There is a 'cheat' where one can got to 12 for 30 minutes. But the
brain needs O2 to function properly. With hard training one can
function adequately at higher altitudes.

Pensacola. Low pressure tank. 30,000 feet pressure. Instructor asks for
volunteer. (I tend to do dumb things) I said OK. "Please take this pencil
and write your name on this clipboard." "How many times?" "Don't worry. I'll
tell you when to stop."

Off comes mask. I write and I write and at the 4th perfect signature I stop --
wondering why am I doing this. "Hell, I can do this for the rest of my life."
(30,00 feet. Yeah) Instructor slams mask back on. My vision instantly
changed from looking down a soda straw to wide open, COLOR!, side vision. Wow!
Last signature was a straight line. (Hmmmm. Looked OK at the time.)

At 30K, one has about 4 seconds of useful consciousness -- unless one holds
one's breath. That may net one a couple of seconds more. No pain. No strain.
Quite pleasant. Been there. Done that. Fast and almost fun.

Humans need a constant supply of oxygen and we store almost none. Lack of
oxygen doesn't hurt. What hurts and gives the feeling of strangulation is an
overabundance of carbon dioxide. Want to know what too much CO2 feels like?
Hold your breath for 3-4 minutes.

Throw in a little exhaust gas and the ceiling easily drops to 20K or lower.

Quent