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 made up of 70% water, this is 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?

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. Bridge workers who worked on the structure in surface-supplied air-filled boxes underwater were affected in the 19th century. For this work and for the military, the first decompression models were developed, which are still used today.

For more on the history of decompression theory, see 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.

What risk are we talking about for recreational divers today?

“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 decedents were low on gas and made emergency ascents with omitted decompression. Six were diving on wrecks, four were spearfishing or hunting, and four were making multiple repetitive dives with short surface intervals. One ran out of gas because his regulator stopped working, one got positive buoyancy when the valve on his drysuit stopped working, and one was dragged down by a speared fish.”