Cabin pressurization is a process in which conditioned air is pumped into the
cabin of an aircraft or
spacecraft, in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually
bled off from the
gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often
cryogenic tanks. The air is cooled, humidified, and mixed with recirculated air if necessary, before it is distributed to the cabin by one or more
environmental control systems. The cabin pressure is regulated by the outflow valve.