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The cabins of commercial aircraft are maintained at an air pressure equivalent to a moderately high altitude on land—up to about 8000 ft (2438 m). Because it is possible to get acute mountain sickness at or even below this altitude, the Boeing Company in collaboration with a US university did a study of acute mountain sickness during simulated long haul flights to assess the risk to passengerspassengers.
Volunteers took 20 hour “flights” in a hypobaric chamber pressurised to the terrestrial equivalent of 650, 4000, 6000, 7000, or 8000 ft. The volunteers' oxygen saturation fell significantly as cabin altitude increased—those at 8000 ft dropped their saturation by a maximum 4.4 percentage points. But air pressure had no measurable impact on the risk of acute mountain sickness, which affected 7.4% (37/502) of volunteers overall.
Those at simulated altitudes (lower air pressures) of 7000 or 8000 ft were significantly less comfortable than the other groups, reporting more malaise, muscle discomfort, and fatigue. The differences appeared after three to nine hours in the hypobaric chamber.
Aircraft cabins are kept at low pressures to save energy and increase fuel efficiency, say the authors. Higher pressures won't prevent mountain sickness, but they could make flying more comfortable.