Ascent through water during a dive:

DCS is best known as an injury that affects underwater divers who breathe gas which is at a higher pressure than surface pressure. The pressure of the surrounding water increases as the diver descends and reduces as the diver ascends. The risk of DCS increases by diving long or deep without slowly ascending and making the decompression stops needed to eliminate the inert gases normally, although the specific risk factors are not well understood. Some divers seem more susceptible than others under identical conditions.

There have been known cases of bends in snorkellers who have made many deep dives in succession. DCS may be the cause of the disease taravana which affects South Pacific island natives who for centuries have dived without equipment for food and pearls.^[27]

Two linked factors contribute to divers' DCS, although the complete relationship of causes is not fully understood:

• deep or long dives: inert gases in breathing gases, such as nitrogen and helium, are absorbed into the tissues of the body in higher concentrations than normal (Henry's Law) when breathed at high pressure.
• fast ascents: reducing the ambient pressure, as happens during the ascent, causes the absorbed gases to come back out of solution, and form "micro bubbles" in the blood. Those bubbles will safely leave the body through the lungs if the ascent is slow enough that the volume of bubbles does not rise too high.

The physiologist John Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without leading to DCS.^[2][28] Bubbles form after every dive: slow ascent and decompression stops simply reduce the volume and number of the bubbles to a level at which there is no injury to the diver.

Severe cases of decompression sickness can lead to death. Large bubbles of gas impede the flow of oxygen-rich blood to the brain, central nervous system and other vital organs.