Why are oxygen masks on airplanes? --- Michael, Toronto, Canada
Marshall Brain Answers:
A typical airliner cruises at an altitude of 30,000 to 35,000 feet. The problem is that it is impossible for humans to survive at this altitude because of lack of oxygen. The oxygen is there, but the air pressure is so low that it can't get into the bloodstream. This altitude calculator shows exactly what is happening:
On the following page you can see how long a person can function at different altitudes without supplemental oxygen:
At 30,000 feet, it's just a minute or two.
This is why the aircraft cabin is pressurized. It allows people to breathe normally even though the conditions outside the plane are lethal.
But what if a window cracks? All the pressure leaks out, and you are suddenly breathing the low-pressure air at 30,000 feet. I have been on a flight where this happened. The overhead masks immediately pop down, and the pilot begins taking the plane down to a safer altitude. What the mask does is allow you to breathe pure oxygen (compared to the 20% oxygen and 80% nitrogen air we normally breathe), which gets enough oxygen into the bloodstream so that you do not pass out.
Where does the oxygen for the mask come from? See: How does an oxygen canister on an airplane work?