O. Sami Saydjari wrote:
Yes, I think so. So if I use a mask, then is there some upper limit?
What am I misunderstanding here?

As the ambient air pressure decreases, you need to increase the percentage of
oxygen in the air you breath to get the same amount of oxygen into your blood.
The nasal canulae will do this up to a point.

Once you get up to about 20,000', the ambient air pressure is so low that the
canula just can't do the job. You need to go with a full mask. Even there, the
less expensive low pressure masks are only good for something like another
8,000'. Then you need a high pressure mask (such as the military used in WWII).
Those will work up to about 35,000'. Above that, you need either a pressure suit
or a pressurized cabin.

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.