Hypoxia is most commonly found amongst the flying community, as it is most easily contracted through high altitudes.
CORRECTION! jasonm informs me that hypoxia occurs more often in children than in pilots. I have not heard this before and am shocked I would make such a grievous error. Unfortunately I don't have any facts at this time to back up the children story. So just pretend.
What: Hypoxia is a physiological condition that occurs without warning of any kind. It is defined as a lack of sufficient oxygen in the body cells or tissues, including the brain.
As you climb in altitude, there is progressively less oxygen in the atmosphere. At 15,000 ft ASL, it will take you two breaths to receive the same amount of oxygen as you would in a single breath at sea level.
The most dangerous fact about hypoxia is that the individual afflicted is unaware that he has any symptoms. The portion of the brain that reports fatigue and lack of oxygen is the first to be affected by hypoxia, and the individual usually feels a general sense of well-being in its place. There is no pain, and there is no warning signs.
Effects: The effects of hypoxia start from euphoria and generally degrades your alertness. After a few minutes, vision is reduced, you experience confusion, inability to concentrate, impaired judgment, and slowed reflexes. Hypoxia eventually leads to unconsciousness.
The retina of the eye is considered a part of the brain, and because of its enhanced functions, it requires more oxygen than any other part of your body. At 5,000 ft ASL, the first effects of hypoxia become apparent - reduced night vision.
The full effects of hypoxia, however, do not generally kick in below 10,000 ft ASL. This is an average figure, and is dependent on your physical fitness, drinking habits, drug use, etc.
At 14,000 ft, people generally become indifferent and very euphoric. Prolonged exposure results in tremors of the hands, clouding of thought, memory and judgment errors, and cyanosis (the discoloring of fingernails).
Shock sets in and the afflicted lose consciousness within minutes at 16,000 ft.
Above 18,000 ft, death usually occurs within five minutes.
Types: There are four major types of hypoxia...
- Hypoxic hypoxia: A normal effect of altitude, this is avoided by using oxygen supplementary devices. These are built into aircraft cabins on airliners, for example, or those dangly-down face masks above your seat. The Canadian Aviation Regulations rule that oxygen must be available for all crew members and passengers above 10,000 ft at day, and 5,000 ft at night.
- Anaemic hypoxia: Caused by an over-abundance of carbon monoxide in the haemoglobin.
Carbon monoxide is colourless, odourless and tasteless. It is a product of fuel combustion and is found in varying amounts in the exhaust from any internal combusion engine. A defect in a vehicle (such as a crack or an opening of any sort) will allow some exhaust to enter the vehicle, subjecting the inhabitants to carbon monoxide.
The higher in altitude you are, the more susceptible to carbon monoxide you are.
Initial symptoms of CO poisoning are feelings of sluggishness and warmness, intense headaches, throbbing in the temples, ringing in the ears, dizziness, and dimming of vision. If left untreated, will eventually result in vomiting, convulsions, coma, and death.
- Stagnant hypoxia: Caused by the brain not receiving enough blood. This usually occurs when the heart doesn't have enough power to pump to the brain, usually during high G-forces.
The first symptoms are deteriorated vision, followed by a grey-out and finally black-out. Sustained long enough, it will result in death.
Tolerance for this effect is dependent on blood sugar levels, physical fitness, hydration, and diet.
At 8,000 feet, even the smoke from a cigarette becomes highly dangerous. Light three of them in a cockpit and you'll be in danger - note I say light them, not smoke them. Just the fumes alone is enough.
- Histotoxic hypoxia: Caused by chemical poisoning and by high blood alcohol. Don't drink so much.
Jeremy F notes that hypoxia isn't limited to the human body. It refers to the starvation of oxygen from ANY cells. There are some studies that have shown that hypoxia is occurring in parts of the Gulf of Mexico, where there is insufficient oxygen required for aquatic life.
Most info gleaned from "From the Ground Up," aviation textbooks, personal experience, and school.