Why decompression theory?
What happens during diving? As soon as we are under water, we expose our body to a higher pressure. Since humans are largely made up of water, that’s not a problem at first—but the gases we have in our bodies react to the increasing pressure. The first effect, namely that gases are compressed, is immediately noticeable during descent: the “pressure on the ears” is nothing more than a tiny gas-filled cavity in which the rising pressure is noticeable. The second effect lasts longer: when we breathe gases under higher pressure, they enter the body in greater quantities. And these dissolved gases are exactly what we are concerned with here.
Decompression theory is about how inert gases – gases that are not used by the body and do not react chemically – behave in the human body, when they might cause harm, and how they can be released as gently as possible. This mainly involves nitrogen, but sometimes helium also comes into play. These gases can form bubbles at the end of the dive, when you ascend and the pressure decreases, and cause a disease that almost only divers know: The “bends”, diver’s fleas, decompression sickness (DCS). This disease is very rare, but kind of disturbing: no one can say exactly when and why you get it, but it’s kind of creepy with the bubbles in your blood and stuff….
To understand the relevance of discussions about the best decompression model, we would first like to take a look at the statistical risk: How high or low is the real risk during a dive? Is diving an “extreme sport”, or rather a relatively gentle, rather low-risk activity?
Risk DCS
Origins: How long can you keep workers diving?
Our current knowledge of decompression comes not primarily from recreational divers like us, but from the military and professional divers. The focus was on how to use them as efficiently as possible without unacceptable damages.
From this context comes the first name of this strange disease that occurs after diving: caisson disease. In the 19th century, this affected bridge workers who worked on the construction in surface-supplied air-filled boxes under water (so-called “caissons”). For this work and for the military, the first decompression models were developed, which are still used today.
You can find out more about the history of deco theory in the SSI Science of Diving course
Lobster fishermen as an example of too cheap labor
Unfortunately, the fact that people suffer serious injuries while working underwater is not just history: even today, decompression accidents are a major risk for those who cannot afford the luxury of safe diving profiles.
An example of this can be found in Honduras – where so many divers from the USA and Europe learn to dive and become instructors in the Bay Islands (Utila and Roatan). The resident lobster fishermen, mostly from the indigenous Miskito group, don’t do their dives for fun, for them it’s about survival.
In the season, the person who brings the most lobsters to the surface earns the most. Long breaks and slow ascents reduce income. This fact plus a lack of knowledge and the absence of dive computers makes the job really dangerous. In order to make a difference here, a number of projects are active in the region – anyone vacationing there might want to consider contributing here.
DAN article on the situation of lobster fishermen in Honduras from 2014.
DCS in accident statistics
Decompression accidents are unpleasant, and of course we try to avoid them. But how great is the risk in the first place? Are we practicing an “extreme sport”, fraught with imponderable dangers, or is diving not rather a quite harmless leisure activity?
The thing that scares most divers, namely the risk of DCS, is fortunately statistically very rare. And if you do get DCS, it is treatable in the vast majority of cases.
You can get an idea of the risk by looking at the analysis of 947 diving deaths between 1992 and 2003 published by a group of diving physicians in 2008. You can read the whole study here: Denoble 2008: OC Recreational Diving Fatalities
Among some other interesting points, one thing becomes clear here: DCS plays a very marginal role in diving fatalities. Among the 947 diving deaths during these 20 years, only 15 were due to DCS, and none were unexplained.
Does that mean you don’t have to deal with the issue at all? No, because decompression illnesses that do not immediately lead to death are also a problem. But maybe it means that an adequate gas supply is simply more important than the perfect decompression strategy.
“Disabling injuries [the injury resulting in inability to act] attributable to DCS were found in 15 cases during dives with a mean depth of 69 m (39-76 m IQR).
Eight deceased had too little gas and made emergency ascents with no decompression. Six were diving on wrecks, four were spearfishing or hunting and four made several repetitive dives with short surface intervals. One ran out of gas because his regulator stopped working, one became positively buoyant when the valve on his drysuit stopped working, and one was dragged to depth by an impaled fish.”
Denoble et al: Common causes of open-circuit recreational diving fatalities. 2008
How high is the DCS risk for recreational divers today?
Apart from the few well-documented serious accidents, DCS naturally occurs much more frequently, usually after completely normal dives. A certain residual risk remains even if you dive within the limits of the computer, there is no dive with zero risk. The statistical risk is not easy to determine because, on the one hand, not every minor DCS is included in data collections and, on the other hand, it is not known how many dives actually take place. However, various qualified estimates repeatedly land at a risk of around 1:10,000, i.e. one in 10,000 dives ends in DCS.
The figure is probably somewhat higher, but diving is certainly far from being a high-risk sport. As with any sport, injuries do occur from time to time, one of which is a condition that only occurs when diving. It is no worse than any other sports injury and should simply be treated as such.
