Air Force calculations suggest that at 8,000 ft the oxygen available beneath the lens may be 35 percent less than that available at sea level at 12,000 ft, oxygen availability may be 60 percent less (O'Neal, 1990). Since the amount of oxygen passing through a contact lens is directly related to the partial pressure of oxygen in the surrounding air, the amount of oxygen available under a contact lens will also diminish at higher altitudes. Again, this pressure differential is held as the craft ascends, resulting in a cabin pressure equivalent to an altitude of 3,500 ft at 30,000 ft and cabin pressure equivalent to an altitude of 8,000 ft at 43,000 ft.Īll things considered, aircrews spend much of their flying time at pressure-equivalent altitudes of 8,000 to 16,000 ft At these altitudes, substantially less oxygen is available relative to sea level. In tanker-transport-bomber-type aircraft, a sea-level atmosphere can often be maintained in the cabin until the pressure difference between the cockpit and outside is about 8.6 psi-at roughly 23,000 ft.

For example, at 30,000 ft, the cabin pressure is equivalent to an altitude of 12,000 ft at 40,000 ft, the cabin pressure is equivalent to an altitude of about 17,000 ft (O'Neal, 1990). This pressure differential is maintained as the aircraft flies higher. The pressure is then held constant until the difference between cockpit pressure and outside pressure is 5 psi-at about 23,000 ft. Generally, high-performance (fighter-attack-reconnaissance) aircraft remain unpressurized to about 8,000 ft (10.9 psi). However, this ability to artificially pressurize the cabin is limited by aircraft engine size and at high altitudes can only lessen, not eliminate, the effects of the thinning atmosphere. To offset these high altitude effects, aircraft cabins are routinely pressurized to mimic lower altitudes. As the concentration of air thus decreases with altitude, so does the concentration of oxygen (measured as its partial pressure) available to crew members and their eyes. Air pressure at 30,000 ft is 4.36 psi, just 30 percent of the ambient pressure at sea level (14.7 psi) at 40,000 ft, air pressure is only 2.72 psi, just 18 percent of that at sea level. Military aircraft frequently fly at altitudes, ranging from 30,000 to 40,000 ft, at which air pressures are greatly reduced from those at sea level.

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