One of the earliest and most well-reported cases of DCS, or Decompression Sickness, was encountered during the building of the Brooklyn Bridge. Workers spent long hours in caissons, compressed-air filled chambers underwater near the supports of the bridge. When they surfaced, they experienced joint pain, abdominal cramps and sometimes death. The condition came to be known as ‘caissons disease’.
In the case of scuba diving, the cause is now fairly well understood. Nitrogen in the air breathed from the tank dissolves into the blood as the diver descends. As the diver ascends, that nitrogen comes out of the blood as a result of the lowered pressure. It forms small bubbles that can grow larger unless re-absorbed into the blood stream or exhaled out through the lungs. When enough of those bubbles get to a certain volume, the results are the well-known symptoms of decompression sickness.
Technically, the phenomenon is an instance of a principle of physics known as Henry’s Law, which is taught in all basic diving courses. It’s similar to what happens when a soda can is popped. The rapid change in pressure causes dissolved gases to bubble out of the liquid. How much gas dissolves, and how quickly it is released later depends on the pressure at each point in the dive.
Permanent health impairment can result, even when the diver survives. DCS is nothing to toy around with, and so avoiding it, is one of the first lessons a novice diver has to learn. Fortunately, that’s very easy with the help of dive tables or a dive computer or even a diving watch. As the diver ascends, time is allowed for remaining neutrally buoyant – neither sinking nor floating – at several levels on the way up. How long to spend at each stop depends on the specific way the diver descended, but on average it amounts to spending from one to three minutes at each 30 feet (9 m) on the way up. For short dives, divers can rise continuously provided they don’t ascend quicker than about 30 feet (10 m) per minute for any dive deeper than 20 feet (6 m). Even then a “safety stop” at 10 to 20 feet (3 to 6 meters) is often performed.
Deeper dives require more stops of longer duration, especially if the diver didn’t spend most of the dive time at one depth. There are other factors, such as age and general physical condition, body type (fat stores nitrogen more effectively), the temperature before and during the dive and others.
The rate of ascent and number of stops changes when the tank air contains a different mix of nitrogen, oxygen and helium. All that complexity can be programmed into a dive computer – a small wristwatch that monitors the dive and calculates a safe rate of ascent and length of stops. That makes them much more useful than the older method, which relied on tables printed on plastic-laminated cards or simply experience. But, both those methods are still in use. Dive tables though, are only partially reliable since they typically assume what’s called a ‘square dive’, in which the diver descends more or less straight down, then stays at one constant depth before ascending.
Since the result can be so serious, investing in a good dive computer is money well spent. Using it and following good diving practice taught by a skilled instructor, is the best way to prevent ruining a vacation and maybe all the ones to come.