<unk>  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. Ascent from depth<unk>  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<unk>  increases when diving for extended periods or at greater depth, without ascending gradually and making the<unk>  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.<unk>  has been confirmed in rare cases of 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 pearls. Two principal factors control the risk of a diver suffering<unk> : # the rate and duration of gas absorption under pressure – 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 – 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. 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<unk> . Leaving a high-pressure environment 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 or a mine that has been pressurized to keep water out, they will experience a significant reduction in ambient pressure. A similar pressure reduction occurs when astronauts 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. The original name for<unk>  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.<unk>  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. On the other side of the Manhattan island during construction of the Hudson River Tunnel contractor's agent Ernest William Moir noted in 1889 that workers were dying due to<unk>  and pioneered the use of an airlock chamber for treatment. Ascent to altitude The most common health risk on ascent to altitude is not<unk>  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<unk> . Altitude<unk>  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 pressurized cabins, which coincidentally controlled<unk> . Commercial aircraft are now required to maintain the cabin at or below a pressure altitude of even when flying above . Symptoms of<unk>  in healthy individuals are subsequently very rare unless there is a loss of pressurization or the individual has been diving recently. 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 represents only 73% of sea level pressure. Generally, the higher the altitude the greater the risk of altitude<unk>  but there is no specific, maximum, safe altitude below which it never occurs. There are very few symptoms at or below unless patients had predisposing medical conditions or had dived recently. There is a correlation between increased altitudes above and the frequency of altitude<unk>  but there is no direct relationship with the severity of the various types of<unk> . A US Air Force study reports that there are few occurrences between and and 87% of incidents occurred at or above . High altitude parachutists may reduce the risk of altitude<unk>  if they flush nitrogen from the body by pre-breathing pure oxygen.
Decompression sickness
Morning sickness
Ski sickness
Wooden chest syndrome
Acute HME syndrome
Simulator sickness
Chronic mountain sickness
Acute chest syndrome