Most people are not aware of the bodys
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
Effects of Hypoxia
People differ in their reactions to hunger,
thirst, and other sensations. An individuals
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