What happens if an airplane cabin depressurizes?

An airliner's cabin is kept at a pressure equivalent to between 1,800 and 2,400 m (6,000-8,000 ft) of altitude, even though the aircraft flies at 11,000. If that pressure is lost, the cabin air equalizes with the outside. In a rapid decompression a brief fog appears from condensing water vapor, along with a loud noise, a blast of cold and pressure in the ears.

The immediate danger is hypoxia. Time of useful consciousness—the seconds during which a person retains the ability to act effectively—drops sharply with altitude: at 12,000 m (40,000 ft) it's just 15 to 20 seconds. That's why crews insist on putting on their own mask before helping anyone else: someone who delays to help another may lose the ability to do both.

Pilots have positive-pressure masks within reach, designed to be put on in under five seconds. After securing their own oxygen, they start an emergency descent: reducing thrust, extending speed brakes and descending at the maximum permitted speed down to 3,000 m (10,000 ft), an altitude at which ambient air is enough to breathe. The descent can be completed in a few minutes.

Passenger masks aren't connected to a shared tank: on most aircraft each panel has a chemical generator that produces oxygen when the mask is pulled, for a limited period covering the time needed to descend. That's why you need to pull the tube firmly, not just bring the mask close to your face. The aircraft itself is designed to fly without problems at low altitude; decompression forces a landing at the nearest suitable airfield, but it doesn't compromise the structure.