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 49,967 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.
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