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