Air travel and oxygen depletion at altitude

For most people, sitting on an airplane is just like sitting on a bus or a train. The view from the window might be different, but the air you breathe is the same. Except it isn’t. Cabin air at altitude contains 25% less oxygen than it does at ground level. That’s a sizeable reduction that savvy fliers need to be aware of. Why the reduction? Atmospheric pressure drops as altitude increases. So as an airplane ascends a pressure difference develops between the cabin air and the air outside, which.