Up to approximately 35,000 feet, an aviator can keep sufficient oxygen in his/her lungs to permit normal activity by use of oxygen equipment that supplies oxygen upon demand (inhalation). The oxygen received by the body on each inhalation is diluted with decreasing amounts of air up to approximately 33,000 feet. Above 33,000 feet and up to approximately 35,000 feet, this equipment provides 100-percent oxygen. At approximately 35,000 feet, inhalation through the DEMAND oxygen system alone will NOT provide enough oxygen. Above 35,000 feet and up to about 43,000 feet, normal activity is only possible by use of PRESSURE DEMAND equipment. This equip- ment consists of a “supercharger” arrangement by which oxygen is supplied to the mask under a pressure slightly higher than that of the surrounding atmosphere. Upon inhalation, oxygen is forced (pressured) into the mask by the system. Upon exhalation the oxygen pressure is shut off automatically so that carbon dioxide can be expelled from the mask. Above 43,000 feet, the only adequate provision for the safety of the aviator is pressurization of the entire body. TYPES OF OXYGEN Aviators breathing oxygen (MIL-0-2721OD) is supplied in two types—type I and type II. Type I is gaseous oxygen and type II is liquid oxygen. Oxygen procured under this specification is required to be 99.5 percent pure. The water vapor content must not be more than 0.02 milligrams per liter when tested at 21.1°C (70°F) and at sea- level pressure. Technical oxygen, both gaseous and liquid, is procured under specification BB-O-925A. The moisture content of technical oxygen is not as rigidly controlled as is breathing oxygen; therefore, the technical grade should never be used in aircraft oxygen systems. The extremely low moisture content required of breathing oxygen is not to avoid physical injury to the body, but to ensure proper operation of the oxygen system. Air containing a high percentage of moisture can be breathed in- definitely without any serious ill effects. The moisture affects the aircraft oxygen system in the small orifices and passages in the regulator. Freezing temperatures can clog the system with ice and prevent oxygen from reaching the user. Therefore, extreme precautions must be taken to safeguard against the hazards of water vapor in oxygen systems. CHARACTERISTICS OF OXYGEN Oxygen, in its natural state, is a colorless, odorless, and tasteless gas. Oxygen is considered to be the most important of all the elements to life. It forms about 21 percent of the atmosphere by volume and 23 percent by weight. The remainder of the atmosphere consists of nitrogen (78 percent) and inert gases (1 percent), of which argon is the most abundant. Of all the elements in our environment, 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 rusting of iron, drying of paints, and the change of alcohol into vinegar. Even fuels in storage are slowly oxidized, the heat usually being rapidly carried away. However, when the heat cannot easily escape, the temperature will rise and a fire may break out. This fire is the result of spontaneous combustion. Oxygen does not burn, but it does support combustion. Nitrogen neither burns nor supports combustion. Therefore, combustible materials burn more readily and more vigorously in oxygen than in air, since air is composed of about 78 percent nitrogen by volume and only about 21 percent oxygen. In addition to existing as a gas, oxygen can exist as a liquid and as a solid. Liquid oxygen is pale blue in color. It flows like water, and weighs 9.52 pounds per gallon. EFFECTS OF LACK OF OXYGEN 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. When the body regains its normal oxygen supply, one may recover from hypoxia. A complete lack of oxygen, which results in permanent physical damage or death, is called ANOXIA. 4-2