Armstrong limit

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

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: 98.6 °F (37.0 °C). 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.^[1] On Earth, this begins at an altitude of approximately 60,000 feet (18,000 m),^[2] or about 12 miles (19 km),^[3] depending on the individual's level of physical fitness.

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.^[2] 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."^[7]

At the nominal body temperature of 98.6 °F (37.0 °C), water has a vapor pressure of 63 hectopascals (47 mmHg); which is to say, at an ambient pressure of 63 hectopascals (47 mmHg), the boiling point of water is 98.6 °F (37.0 °C). A pressure of 63 hPa—the Armstrong limit—is about 1/16 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 shows that 63 hPa is typically found at an altitude of 63,100 feet (19,200 m).

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 63 hPA pressure level occurs (about 60,000 feet (18,000 m)), 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 hectopascals (0.91 psi) of ambient pressure. This means that for an individual with a diastolic low blood pressure, typically 80 hPa (60 mmHg), the person's blood pressure would be 143 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 60,000 feet (18,000 m) while the heart is still beating.^[2][7]

Hypoxia below the Armstrong limit

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

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 49,000 feet (15,000 m) for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins.^[8] The prompt physiological reaction when breathing pure oxygen through a face mask in an unpressurized cockpit at altitudes greater than 49,000 feet (15,000 m) above sea level is hypoxia—inadequate oxygen level causing confusion and eventual loss of consciousness. Air contains 20.95% oxygen. At 49,000 feet (15,000 m), 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 15,400 feet (4,700 m) above sea level.

Commercial jetliners are required to maintain cabin pressurization at a cabin altitude of not greater than 8,000 feet (2,400 m). 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 a half hour at an altitude above 12,500 feet (3,800 m). General aviation pilots must be on supplemental oxygen if the plane spends any time above 14,000 feet (4,300 m), and even the passengers must be provided with supplemental oxygen at 15,000 feet (4,600 m).^[9] Skydivers, who are at altitude only briefly before jumping, do not normally exceed 15,000 feet (4,600 m).^[10] Since approximately 49,000 feet (15,000 m) 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 15,000 feet (4,600 m), an altitude of 49,000 feet (15,000 m) 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 59,000 feet (18,000 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.^[11]

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 49,970 feet (15,230 m) flying a Bristol Type 138 while wearing a pressure suit. Two years later Italian military officer Mario Pezzi set an altitude record of 56,860 feet (17,330 m), 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

Notes

1. ↑ 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.

References

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”