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prediction in 1917, was not described until the early 1930s. Indeed altitude decompression sickness was not widely
recognised until the last two years of the third decade of the last century (Armstrong, 1939). Paul Bert (1878) not only
proved that the hypoxia induced by breathing air at altitude was due to the fall of the partial pressure of oxygen in the
inspired gas, but also conducted the first demonstrations using human subjects that the altitude hypoxia can be
prevented by raising the concentration of oxygen in the gas breathed. Furthermore Paul Bert must also be recognised as
the first to propose that pressurisation of the crew compartment of an aerial vehicle would protect the occupants against
the effects of exposure to high altitude. Thus the methods by which the occupants of aircraft flying at altitude could be
protected against the effects of exposure to low environmental pressure had been proposed before the end of the 19 th
century and indeed before the first flight of a heavier than air machine.
The first two decades of the last century spurred by the increasing performance of aircraft saw the firm establishment
both in the laboratory (in hypobaric chambers) and in flight of the basic requirements for supplemental oxygen
(Haldane 1920). By the 1930s the requirements for full pressure suits to prevent hypoxia at altitudes above 40,000 feet
had been recognised and during the third decade full pressure suits were worn by aviators attempting high altitude
records in open cockpit aircraft. The earliest exposure to very high altitude (84,000 feet) whilst wearing a full pressure
suit inflated with oxygen was conducted in a hypobaric chamber by Haldane in 1933 (Haldane and Priestly 1935).
Whilst positive pressure breathing had been employed to treat a variety of clinical conditions its use to prevent
significant hypoxia breathing oxygen at altitudes between 40,000 and 50,000 feet was exploited by Gagge (Gagge et al
1945) in the United States and by Bazett initially in Canada and later in the United Kingdom (Bazett and Macdougall,
1942). The development of partial pressure suits employing pressure helmets or pressure demand masks to provide
protection at altitudes between 40,000 and 100,000 feet occurred principally in the twenty years or so following World
War II (Jacobs & Karstens, 1948; Ernsting, 1966). The last twenty years has seen the further development of improved
partial pressure suit assemblies in the United States and Canada (Shaffstall et al, 1995; Holness et al, 1980; Goodman et
al, 1993) and in the United Kingdom (Gradwell, 1991) for the new generation of agile combat aircraft.
Whilst the concept of pressurising the cabin of an aircraft so that the occupants are not exposed to the environmental
pressure at altitude is older than powered flight itself, the first attempt to do so did not occur until 1921 when the US
Army Air Corps conducted a test flight in which the pilot was enclosed in a tank which was pressurised with air. The
flow capacity of the discharge valve fitted to the tank was, however, totally inadequate so that during flight at 3,000
feet, the pressure within the tank increased to the equivalent of 7,000 below sea level. The pilot suffered severe otitic
barotrauma and the temperature within the cabin rose to 66ºC. This failure of engineering design delayed further
attempts in the United States at cabin pressurisation. Aircraft fitted with experimental pressure cabins were, however,
developed and flown during the early 1930s by several European nations, including Germany and France. The parallel
concept of the sealed cabin pressurised with oxygen carried with the vehicle was, however, developed and exploited by
high-altitude balloonists such as the Belgian Piccard who ascended to an altitude of 51,000 feet in 1931, and Stevens
and Anderson in the United States who ascended to an altitude of 72,395 feet in 1935.
The early attempts to pressurise the crew compartment of an aircraft revealed the major factors that had to be taken into
account in producing an acceptable pressure environment for the occupants. It was recognised that the absolute pressure
to be maintained within the cabin during flight was a function of the physiological effects of altitude, specifically
hypoxia, and whether the occupants would be using supplemental oxygen. The earliest studies had shown the
importance of adequate control of the differential pressure of the cabin. It was also recognised that the strength of the
pressure-holding structure was a vital consideration, both with regard to the integrity of, and the increased weight
penalty imposed by, the pressure cabin. The possibility of a sudden failure of the pressure cabin in flight was
considered and led to the extensive studies of rapid decompression performed in the late 1930s and the 1940s. The
importance of adequate control of the ventilation of the pressure cabin and the temperature within it was established.
The fundamental aeromedical requirements for the pressure cabin were specified in the classic report by Armstrong in
1935. These requirements were embodied in the design of the XC-35 sub-stratosphere airplane built by Lockheed for
the US Army Air Corps. This aircraft completed a very successful flight test programme in 1937 which provided a firm
basis for the pressurisation of the crew and passenger compartments of future aircraft.
World War II saw the development and introduction into service in the United States and the United Kingdom of
fighter and bomber aircraft equipped with pressure cabins. The requirement for minimum aircraft mass and the
likelihood of rapid decompression of the cabin due to enemy action led to the adoption of a low pressure differential for
fighter aircraft – typically of the order of 2.0 to 2.75 Lb in -2 , whilst the advantages of being unencumbered with oxygen
equipment throughout flight led to the selection of a pressure differential of 6.5 to 7.5 Lb in -2 for bomber aircraft. The
aeromedical requirements which determined the pressurisation schedules of the military aircraft constructed during
World War II were well summarised by Lovelace and Gagge (1946). These set the maximum cabin altitude without
supplemental oxygen at 10,000 feet [5,000 feet for night vision] and the maximum allowable cabin altitude to avoid
decompression sickness (aeroembolism) at 25,000 to 30,000 feet. The use of a limit to the Relative Gas Expansion