Histotoxic Hypoxia refers to hypoxia specifically caused by toxins in the blood that interfere with the ability of hemoglobin to absorb oxygen even in the presence of sufficient quantities and at normal atmospheric pressure (Jepperson, 2007; USDOT,

2003). In that regard, alcohol is the most likely toxin to affect pilots, but other poisonous substances like cyanide and certain narcotics and other medications (including some sold over-the-counter) can also cause histotoxic hypoxia. Finally, Stagnant Hypoxia refers to insufficient oxygen absorption caused by underlying circulatory problems that reduce blood flow, and therefore, the efficient transport of oxygen, even where the quality of air, atmospheric conditions, and oxygen absorption by hemoglobin are normal (Jepperson,

2007; USDOT, 2003).

Signs and Symptoms of Altitude-Induced Hypoxia:

One of the most dangerous aspects of all forms of hypoxia is that its onset is not noticed by the pilot. Another danger is that while hypoxia severely reduces physical and cognitive capabilities necessary to execute in-flight procedures, it also produces a corresponding confidence in one's abilities and even a giddiness that in many ways is similar to alcoholic intoxication (Reinhart, 2008; USDOT, 2009). Moreover, because hypoxia affects different body functions in very subtle ways, a lone pilot has very little chance of recognizing hypoxia onset once it occurs.

Below 10,000 feet, the symptoms of hypoxia are so mild that many recreational pilots fail to realize that their ability to read gauges accurately and monitor landscapes is already reduced significantly, especially over relatively long flights. This is very easily demonstrated with dramatic effect by conducting an extended (i.e. several hours) "low level" un-pressurized flight above 5,000 but well below 10,000 feet at night. After noting the visible landmarks to the best of his ability, the pilot is presented with a pressurized oxygen bottle and allowed to breath from it for a few minutes, after which the pilot is asked to take note once again of all visible landmarks.

Invariably, pilots are completely astonished to see how many additional features they recognize after concentrated oxygen inhalation that they missed previously. Reduced night vision is the first physiological effects of hypoxia because the retinas of the eye require more oxygen than any other human tissue, followed by the brain (Reinhart, 2008).

At 10,000 feet, the reduction in night vision can reach twenty-five percent and rational judgment becomes significantly impaired, whereas the pilot's confidence in his abilities remains absolutely unaffected...

daytime) vision also begins to degrade. Several thousand feet higher is considered the absolute cut-off point above which any longer than momentary un-pressurized exposure to atmospheric conditions results in serious physical and mental incapacitation that mirrors many of the effects of extreme alcohol intoxication, including euphoria, laughter, irrationality, belligerence, and spacial disorientation. By an altitude of 18,000 feet, the pilot virtually completely incapable of any rational function altogether, while still maintaining the characteristic confidence in his abilities. At 20,000 feet, unconsciousness follows within less than fifteen minutes and at 25,000 feet, the onset of unconsciousness occurs in less than five minutes (Reinhart,
2008; USDOT, 2009).

Altitude-Induced Hypoxia Prevention:

About the only possible way of preventing altitude-induced hypoxia is to understand the relationship between altitude and the physiology of oxygen absorption in the human body. The understanding of the relevance of prolonged exposure to specific altitudes enables pilots of un-pressurized aircraft to avoid subjecting themselves to potential hypoxia by carefully monitoring their altitude in relation to flight times

(Reinhart, 2008; USDOT, 2009). Similarly, the awareness of the contributing effects of exposure to cigarette smoke and of the consumption of alcohol and narcotics or other medications (even when used for authorized medical purposes) allows pilots to avoid reducing their relative tolerance to the potential effects of altitude-induced hypoxia.

Ultimately, because one of the symptoms of hypoxia is failure to perceive danger and unaffected confidence in one's abilities, the only genuine method of avoiding altitude-induced hypoxia remains scrupulously adhering to responsible flight plans and avoiding irresponsible exposure to altitudes where its onset is known to occur, especially at what are traditionally considered to be "low altitudes."