At what altitude does a human being need to go onto oxygen?

18000Ft

In aviation the passenger cabin pressure is the equivalent to being at8000feet, passengers would need to go onto oxygen (if the aircraft became unpressurised) at12000feet and the Flight Crew at10000feet. People can survive above these altitudes, for example mountain climbers in the Himalayas frequently climb to higher levels but they have to undergo periods of aclimatising at regular intervals before and during their ascent.