Dredging the old memory cells from military times. My understanding is that at about 35,000' 100% oxygen will be equivalent to the partial pressure of oxygen in the air at 10,000'. That is liveable, but you wouldn't want to start doing any vigorous exercise. Above that level, pressure breathing is required, and that is satisfactory up to about 50,000' ( I never flew that high, so stand to be corrected). After that, a pressure suit is required. I think draglift's figures are about correct. In the pressure chamber at 40,000+, the removal of the mask caused almost instantaneous loss of critical faculties.