I've seen reports of flight passengers/crew suffering hypoxia, and many times without knowing it.
My question is, how could you not be aware of it? For example, at above 10000ft, if your body is suffering from a lack of oxygen, doesn't it make you breathe faster and harder? (which in turn, makes you aware that something is wrong in the air you're breathing)
Second question: if you're dealing with air containing half as much oxygen, is it not enough (sustainable) to inhale (faster) twice as much air when breathing (without oxygen mask of course) ?