Airplanes cabin pressurization is the active pumping of air into an aircraft cabin to increase the air pressure within the cabin. Planes are pressurized as cruising altitudes are freezing and lack sufficient oxygen to breathe inside.
During rare instances the aircraft may lose cabin pressurization at high altitude. Forcing the plane into a rapid descent to 10,000 ft after a sudden loss of pressure is a standard emergency procedure. 10,000 ft is an altitude that can be tolerated without supplemental oxygen.
Wherein there is a loos in cabin pressure, from a medical point of passengers are in the risk of lack of oxygen which is compensated by the oxygen masks.
To reduce the amount of expansion and contraction that the fuselage must endure (and reduce metal fatigue) the cabins are usually regulated for an equivalent of 8,000 ft of altitude.
Cabins are pressurized by an environmental control system (ECS) using air provided by compressors or bleed air. Some aircraft, such as the Boeing 787, use electric compressors to provide pressurization. Control systems maintain air pressure equivalent to 2,500 m (8,000 ft) or below, even during flight at altitudes above 13,000 m (43,000 ft).