Editing Decompression sickness (section)

== Causes ==
{{Further|Decompression (diving)#Bubble formation, growth and elimination}}
DCS is caused by a reduction in [[ambient pressure]] that results in the formation of bubbles of [[inert gases]] within tissues of the body. It may happen when leaving a high-pressure environment, ascending from depth, or ascending to altitude. A closely related condition of bubble formation in body tissues due to [[isobaric counterdiffusion]] can occur with no change of pressure.

=== Ascent from depth ===
DCS is best known as a [[diving disorder]] that affects divers having breathed gas that is at a higher pressure than the surface pressure, owing to the pressure of the surrounding water. The risk of DCS increases when diving for extended periods or at greater depth, without ascending gradually and making the [[decompression stops]] needed to slowly reduce the excess pressure of inert gases dissolved in the body. The specific risk factors are not well understood and some divers may be more susceptible than others under identical conditions.{{r|38uhms | Benton}} DCS has been confirmed in rare cases of [[apnea|breath-holding]] divers who have made a sequence of many deep dives with short surface intervals, and may be the cause of the disease called [[taravana]] by South Pacific island natives who for centuries have dived by breath-holding for food and [[pearl]]s.{{r|Wong1999}}

Two principal factors control the risk of a diver developing DCS:

# the rate and duration of gas absorption under pressure&nbsp;– the deeper or longer the dive the more gas is absorbed into body tissue in higher concentrations than normal ([[Henry's Law]]);
# the rate and duration of outgassing on depressurization&nbsp;– the faster the ascent and the shorter the interval between dives the less time there is for absorbed gas to be offloaded safely through the lungs, causing these gases to come out of solution and form "micro bubbles" in the blood.{{sfn|Lippmann & Mitchell|pp=65–66}}

Even when the change in pressure causes no immediate symptoms, rapid pressure change can cause permanent [[bone]] injury called [[dysbaric osteonecrosis]] (DON). DON can develop from a single exposure to rapid decompression.{{r|Ohta1974}}

=== Leaving a high-pressure environment ===
[[File:Caisson Schematic.svg|thumb|300px|alt=Schematic of a caisson|The principal features of a caisson are the workspace, pressurised by an external air supply, and the access tube with an airlock]]

When workers leave a pressurized [[Caisson (engineering)|caisson]] or a [[mining|mine]] that has been pressurized to keep water out, they will experience a significant reduction in [[ambient pressure]].{{r|38uhms | Elliott}} A similar pressure reduction occurs when [[astronaut]]s exit a space vehicle to perform a space-walk or [[extra-vehicular activity]], where the pressure in their [[spacesuit]] is lower than the pressure in the vehicle.{{r|38uhms | Dehart | Pilmanis | Vann2}}

The original name for DCS was "caisson disease". This term was introduced in the 19th century, when caissons under pressure were used to keep water from flooding large engineering excavations below the [[water table]], such as bridge supports and tunnels. Workers spending time in high ambient pressure conditions are at risk when they return to the lower pressure outside the caisson if the pressure is not reduced slowly. DCS was a major factor during construction of [[Eads Bridge]], when 15 workers died from what was then a mysterious illness, and later during construction of the [[Brooklyn Bridge]], where it incapacitated the project leader [[Washington Roebling]].{{r|Eads}} On the other side of the Manhattan island during construction of the [[Downtown Hudson Tubes|Hudson River Tunnel]], contractor's agent [[Ernest William Moir]] noted in 1889 that workers were dying due to decompression sickness; Moir pioneered the use of an [[airlock]] chamber for treatment.{{r|Hudson}}

=== Ascent to altitude and loss of pressure from a pressurised environment===
The most common health risk on ascent to altitude is not decompression sickness but [[altitude sickness]], or acute mountain sickness (AMS), which has an entirely different and unrelated set of causes and symptoms. AMS results not from the formation of bubbles from dissolved gasses in the body but from exposure to a low partial pressure of oxygen and [[alkalosis]]. However, passengers in unpressurized aircraft at [[high altitude]] may also be at some risk of DCS.{{r|38uhms | Dehart | Pilmanis | Gerth}}

Altitude DCS became a problem in the 1930s with the development of high-altitude balloon and aircraft flights but not as great a problem as AMS, which drove the development of [[cabin pressurization|pressurized cabins]], which coincidentally controlled DCS. Commercial aircraft are now required to maintain the cabin at or below a [[pressure altitude]] of {{convert|2400|m|ft|abbr=on}} even when flying above {{convert|12000|m|ft|abbr=on}}. Symptoms of DCS in healthy individuals are subsequently very rare unless there is a [[Cabin pressurization#Unplanned decompression|loss of pressurization]] or the individual has been diving recently.{{r|NASA1999 | RRR1181}} Divers who drive up a mountain or fly shortly after diving are at particular risk even in a pressurized aircraft because the regulatory cabin altitude of {{convert|2400|m|ft|abbr=on}} represents only 73% of [[sea level pressure]].{{r|38uhms | Dehart | Vann1}}

Generally, the higher the altitude the greater the risk of altitude DCS but there is no specific, maximum, safe altitude below which it never occurs. There are very few symptoms at or below {{convert|5500|m|ft|abbr=on}} unless the person had predisposing medical conditions or had dived recently. There is a correlation between increased altitudes above {{convert|5500|m|ft|abbr=on}} and the frequency of altitude DCS but there is no direct relationship with the severity of the various types of DCS. A US Air Force study reports that there are few occurrences between {{convert|5500|m|ft|abbr=on}} and {{convert|7500|m|ft|abbr=on}} and 87% of incidents occurred at or above {{convert|7500|m|ft|abbr=on}}. {{r|FAA2005}} [[High-altitude military parachuting|High-altitude parachutists]] may reduce the risk of altitude DCS if they flush nitrogen from the body by [[Oxygen prebreathing|pre-breathing pure oxygen]].{{r|pmid14620473}} A similar procedure is used by astronauts and cosmonauts preparing for extravehicular activity in low pressure [[space suit]]s.