Have you ever had a weird feeling in your ears when you're driving up or down a hill? This is a result of different air pressures at different altitudes. As you go higher up the air pressure decreases which means that the air becomes less dense. When you go up a hill the air pressure outside decreases but the pressure in your ear stays the same which results in discomfort. This is why your ears sometimes "pop". You can imagine how painful it would be to travel several thousand feet above the grounds. In fact, at around 25,000 feet you will pass out from lack of oxygen. In order to fix this problem airplanes use pressurized cabins. Outside air is pressurized by the jet engine and then circulates around the cabin (for a more detailed explanation watch the video).
If the cabin pressurization failed becuase a window broke then oxygen masks would deploy and the pilot would immediatley descend to a safer altitude. Without pressurized cabins passengers could suffer from hypoxia, altitude sickness, decompression sickness, barotrauma, or many other physiological problems.
1. Why do your ears pop when you go up a hill?
2. Why do people use oxygen tanks when they climb Mt. Everest?
3. Why do some pilots in the military use oxygen masks when flying their jets?