Bucky wrote:

I don't get it. Can't a person stay conscious for longer than 30
seconds without breathing? Most people can hold their breath for over
a minute.

If the aircraft loses pressure at 35,000', you cannot hold your breath. The
pressure difference will force you to exhale. Nothing you can do about it.

But that's not the biggest deal. The biggest problem is the fact that the
ambient pressure at that altitude is too low to keep oxygen in your bloodstream.
All the oxygen immediately begins to flow out of your bloodstream into your
lungs and out into the surrounding air. It doesn't matter whether you breathe
or not.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.