Oxygen above 20,000 feet

Most people I know seldom go very high in their turbo airplanes -- 02
is just too big a hassle.
Try to buy it on Sunday. Many places require a mechanic to fill.

So, with you experts on board here, I want to investigate this cannula
business a bit.

Using a mountain high pulse system, with a rebreather cannula at 18,000
feet, I easily get measured
low 90s saturation and it is still using only brief pulses on each
inhalation.
BTW, this thing really saves O2 & you feel good.

So the simple question is: So long as I climb higher and still
maintain the measured 02 saturation,
what's the harm in using the cannula? Why not at 210?

What else don't we know?

Bill Hale