A pressurized aircraft cabin maintains a higher pressure in the cabin than that of the atmosphere found outside at altitude. It does this by pressurizing the cabin. Many aircraft can maintain a cabin altitude of 8,000 feet at the cruising altitude of the aircraft. At 8,000 feet, passengers and crew do not require oxygen to breathe, even though the aircraft may be flying at an altitude that would normally require oxygen use. The pressure differential, in pounds per square inch, is the difference between the higher cabin and lower outside pressure.