Cabin pressurization is used to create a safe and comfortable environment for aircraft passengers and crew flying at high altitude by pumping conditioned air into the cabin. This air is usually bled off from the engines at the compressor stage. The air is then cooled,humidified, mixed with recirculated air if necessary and distributed to the cabin by one or more environmental control systems.[1] The cabin pressure is regulated by the outflow valve. Source : http://en.wikipedia.org/wiki/Cabin_pressurization