You essentially won't be getting the oxygen. In reality - those yellow masks are borderline ineffective anyway. You basically need an airtight seal around your

You essentially won't be getting the oxygen. In reality - those yellow masks are borderline ineffective anyway. You basically need an airtight seal around your entire nose and mouth otherwise the pure O2 will simply vent away. But at the altitudes that commercial aircraft fly at, it is better than nothing. Below about 25,000 feet they provide a little more time before onset of hypoxia, but above 35,000 they would be useless. Best bet if you are at higher altitudes is to hold the mask on as tight as possible and try to eliminate any gaps. If you notice the oxygen masks that the pilots use (and that fighter pilots use), they are a very different beast, and have secure latching mechanisms to hold the mask tightly against your face with NO gaps at all. (Also a reason why beards are banned for flight crew in the air force - can't have ANY gaps where the O2 can leak out). My old flight instructor used to fly jets in the Air Force back in the 50's and 60's, when the cockpits were unpressurised, and he said that at altitudes around 60,000+ feet, if your oxygen mask failed, you had about 1 second of consciousness. In fact, he said at those altitudes, the pressure differential was so great that your diaphragm actually couldn't expand to enable normal breathing, and they used to have liquid oxygen pressure fed through their masks. If they wanted to breath, they just opened their mouth and freezing cold LOX (which vaporised almost instantly at that altitude) would be rammed down their gullets...