Lack of oxygen is the greatest single danger to man at high altitudes, despite the importance of pressure and temperatures. The shortage of oxygen in the human body results in a condition called hypoxia, which simply means oxygen starvation. When a pilot inhales air at high altitudes, there isn't enough oxygen pressure to force adequate amounts of this vital gas through the membranes of the lungs into the blood stream, so that it can be carried to the tissues of the body. The function of various organs, including the brain, is then impaired.

Unfortunately, the nature of hypoxia makes you, the pilot, the poorest judge of when you are its victim. The first symptoms of oxygen deficiency are misleadingly pleasant, resembling mild intoxication from alcohol. Because oxygen starvation strikes first at the brain, your higher faculties are dulled. Your normal self-critical ability is out of order. Your mind no longer functions properly; your hands and feet become clumsy without being aware of it; you may feel drowsy, languid, and nonchalant; you have a false sense of security; and, the last thing in the world you think you need is oxygen.

As the hypoxia gets worse, you may become dizzy or feel a tingling of the skin. You might have a dull headache, but you are only half aware of it. Oxygen starvation gets worse the longer you remain at a given altitude, or if you climb higher. Your heart races, your lips and the skin under the fingernails begin to turn blue, your field of vision narrows, and the instruments start to look fuzzy. But hypoxia - by its nature, a grim deceiver - makes you feel confident that you are doing a better job of flying than you have ever done before. You are in about the same condition as the fellow who insists on driving his car home from a New Year's Eve party when he can hardly walk. Regardless of his acclimatization, endurance, or other attributes, every pilot will suffer the consequences of hypoxia when he is exposed to inadequate oxygen pressure.

What do you do about it?  There is one general rule:  Don't let hypoxia get a foot in the door. Carry oxygen and use it before you start to become hypoxic. Don't gauge your "oxygen hunger" by how you feel. Gauge it by the altimeter.

Here are some general suggestions which apply to young healthy flyers.
 
    1. Carry oxygen in your plane or don't fly above 12,500 feet. If bad weather lies ahead, go around it if you can't get over it.
    2. Use oxygen on every flight above 12,500 feet. You'll probably need it, and when you do, you might not realize it.
    3. Use oxygen on protracted flight near 12,500 feet. It won't hurt you and you'll be a lot sharper pilot.
    4. Use oxygen on all night flights above 5,000 feet. If you want to give your night vision the best protection, use oxygen from the ground up.
      5. Breathe normally when using oxygen. Rapid or extra deep breathing can cause loss of consciousness also. (See Chapter 5,
HYPERVENTILATION.)

Flying above 12,500 feet without using oxygen is like playing Russian roulette - the odds are than you may not get hurt, but it's a deadly game!  At 20,000 feet your vision deteriorates to the point that seeing is almost impossible. The engine sounds become imperceptible, breathing is labored, and the heart beats rapidly. You haven't the vaguest idea what is wrong, or whether anything is wrong. At 25.000 feet you will collapse and death is imminent unless oxygen is restored.

No one is exempt from the effects of hypoxia. Everyone needs an adequate supply of oxygen. Some pilots may be able to tolerate a few thousand feet more of altitude than some others, but no one is really very far from average. Remember this:  Serious trouble is waiting for the pilot who tries to test himself to prove how much higher he can fly or how much longer he can function without supplemental oxygen. Pilots who are older, fatter, out of condition, or smoke heavily should limit themselves to a ceiling of 8,000 to 10,000 feet unless oxygen is available.