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 (1.9 inHg)) 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. The altitude is variously reported as being between 18,900–19,350 meters (62,000–63,500 feet, or about 12 miles (10 nmi)).
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 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 47 mmHg, 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 47 mmHg of ambient pressure. This means that for an individual with a diastolic low blood pressure, typically 60 mmHg, its blood pressure would be 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
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 meters 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 meters above sea level.
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 (3,810 meters). General aviation pilots must be on supplemental oxygen if the plane spends any time above 14,000 feet (4,270 meters), and even the passengers must be provided with supplemental oxygen at 15,000 feet (4,570 meters). Sky divers, 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 meters 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 49,967 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 56,850 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.
- 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.
- NAHF - Harry Armstrong
- NASAexplores Glossary on web.archive.org
- Ask an Astrophysicist: Human Body in a Vacuum
- Human Exposure to Vacuum
- Dryden Research Center: “A Brief History of the Pressure Suit”
- Aviation Week & Space Technology, July 18/25, 2011, p. 35, “Stealthy Danger: Hypoxia incidents troubling Hornets may be related to F‑22 crashes”
- 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”