Hypoxia

Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. In aviation it is most commonly caused by exposure to reduced partial pressure of oxygen at altitude (hypoxic hypoxia), but it can also result from carbon monoxide poisoning displacing oxygen in the blood (hypemic hypoxia), impaired circulation (stagnant hypoxia), or the body's inability to use available oxygen, as with alcohol or certain drugs (histotoxic hypoxia).

Early symptoms are insidious and easy to miss: they include a false sense of well-being, poor judgment, drowsiness, and mild euphoria, well before more overt signs like cyanosis (blue lips and fingernails) or loss of consciousness appear. This is precisely why hypoxia is dangerous — the pilot experiencing it often does not recognize the impairment.

The FAA sets specific supplemental oxygen requirements under 14 CFR 91.211 tied to cabin pressure altitude and duration of exposure, and knowledge tests expect candidates to know both the categories of hypoxia and the regulatory thresholds at which oxygen becomes required.