In simple terms, hypoxia is the result of insufficient oxygen in the blood stream. Many are prone to associate hypoxia only with flights at high altitude. While it is true that there is a progressive decrease of oxygen with an increase in altitude, there are many other conditions or situations which can and do interfere with the blood's ability to carry oxygen to the brain. Many drugs, alcohol, and heavy smoking will either diminish the blood's ability to absorb oxygen or the ability of the brain to tolerate hypoxia.

Because of wide individual variations in susceptibility to hypoxia, it is impossible to predict exactly when, where, or how hypoxia reactions will occur in each pilot. As a general rule, however, flights below 10,000 feet MSL without the use of supplemental oxygen can be considered safe, though night vision is particularly critical, and impairment of sight can occur at lower altitudes - especially for heavy smokers. The onset of hypoxia is insidious and progresses slowly. Impaired reactions, confused thinking, poor judgment, unusual fatigue, and dull headaches are typical reactions. Sometimes there is a sense of "well being" characterized by high spirits or by the feeling that "things could not be better" (euphoria), but the individual thus involved may have little if any insight into his or her actual condition. Because of this and because many pilots never experience this false, happy, carefree feeling, it is not always a symptom one can rely upon to warn of an active or incipient hypoxic condition.