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If a pilot flew above the Armstrong limit using only an oxygen mask and no pressure suit, the water wetting his lungs would boil, as would the saliva in his mouth.

The Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low (0.0618 atmosphere or Predefinição:Convert/LonAonDbSoff) 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 the first to recognize this phenomenon, which occurs at an altitude beyond which humans absolutely cannot survive in an unpressurized environment. The altitude is variously reported as being between 18,900 and 19,350 meters (62,000 and 63,500 feet, or about 12 milhas (10 nmi)).

Effect on bodily liquids

Atmospheric pressure comparison

Location	Pressure
Olympus Mons summit	0,03 kilopascals (0,0 044 psi)
Mars average	0,6 kilopascals (0,087 psi)
Hellas Planitia bottom	1,16 kilopascals (0,168 psi)
Armstrong limit	6,25 kilopascals (0,906 psi)
Mount Everest summit	33,7 kilopascals (4,89 psi)
Earth sea level	101,3 kilopascals (14,69 psi)
Dead Sea level	106,7 kilopascals (15,48 psi)
Surface of Venus	9 200 kilopascals (1 300 psi)

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.{{carece de fontes}} 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 37,0 °C (98,6 °F), water has a vapor pressure of Predefinição:Convert/LoffAoffDflipSoff; 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 one‑sixteenth that of the standard sea level atmospheric pressure of 1 013 hectopascals (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{{carece de fontes}} shows that 63 hPa is typically found at an altitude of Predefinição:Convert/LoffAoffDflipSoff.

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 when 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 80 hPa (60  mmHg), their blood pressure would be 140 hPa (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 19,000 meters while the heart is still beating.

Hypoxia below the Armstrong limit

A biplane pilot in the late 1930s wearing a pressure suit to avoid hypoxia.

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 around 15,000 metros (49,21 pé) 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 meters above sea level is hypoxia—inadequate oxygen causing confusion and eventual loss of consciousness. Air is 20.95% oxygen. At 15,000 meters 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 metros (15 000 pé) above sea level.

Commercial jetliners are required to pressurize their cabins to an equivalent altitude not greater than 8,000 feet (2,438 m).{{carece de fontes}} 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 3,810 meters (12,500 feet). General aviation pilots must be on supplemental oxygen if the plane spends any time above 4,270 meters (14,000 feet), and even the passengers must be provided with supplemental oxygen at 4,570 meters (15,000 feet). Skydivers, who are at altitude only briefly before jumping, do not normally exceed 4,500 meters. Since 15,000 meters 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 4,700 meters, an altitude of 15,000 meters 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 metros (59,06 pé) 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 1936, Francis Swain of the Royal Air Force reached 15,230 metros (49,97 pé) flying a Bristol Type 138 while wearing a pressure suit.{{carece de fontes}} Two years later Italian military officer Mario Pezzi set an altitude record of 17,330 metros (56,86 pé), 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.

See also

Comparison of a graph of International Standard Atmosphere temperature and pressure with the Armstrong limit and approximate altitudes of various objects

• Altitude sickness
• Effect of spaceflight on the human body
• Decompression illness

External links

• US Naval Flight Surgeon's Manual. Chapter 1 is Physiology of Flight
• Ebullism at 1 Million Feet: Surviving Rapid/Explosive Decompression
• The Engineering ToolBox: “Air Pressure and Altitude above Sea Level”