fred wrote:

The question was what happens when breathing air from the mask rather than
O2 and the answer is nothing at all at a cabin pressure of 10K feet.

But the question was asked about pilots that were at 35,000'. I believe that
George B. is correct that the partial pressure would be too low and the pilots
would pass out.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.