Some of you guys picked up on my comment that the FAA requires a "positive
pressure" mask above 18K...

One person may be
>  fine with a cannula up to 35,000ft; another might not make it much
> past  18,000ft -- the altitude at which the FAA requires one to wear a
> mask  providing positive pressure.

Mr. Simon pointed out that the masks usually found in light aircraft don't
seal and don't provide positive pressure.

You guys are right, I misspoke. The masks DON'T provide positive pressure
because they don't seal. But, they do ensure the MIXTURE you're breathing
contains a higher percentage of oxygen than would be provided by a cannula.
The regulator is constantly pushing oxygen into the mask and the bag
attached forcing ambient air out, so at least the volume of the mask and bag
is 100% oxygen.

100% oxygen will delay the point at which the dropping pressure results in
hypoxia, but eventually you WILL reach an altitude where the ambient
pressure no longer forces a gas exchange in the lungs. Not even 100% oxygen
will help at that point.
 
So at what altitude does the reduced efficiency of the gas exchange result
in dangerously low blood oxygen? Again, everyone is different. If you climb
mountains regularly or live in Breckenridge, CO (around 12K MSL IIRC) you
may not need ANY supplemental oxygen until well above 18K. If you're a
little older and a little out of shape -- with the resultant loss in aerobic
efficiency -- you may want to give yourself every chance you can to remain
conscious.

The bottom line in my book -- the mask is necessary above 18K, eschew its
use only if you have a good reason and know why you're doing so. If you plan
to fly regularly above 18K with only a cannula I highly recommend the use of
an oximeter on EVERY flight. Don't use it one time and think you have it
licked. Blood/oxygen efficiency is affected by MANY factors and may be very
different from one flight to the next.

Again, sorry for the misstep.

Mark Sletten