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.

Hypoxia Most people are not aware of the body's enormous increase in oxygen requirements caused by an increase in physical activity. Strenuous exercise like cross-country running results in a greatly increased need for oxygen, which is evidenced by deep and rapid breathing. Even mild exercise like getting up and walking around a room may double the air intake. In the case of the aviator, leaking of an oxygen mask, which may go completely unnoticed while the wearer is at rest, may lead to collapse and unconsciousness when an attempt is made to move from one station to another in the aircraft. A walkaround (portable) oxygen bottle sufficient for 24 minutes of quiet breathing maybe emptied by 17 minutes of use when the user is moving around inside the aircraft.

Effects of Hypoxia People differ in their reactions to hunger, thirst, and other sensations. An individual's reactions vary from time to time under similar 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 hypoxia on a given person cannot be accurately predicted. For example, a person may be relatively unaffected one day, but highly susceptible the next. It is difficult to detect hypoxia, because its victims are seldom able to judge how seriously they are affected, or if they are affected at all. The unpleasant sensations experienced in suffoca-tion 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 go unnoticed. While still conscious, the aviator may lose all sense of time and spend his/ her last moments of consciousness in some apparently meaningless activity. In such a condition, a person is a menace to the crew as well as to the himself. Since the aviator understands that it is the reduced air pressure at higher altitudes that determines the effect on the body, dependence should be upon the altimeter rather than sensations or judgment to determine when oxygen is needed. The effects of hypoxia at various altitudes are discussed in the following paragraphs.

BELOW 10,000 FEET.- At or below 10,000 feet, some effects of hypoxia may be present. Generally, the eye is the first part of the body to suffer effects of hypoxia. Even at a relatively low altitude of approximately 5,000 feet, where no other effect of hypoxia can be detected, night vision is appreciable reduced. At 10,000 feet, night operations may be seriously handicapped by poor night vision, which is due to mild oxygen starvation. Thus, the use of supplemental oxygen on night flights above 5,000 feet is required. Although hypoxia affects the eyes in the daytime as well as at night, the results during the day are usually not as noticable below 10,000 feet.

BETWEEN 10,000 AND 15,000 FEET.- Although efficiency may be considerably impaired at 10,000 to 15,000 feet, death from oxygen starvation at these altitudes is virtually unknown. The greatest dangers are from errors in judgment or performance due to drowsiness or mental confusion. At these altitudes, long flights without oxygen produce persistent drowsiness and excessive fatigue for many hours afterward. Frequently, persistent headaches develop soon after completion of the flight. For these reasons, the use of oxygen on flights above 10,000 feet is required. Portable oxygen systems are available for aircraft that do not have oxygen equipment.

BETWEEN 15,000 AND 20,000 FEET.- Flights at 15,000 to 20,000 feet, even for short periods, must not be attempted without the use of oxygen. Collapse and unconsciousness are common. Failure to use oxygen could result in death, especially when the situation is complicated by loss of blood in combat or by shock due to pain or fear.

BETWEEN 20,000 AND 25,000 FEET.- During World War II, most military flying was done in unpressurized aircraft at altitudes of between 20,000 and 25,000 feet. Most of the resulting anoxia deaths occurred in this altitude range. The general symptoms of drowsiness, mental confusion, dim vision, and dizziness occur here, as at lower altitudes, but they come on much more quickly, allowing less opportunity for corrective action. Consequently, under no circumstances should aircraft ascend to these altitudes, even for short periods, without the use of oxygen by all persons aboard. The movement of personnel in the aircraft requires the constant use of walkaround equipment. Unusual actions or failure of a crew member to respond quickly and clearly, when called, require immediate investigation.

BETWEEN 25,000 AND 30,000 FEET.- Between 25,000 and 30,000 feet, collapse, unconsciousnes, and death quickly follow interruption of the oxygen supply. Mask leakage at these altitudes may cause a degree of hypoxia that, although not noticed during flight, can produce considerable fatigue and have serious cumulative effects.

ABOVE 30,000 FEET.- Above 30,000 feet, unconsciousness and death strike rapidly and often without warning. At such altitudes, it is imperative that all oxygen equipment be functioning correctly and that each breath be taken through a properly fitted oxygen mask. Above a pressure altitude of 35,000 feet, pressure breathing oxygen equipment is required.