1. The production of gas bubbles in body fluids, arising from the sudden reduction in ambient pressure which may occur in decompression sickness and is a hazard for astronauts on the moon, where there is no atmosphere.
2. The formation of bubbles in body fluids under sharply reduced environmental pressure.
3. Formation of water vapor bubbles in the tissues brought on by an extreme reduction in barometric pressure.

It occurs if the body is exposed to pressures which are found above an altitude of 63,000 feet, or if a diver rises rapidly from a great depth in a water environment to the surface.