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