Hypoxia is oxygen starvation. Lack of oxygen impairs the whole body but most importantly the brain. The first part of the body to show significant effect from oxygen deficiency is the retina of the eye. Every individual is affected but in different ways and to different degrees. The danger in hypoxia is that it occurs insidiously below the conscious threshold. Hypoxia makes you happy and such happiness in the cockpit is very dangerous. The best warning indicator for hypoxia is the altimeter. You will quickly recover by descent to a lower altitude.

Since hypoxia is due to reduced barometric pressure, low-grade hypoxia begins on takeoff. The percentage of oxygen is same but less is reaching the blood stream. Any stress or increase in activity requires more oxygen, up to 8 times more. Pilot performance deterioration begins at takeoff, as well. Slowed response times and inability to deal with complexities due to hypoxia compromise safety. Noticeable oxygen deficiency effects begins at 4000’ safety margins are beginning to erode. Hypoxic symptoms of difficulty breathing or headache may not be obvious or may not occur at all even though there are the foregoing changes in mental status.

I have seen complete personality changes occur after a couple of hours around 12,000’. Symptoms such as headache, drowsiness, dizziness, euphoria, tingling, perspiration, or belligerence are typical. Tunnel vision and blue fingernails occurs with times as little as 15 minutes above 15,000’. At 16,000’ disorientation, lapses of judgment, loss of impulse control, risk-taking behavior, decreased problem solving abilities, impaired memory, mood disturbances, and lowered coordination are common. Unconsciousness occurs in 10 minutes at 20,000’.

All effects are made worse and happen at lower altitudes with fatigue, age, smoking, health habits, and drinking. Oxygen recommended above 10,000 day and 5,000 night. If oxygen is being used, the pilot must be knowledgeable about the operation of the system and be able to recognize his and the system’s warnings of oxygen deficiency. FARs require oxygen if ½ hour above 12,500’, crew above 14,000’, everybody above 15,000’.

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