oxygen is forced into the lungs by the system pressure. Upon exhalation, the oxygen flow is shut off automatically so that carbon dioxide can be expelled from the mask. Normal activity is possible to 50,000 feet with the use of a pressure breathing oxygen regulator. Above 50,000 feet, the only adequate provision for the safety of the aviator is pressurization of the en- tire body. Up to about 35,000 feet, an aviator can keep a sufficient concentration of oxygen in his/her lungs to permit normal activity by use of demand oxygen equipment, which supplies oxygen upon demand (inhalation). The oxygen received by the body on each inhalation is diluted with decreasing amounts of air up to about 30,000 feet. Above this altitude up to about 35,000 feet, this equipment provides 100-percent oxygen. At about 35,000 feet, inhalation alone will not provide enough oxygen with this equipment. EFFECTS OF HYPOXIA A decrease in the amount of oxygen per unit volume of air results in an insufficient amount of oxygen entering the bloodstream. The body reacts to this condition rapidly. This deficit in oxygen is called HYPOXIA. A complete lack of oxygen, which causes death, is called ANOXIA. If the body is returned to its normal oxygen supply, one may recover from hypoxia, but can- not recover from anoxia. Many persons are not aware of the enormous increase in the need for oxygen caused by an increase in physical activity. Strenuous calisthenics or a cross-country run results in deep and rapid breathing. Even so mild an exercise as getting up and walking around a room may double the air intake. In the case of the aviator, a leaking oxygen mask that may go completely unnoticed while the wearer is at rest may lead to collapse and unconsciousness when he/she attempts to move about from one station to another in the aircraft. A walkaround (portable) oxygen bottle sufficient for 24 minutes of quiet breathing may be emptied by 17 minutes of use when the user is moving about the aircraft. People differ in their reactions to hunger, thirst, and other sensations. Even an individual’s reactions vary from time to time under different circumstances. Illness, pain, fear, excessive heat or cold, and many other factors govern what the response will be in each particular case. The same thing is true of individual reactions to oxygen starvation. The effects of a certain degree of hypoxia on a given person cannot be accurately predicted. For instance, a person maybe relatively resistant on one day, but highly susceptible the next. It is difficult to detect hypoxia, because its victim is seldom able to judge how seriously he/she is affected, or often that he/she is affected at all. The unpleasant sensations experienced in suffocation are absent in the case of hypoxia. Blurring of vision, slight shortness of breath, a vague weak feeling, and a little dizziness are the only warnings. Even these may be absent or so slight as to be unnoticeable. While still conscious, the aviator may lose all sense of time and spend his last moments of consciousness in some apparently meaningless activity. In such a condition, the aviator is a menace to the crew as well as to himself. Since the aviator understands that it is the reduced air pressure at higher altitudes that determines the effect upon the body, he/she depends upon the altimeter rather than sensations or judgment to tell when oxygen is needed. CHARACTERISTICS OF OXYGEN Oxygen, in its natural state, is a colorless, odorless, and tasteless gas. Oxygen is considered to be the most important to life of all the elements. It forms about 21 percent of the atmosphere by volume and 23 percent by weight. Of all the elements in the universe, oxygen is the most plentiful. It makes up nearly one-half of the earth’s crust and approximately one-fifth of the air we breathe. Oxygen combines with most of the other elements. The combining of an element with oxygen is called oxidation. Combustion is simply rapid oxidation. In almost all oxidations, heat is given off. In combustion, the heat is given off so rapidly it does not have time to be carried away; the temperature rises extremely high, and a flame appears. Some examples of slow oxidation are the rusting of iron, drying of paints, and the changing of alcohol into vinegar. Even fuels in storage are 11-2