Well, from what I know..
In high elevations, there are fewer air molecules above a given surface than a similar surface at lower levels. Most of the atmospheres molecules are held close to the earth's surface by the force of gravity and hence air pressure decreases rapidly at first, then more slowly at higher levels.
Sooooo if the pressure in an airplane cabin isn't changed, it can cause injury or harm to passengers and oxygen equipment is required above 8,000 altitude.
