How is decompression sickness caused and how can it be prevented?

Answer 1

Decompression Sickness (DCS) is caused by the body's tissues absorbing gas breathed under pressure. Normally the absorbed gas is Nitrogen, however, for some technical divers breathing Helium mixtures, Helium is also absorbed into the tissues and can cause DCS.

There are two primary ways to avoid DCS. The first, and most common way, is to only dive within the 'no decompression limits' (NDLs) on your dive tables or your dive computer. Stay shallow, or keep your dives short. Staying within NDLs will not guarantee that a diver will never get DCS, but it makes that liklihood very small. Many divers also do an additional 'safety stop' at 15 feet/5 metres before ascending to guard against even that reduced risk.

For divers who exceed NDLs (either accidentally or deliberately), they avoid the risk of DCS by conducting "staged decompression", ie. ascending to progressively shallower depths, reducing the pressure on their bodies, and allowing the gases to dissolve out of their tissues slowly, avoiding the formation of bubbles in the bloodstream that cause DCS. Sometimes these staged decompression stops can be relatively short, a few minutes at 10 feet or 20 feet. However, for long and deep dives, sometimes they be many times longer than the substantive dive itself. For lengthy decompressions, it is quite common for technical divers to use oxygen enriched mixes (or even pure oxygen) to speed up the decompression process. However, this practice can be very dangerous if attempted by untrained divers due to the risk of oxygen toxicity.

Small addition

The above is a nice description. The only correction I would make is doing proper decompression (or safety) stops does not allow gas to be released slowly (a misconception that was unfortunately taught). In fact, proper stops allow the gas to be rapidly eliminated. This is why safety stops are one of the best things you can do to help prevent decompression sickness ... because you are rapidly eliminating gas before you surface.

In addition to the obvious answer of doing safety stops, staying well within time limits, and making slow ascents (i.e. no faster than 30'/min), an often over looked way to help prevent decompression sickness is to not engage in any exertion either before or after a dive and minmize any "work" that is done. Unfortunately this is not completely possible since just walking with Scuba gear on does this, but you can try to minimize it. Such work can actually excite bubbles into forming.