||6.25 kPa (0.906 psi)|
|Mount Everest summit||33.7 kPa (4.89 psi)|
|Earth sea level||101.3 kPa (14.69 psi)|
|Dead Sea level||106.7 kPa (15.48 psi)|
|Surface of Venus||9,200 kPa (1,330 psi)|
At or above the Armstrong limit, exposed bodily liquids such as saliva, tears, urine, blood 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 6.3 kilopascals (47 mmHg); which is to say, at an ambient pressure of 6.3 kilopascals (47 mmHg), the boiling point of water is 37 °C (98.6 °F). A pressure of 6.3 kPa—the Armstrong limit—is about 1/16 of the standard sea-level atmospheric pressure of 101.3 kilopascals (760 mmHg). 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 6.3 kPa is typically found at an altitude of 19,000 m (62,000 ft).
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 hectopascals (0.91 psi), that is, it will have the ambient pressure at the altitude where the 6.3 kPA pressure level occurs (about 18 km (60,000 ft)), both inside and out of it. If one inflates the tire to non-zero gauge pressure, this internal pressure is in addition to those 6.3 kilopascals (0.91 psi) of ambient pressure. This means that for an individual with a diastolic low blood pressure, typically 8.0 kPa (60 mmHg), the person's blood pressure would be 14.3 kPa (107 mmHg), 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 18 km (60,000 ft) while the heart is still beating.
Hypoxia below the Armstrong limit
The altitude of the Armstrong limit is not the same as the altitude at which it first becomes necessary to wear a pressure suit. A pressure suit is normally required at around 15,000 m (49,000 ft) 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 15,000 m (49,000 ft) above sea level is hypoxia—inadequate oxygen level causing confusion and eventual loss of consciousness. Air contains 20.95% oxygen. At 15,000 m (49,000 ft), breathing pure oxygen through a face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at around 4,700 m (15,400 ft) above sea level.
Commercial jetliners are required to maintain cabin pressurization at a cabin altitude of not greater than 2,400 m (8,000 ft). U.S. regulations on general aviation aircraft (that is, non-airline, non-government flights) require that the pilot, but not the passengers, be on supplemental oxygen if the plane spends more than half an hour at a cabin altitude above 3,800 m (12,500 ft). General aviation pilots must be on supplemental oxygen if the plane spends any time above a cabin altitude of 4,300 m (14,000 ft), and even the passengers must be provided with supplemental oxygen above a cabin altitude of 4,500 m (15,000 ft). Skydivers, who are at altitude only briefly before jumping, do not normally exceed 4,500 m (15,000 ft). Since approximately 15,000 metres (49,000 ft) 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 4,500 m (15,000 ft), an altitude of 4,500 m (15,000 ft) or higher requires increasing the pressure delivered into the lungs—as well as outside the lungs to make breathing comfortable; 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 18,000 m (59,000 ft) 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.
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 1936, Francis Swain of the Royal Air Force reached 15,230 m (49,970 ft) flying a Bristol Type 138 while wearing a pressure suit. Two years later Italian military officer Mario Pezzi set an altitude record of 17,083 m (56,047 ft), wearing 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.
- 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.
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