most "standard" masks are not sealed.. but have rebeather bags.. and are not
suited about 25,000ft

above 25,000 unpressurized you should have a military aviators style mask
with pressure regulator

BT

"Mike W." wrote in message
...

"O. Sami Saydjari" wrote in message
...
Yes, I think so. So if I use a mask, then is there some upper limit?
This seems counter-intuitive. It seems like nasal canulas deliver
oxygen more directly into the lungs (assuming you are a nose
breather)...especially if you use a pusher (a little device that detects
when you breath and pushes O2 to you...and then shuts it off when you
are not breathing). What am I misunderstanding here?

-Sami


A mask is more of a sealed system, whereas a canula allows a leak out of
your nose. With the air that thin, I would think the oxygen coming out
would
migrate towards the atmosphere a lot faster than into you.
Another thing to ponder might be the physical strength of the O2 bottle,
it
may not be safe at the reduced atmospheric pressure at higher altitudes.