I don't recall the exact altitude, but from what I remember from altitude chamber classes while in the Air Force, one has to "pressure breath" 100% O2 when the ambient pressure drops below a certain level.
Only the cockpit masks are designed to do this.
The passenger masks are not designed, nor are able, to force oxygen under pressure through the lungs and into the hemoglobin above a certain pressure level.
I seem to remember that the critical level below which the passenger masks start to become effective is somewhere around 30,000 ft. (could be mistaken)
Last edited by wanabee777; 12th Mar 2016 at 08:32.