Hypoxia and the time of useful consciousness

(Human Physiology and High Altitudes)

Hypoxia is a lack of sufficient oxygen reaching the body tissues, especially the brain. It is the most important physiological hazard in the subject, and the most dangerous, for one reason: it sneaks up on you, and one of its first effects is a feeling of euphoria and well-being that destroys your ability to notice it. A hypoxic pilot typically feels like they are doing a great job. That is the trap.