Hypoxia
I picked up an old book published by AOPA from 1978-1986, Defensive Flying by Norbert Slepyan and it has some interesting info on hypoxia.
At 25,000 feet, there is the same percentage of oxygen in the atmosphere as at sea level, but there is much less atmospheric pressure to force sufficient oxygen into our lungs. At 10,000 feet, each inhalation gives us only two-thirds the oxygen molecules available at sea level, and only half at 18,000. Less oxygen per breath means less for oxidation burning of food to produce heat and energy. Unless we use supplemental oxygen at higher altitudes, our bodies become starved. At 45,000 feet, it takes only nine to 15 seconds for the average person to pass out with inadequate oxygen; at 30,000, one to two minutes.
Charts show about 30 minutes of effective performance at 20,000, but a trip in an altitude chamber will demonstrate that people show signs of hypoxia long before that. Thin air is like thin ice—if offers little support.
Hypoxia symptoms are not easily recognized. Unless a pilot is aware of and expecting them, he may feel excellent and in control. He may calculate badly and copy erratically and illegibly while his fingernails turn blue and he stares dumbly at the panel, but he may still insist that all is fine.
Hypoxia can be caused by poor transportation of oxygen through the body or by the body’s inability to use it. Drugs, alcohol, and tobacco aggravate the condition. One cigarette lowers the oxygen content of the blood and raises the apparent altitude of a person at sea level to nearly 7,000 feet. More smoking adds altitude to cabin altitude in flight. Excessive smoking can cause such hypemic hypoxia. Anemia, donating blood, and carbon monoxide poisoning have the same effect. Poor blood circulation brought on by excessive g forces or long periods of positive pressure breathing of oxygen (necessary above 25,000 feet) can cause stagnant hypoxia. Nor is oxygen a remedy for the histoxic hypoxia suffered by a hungover pilot. Only detoxification over time can restore normality, so shrinking from drinking is the only defense. One drink raises one’s personal altitude by 2,000 feet. Poor nutrition, obesity, fatigue, illness, and poor general physical condition also affect our susceptibility to hypoxia. Threatening conditions can differ from day to day and from individual to individual.
The Air Force requires its pilots to use oxygen from 10,000 feet up and from the ground up at night, for lack of oxygen can hinder night vision, including color perception and peripheral acuity. The FAA recommendation of oxygen use from 5,000 feet at night is conservative and should certainly be followed.
The FAA’s oxygen requirements begin with flight crew in unpressurized aircraft flying between 12,500 and 14,000 for more than 30 minutes. Above 14,000, oxygen is required for the pilot (or minimum crew) for the entire flight. Above 15,000, it must be provided for passengers. If the aircraft is pressurized, above FL 250, a ten-minute supply is required for each occupant. Above FL 350, at least one pilot must use an oxygen mask if the cabin exceeds 14,000 feet. If two pilots are at the controls, quick-lonning masks are sufficient below FL 410.