Explosive Decompression

Decompression is defined as the inability of the airplane's pressurization
system to maintain its designed pressure differential. This can be caused
by a malfunction in the pressurization system or structural damage to the
airplane. Physiologically, decompression's fall into two categories; they
are:

Explosive Decompression - Explosive decompression is defined as a
change in cabin pressure faster than the lungs can decompress;
therefore, it is possible that lung damage may occur. Normally, the
time required to release air from the lungs without restrictions,
such as masks, is 0.2 seconds. Most authorities consider any
decompression that occurs in less than 0.5 seconds as explosive and
potentially dangerous.
Rapid Decompression - Rapid decompression is defined as a change in
cabin pressure where the lungs can decompress faster than the cabin;
therefore, there is no likelihood of lung damage. During an explosive
decompression, there may be noise, and for a split second, one may
feel dazed. The cabin air will fill with fog, dust, or flying debris.
Fog occurs due to the rapid drop in temperature and the change of
relative humidity. Normally, the ears clear automatically. Air will
rush from the mouth and nose due to the escape of air from the lungs,
and may be noticed by some individuals.

The primary danger of decompression is hypoxia. Unless proper utilization
of oxygen equipment is accomplished quickly, unconsciousness may occur in a
very short time. The period of useful consciousness is considerably
shortened when a person is subjected to a rapid decompression. This is due
to the rapid reduction of pressure on the body—oxygen in the lungs is
exhaled rapidly. This in effect reduces the partial pressure of oxygen in
the blood and therefore reduces the pilot's effective performance time by
one-third to one-fourth its normal time. For this reason, the oxygen mask
should be worn when flying at very high altitudes (35,000 feet or higher).
It is recommended that the crew members select the 100 percent oxygen
setting on the oxygen regulator at high altitude if the airplane is
equipped with a demand or pressure demand oxygen system.

Another hazard is being tossed or blown out of the airplane if near an
opening. For this reason, individuals near openings should wear safety
harnesses or seat belts at all times when the airplane is pressurized and
they are seated.

Another potential hazard during high altitude decompression is the
possibility of evolved gas decompression sicknesses. Exposure to wind
blasts and extremely cold temperatures are other hazards one might have to
face.

Rapid descent from altitude is necessary if these problems are to be
minimized. Automatic visual and aural warning systems are included in the
equipment of all pressurized airplanes.