Answer:
In higher elevation, there is the presence of lesser molecules of air above a given surface in comparison to the similar surface at the lower levels. The majority of the molecules or atmosphere are held close to the surface of the Earth due to the gravitational forces, and thus, air pressure declines briskly at the initial stage, and then more gradually at the higher levels.
Thus, if the pressure in an airplane cabin does not change, then it can even result in harm or injury to the passengers and there will be a requirement of oxygen equipment at the height of 8000 meters.
