Peter Duniho wrote:

When they do that, they are using up a small reserve of oxygen-filled air
contained in their lungs. Exhale as much air from your lungs as you can,
and THEN see how long you can hold your breath.

Even then, there's still a lot of oxygen in the lungs, the human lung is
pretty inefficient. What happens at altitude is that you don't feel the
absence of oxygen, hence just continue to breath normally. This empties
your oxygen reserves rather quickly.

On airliners, though, there are instruments which alarm the pilot of
pressure loss and drop the mask immediately. So there must have been
another problem.

Stefan