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Decompression (altitude) refers to the reduction in ambient pressure due to ascent above sea level. Decompression has physical effects on gas filled spaces and on liquids, particularly when they contain dissolved gases. Physiological effects of decompression are due to these physical effects and the consequential effects on the living tissues, mostly as a result of the formation and growth of bubbles, and the expansion of gas filled spaces. Formation and growth of bubbles due to reduced pressure can be due to reduction in solubility as described by Henry's Law, nucleation and growth of bubbles in supersaturated liquids and boiling of liquids when the pressure is reduced below the vapour pressure for the temperature of the liquid.
Altitude decompression may be a natural consequence of unprotected elevation to altitude, or due to intentional or unintentional release of pressurisation of a pressure suit or pressurised compartment, vehicle or habitat, and may be controlled or uncontrolled.
There are three principal physiological effects arising from decompression at altitude:
- Decompression illness, which includes decompression sickness due to bubble formation in the tissues similar to those caused by decompression after exposure to pressures higher than sea level atmospheric pressure. There is little evidence of altitude decompression occurring among healthy individuals at altitudes below 18,000 feet (5,500 m).
- Barotrauma caused by the over-expansion of gas-filled spaces
- Altitude sickness, also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, the altitude bends, or soroche, is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen and blood alkalosis arising from the low partial pressure of carbon dioxide at high altitude.