Though partially funded by NASA, it is still a private site. Therefore, before downloading any materials for use, you need to ask permission. While the percentage of oxygen in the atmosphere remains fairly constant up to 70,000 feet, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet. At 18,000 feet the pressure is 7.34 psi.
Hypoxia is the effects of an insufficient supply of oxygen to the body.
The early signs of hypoxia generally begin at 10,000 feet. U.S. Air Force aircrews must use supplemental oxygen when the cabin pressure of the aircraft reaches this altitude. The Time of Useful Consciousness is a function of altitude. At 20,000 feet, an average individual will have 5 to 12 minutes. At 25,000 feet, this time is reduced to 3 to 5 minutes.