Altitude sickness isn’t caused by your actual elevation, but by the low air pressure and relatively low amount of oxygen. To combat this, the cabins are sealed and pressurized to keep passengers from dealing with the lack of air.
In the case of a pressure breach (so, a hole in the plane/etc,) there are masks that drop down to supply pressurized air for breathing.
Commercial airplanes are pressurized to keep the air inside the cabin at a level that does not cause people to suffer from high altitude sickness. Most airliners maintain a pressure inside that is roughly the same as being at about 8000’ above sea level. The newest generations of airliners keep the pressure about the same as being at more like 6000’.
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