Air pressure in a airplane

Answer 1

As an airplane climbs higher in the atmosphere, air pressure, both outside and inside an unpressurized airplane, decreases. Reduced air pressure causes less oxygen to be forced through membranes in the lungs and into the blood stream of crew and passengers, resulting in oxygen deprivation or "hypoxia". Use of a higher concentration of oxygen can counter act this effect to an altitude where use of even 100% oxygen will not provide enough oxygen to pass into the blood stream. Use of a pressurized mask or cabin pressurization is required.

Pressurized aircraft pump outside air into the cabin by use of a turbocharger or supercharger in piston powered airplanes or by directing compressed air from the front, compressor stage, of a turbine (jet) engine. An outflow valve constricts the flow of air escaping from the cabin, resulting in a higher cabin air pressure. On commercial jets, the cabin is generally restricted to a pressure that equates to an altitude of 8,000 feet above sea level, or lower, while at cruise altitude.