Why do airplanes have pressurized cabins?

In 1937, the U.S. Army Air Corps began research flights in a modified Lockheed Electra; the XC-35 was the first airplane built with a pressurized cabin. ... Two years later, Boeing submitted a design to the Air Corps for a long-range bomber, the B-29 Superfortress, which would have pressurized compartments for the crew. read more

The air pressure in an aircraft(cabin pressure) is kept higher than the atmospheric pressure at high altitudes to avoid discomfort to passengers due to low outside air pressure. This process is called cabin pressurization. Air pressure within the aircraft is called cabin pressure. read more

Because the air is very thin at the altitude they fly. The average passenger jet has a causing altitude of about 30,000 to 40,000 feet. read more

To maintain the pressure in the cabin equal to that at low altitude, even while the airplane is at 30,000 feet, the incoming air is held within the cabin by opening and closing an outflow valve, which releases the incoming air at a rate regulated by pressure sensors. read more