Hypoxia is when the cells of the body do not receive enough oxygen; a person who suffers from this is said to be hypoxic.

The form of hypoxia most commonly experienced by pilots is when there is not enough oxygen in the lungs, or when the lungs are unable to transfer oxygen in sufficient amounts to the bloodstream.1  Here is a description of the condition:

In all (cases) . . the net effect is the same—reduced oxygen to the body, more importantly to the brain and eyes, causing a reduction in performance capability.  As hypoxia increases, you become less and less able to function properly both mentally and physically.  Mentally, as less oxygen reaches the brain, your thinking becomes confused and you are less able to make good judgement calls.  Physically, your body increases its respiration in an attempt to get more oxygen.  You may also start to feel dizzy and nauseous and perhaps get a headache.  You also start losing motor-skill co-ordination and, in extreme cases, may pass out completely . .     Hypoxia is an insidious problem in aviation; its effects creep up on pilots without their knowing it.  Compounding the problem is the fact that one of the symptoms is a feeling of well-being; not only does hypoxia impair your ability to fly well, but it also makes you feel good at the same time.  How you perceive your performance may be quite different from how everyone else sees it (Transport Canada’s Human Factors for Aviation—Basic Handbook, Pp. 46-47).


   Times of Useful Consciousness

10,000 ft.


20,000 ft.

   5 to 12 minutes

30,000 ft.

   45 to 75 seconds

40,000 ft.

   3 to 30 seconds

45,000 ft and above

   12 to 15 seconds or less


Hypoxia is practically measured by the time at which a person can maintain useful consciousness—function with reasonable competence.  As you can see in the table to the right, performance ability decreases rapidly with altitude.

The more physically active you are at altitude, the shorter the time of useful consciousness.

Smokers have shorter useful consciousness time—an altitude of 5000’ the symptoms and effects for a smoker are equivalent to those experience by a non-smoker at 10000’.

1 This is referred to as hypoxic hypoxia, while a second form of hypoxia—anemic hypoxia—is when there is sufficient oxygen in the lungs, but the blood is unable to distribute it to the body in sufficient quantities.  Anemic hypoxia occurs in carbon monoxide poisoning.