With slow oxygen deprivation, like an airplane climbing, you are not aware of it for a long time. My grandpa, a WWII veteran who was a waist gunner in a B-24, said one time the other waist gunner fell down on the floor and turned blue in the face. Ice had accumulated in the oxygen tube, so he wasn't getting enough oxygen. When my grandpa went to switch his oxygen mask, he didn't know why my grandpa was messing with him. He wasn't even aware something was wrong.