Originally Posted by Milt
Ever since I have tried to determine the minimum blood oxygen percentage which will just keep an average human conscious. It seems to be about 65 to 70 %.
You may find
this paper which was prepared during consideration of construction of ALMA, a very high-altitude observatory, to be helpful. In particular the graph listed as figure 1 in section 2.1 appears to summarize the conventional wisdom on the question you raise.
Everest climbers are physiologically different from the rest of us. They find Everest Base Camp a recuperation site. For not a few of us it would be rather rapid death.
[disclosure: I'm not a pilot, but I did take an course in cardiovascular pathophysiology some years ago]