Here is a wikipedia quote that better tells it.
Aircraft[edit]
In airliners, cabin altitude during flight is kept above sea level in order to reduce stress on the pressurized part of the fuselage; this stress is proportional to the difference in pressure inside and outside the cabin. In a typical commercial passenger flight, the cabin altitude is programmed to rise gradually from the altitude of the airport of origin to a regulatory maximum of 8,000 ft (2,400 m). This cabin altitude is maintained while the aircraft is cruising at its maximum altitude and then reduced gradually during descent until the cabin pressure matches the ambient air pressure at the destination.[citation needed]
Keeping the cabin altitude below 8,000 ft (2,400 m) generally prevents significant hypoxia, altitude sickness, decompression sickness, and barotrauma.[5] Federal Aviation Administration (FAA) regulations in the U.S. mandate that under normal operating conditions, the cabin altitude may not exceed this limit at the maximum operating altitude of the aircraft.[6] This mandatory maximum cabin altitude does not eliminate all physiological problems; passengers with conditions such as pneumothorax are advised not to fly until fully healed, and people suffering from a cold or other infection may still experience pain in the ears and sinuses.[citation needed] The rate of change of cabin altitude strongly affects comfort as humans are sensitive to pressure changes in the inner ear and sinuses and this has to be managed carefully. Scuba divers flying within the “no fly” period after a dive are at risk of decompression sickness because the accumulated nitrogen in their bodies can form bubbles when exposed to reduced cabin pressure.
The cabin altitude of the Boeing 767 is typically about 7,000 feet (2,100 m) when cruising at 37,000 feet (11,000 m).[7] This is typical for older jet airliners. A design goal for many, but not all, newer aircraft is to provide a lower cabin altitude than older designs. This can be beneficial for passenger comfort.[8] For example, the Bombardier Global Express business jet can provide a cabin altitude of 4,500 ft (1,400 m) when cruising at 41,000 feet (12,000 m).[9][10][11] The Emivest SJ30 business jet can provide a sea-level cabin altitude when cruising at 41,000 feet (12,000 m).[12][13][unreliable source?] One study of eight flights in Airbus A380 aircraft found a median cabin pressure altitude of 6,128 feet (1,868 m), and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of 5,159 feet (1,572 m).[14]
Before 1996, approximately 6,000 large commercial transport airplanes were assigned a type certificate to fly up to 45,000 ft (14,000 m) without having to meet high-altitude special conditions.[15] In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin pressure specifications for new-type aircraft designs. Aircraft certified to operate above 25,000 ft (7,600 m) “must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 ft (4,600 m) after any probable failure condition in the pressurization system”.[16] In the event of a decompression that results from “any failure condition not shown to be extremely improbable”, the plane must be designed such that occupants will not be exposed to a cabin altitude exceeding 25,000 ft (7,600 m) for more than 2 minutes, nor to an altitude exceeding 40,000 ft (12,000 m) at any time.[16] In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of 40,000 ft (12,000 m) on the majority of newly designed commercial aircraft.[17][18] Aircraft manufacturers can apply for a relaxation of this rule if the circumstances warrant it. In 2004, Airbus acquired an FAA exemption to allow the cabin altitude of the A380 to reach 43,000 ft (13,000 m) in the event of a decompression incident and to exceed 40,000 ft (12,000 m) for one minute. This allows the A380 to operate at a higher altitude than other newly designed civilian aircraft.[17]
