Armstrong limit

At or above the Armstrong limit, exposed body fluids such as saliva, tears, urine, and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away without a full-body pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes.[1] 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 subject later reported that ... his last conscious memory was of the saliva on his tongue beginning to boil."[7]

At the nominal body temperature of 37 °C (99 °F), water has a vapour 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 (99 °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). At higher altitudes water vapour from ebullism will add to the decompression bubbles of nitrogen gas and cause the body tissues to swell up, though the tissues and the skin are strong enough not to burst under the internal pressure of vapourised water. 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).

Mario Pezzi, a pilot in 1937 wearing a pressure suit to avoid hypoxia

Well below the Armstrong limit, humans typically require supplemental oxygen in order to avoid hypoxia. For most people, this is typically needed at altitudes above 4,500 m (15,000 ft). 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 (non-airline, non-government flights) require that the minimum required flight crew, 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). The minimum required flight crew 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).[8] Skydivers, who are at altitude only briefly before jumping, do not normally exceed 4,500 m (15,000 ft).[9]

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

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.[10] 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 Caproni Ca.161bis biplane even though he was well below the altitude at which body-temperature water boils.

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.[11] In an unpressurized cockpit at altitudes greater than 11,900 m (39,000 ft) above sea level, the physiological reaction, even when breathing pure oxygen, is hypoxia—inadequate oxygen level causing confusion and eventual loss of consciousness. Air contains 20.95% oxygen. At 11,900 m (39,000 ft), breathing pure oxygen through an unsealed face mask, one is breathing the same partial pressure of oxygen as one would experience with regular air at around 3,600 m (11,800 ft) above sea level. At higher altitudes, oxygen must be delivered through a sealed mask with increased pressure, to maintain a physiologically adequate partial pressure of oxygen. If the user does not wear a pressure suit or a counter-pressure garment that restricts the movement of their chest, the high pressure air can cause damage to the lungs.

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.[12]

• Altitude sickness – Medical condition due to rapid exposure to low oxygen at high altitude
• Death zone – Altitudes above about 8,000m (26,000 ft)
• Decompression (altitude) – Reduction in ambient pressure due to ascent above sea level
• Decompression illness – Disorders arising from ambient pressure reduction
• Effect of spaceflight on the human body – Medical issues associated with spaceflight
• Kármán line – Proposed definition for the boundary of outer space

• "Chapter 1:Physiology of Flight". US Naval Flight Surgeon's Manual (PDF).
• "Ebullism at 1 Million Feet: Surviving Rapid/Explosive Decompression". Archived from the original on 2000-09-19.
• "Air Pressure and Altitude above Sea Level". The Engineering ToolBox. Archived from the original on 2016-11-09.