Why do you get DCS?

Whenever it’s mentioned somewhere that someone had symptoms of decompression sickness (DCS), it doesn’t take five minutes for the discussion to shift. In forums and on social media, it follows an amazingly reliable pattern: first sympathy, then analysis, then the attempt to “categorize” the case. Dehydrated. Too old. Not fit enough. Too fat. Definitely a PFO. Or cold. In the end, there is almost always the implicit message: There was a clear reason for it—and it doesn’t apply to me.
Why people react this way is something we discuss in great detail on the “Talking about DCS” page. There, we are also gradually presenting the results of our own survey on DCS.

If we seriously want to understand what makes DCS more likely, we have to endure two things simultaneously: We know that bubbles alone are not a reliable indicator of DCS. And we know just as certainly that there must be reasons why some people develop symptoms during a specific dive profile while others—and sometimes even they themselves on previous dives—remain completely unaffected. However, despite all the research, these reasons remain largely unclear so far.

This is exactly where the topic of risk factors begins. Which things have a verifiable influence on the risk? Which physical conditions, which behaviors, and which dive profiles make DCS more likely? And it is worth making a clean distinction between what we really know and what we plausibly suspect—and also clarifying which factors show no detectable influence.

The study from which most of the information comes and which we cite repeatedly is the large DAN Europe data collection. Nearly 40,000 evaluated dive profiles, almost 1,000 bubble measurements, about 320 DCS cases—this is by far the most significant current empirical data from real recreational dives. Further texts on the subject are linked at the very bottom of the article.

Your own body

What really has an influence

Age and body fat

In the largest analysis of real recreational dives, conducted by DAN Europe, it is shown that with increasing age and higher body fat percentage, the risk of developing bubbles after diving increases. The rate of DCS cases also grows. In addition to age and body fat, gender also plays a role here: women get DCS slightly more often than men.
This does not mean that diving is “dangerous” for older people or those with a bit more body fat. The risk still remains very low—it is just a little higher than for young, slim, top-fit people. This applies to quite a few activities, and an appropriate way to handle it is to test fewer limits as you get older. Most people do this quite automatically—in diving, it means eventually setting the computer a bit more conservatively and perhaps not joining every single dive anymore.

What could be

The statement “it’s simply just age and body fat” probably falls short, even if other factors are really hard to prove. Fundamentally, it is very likely that general fitness and overall health also have an influence on how high the risk is that a dive ends with DCS. The problem is: to determine more precisely how much influence which factors have exactly, data is simply lacking.

Pre-existing conditions, chronic inflammatory processes, endothelial dysfunction, scar tissue, individual differences in the immune system—all these are factors for which there is good evidence that they influence how the body reacts to decompression stress. They could explain why bubbles remain silent in some people and trigger symptoms in others.

This entire complex of topics has been researched under the keyword “individual decompression” for many years. This is linked to the hope that divers can better understand exactly where their individual stress limit lies. Perhaps there will be new insights in this area in the coming years—for the moment, we unfortunately have to accept that much here remains unclear.

Diving conditions

What verifiably reduces the risk

Lower supersaturation = lower risk

It sounds banal, but: Yes, the biggest risk factor can be read from the dive profile. Unfortunately, not in a way that one could clearly say which profile will result in DCS—at least not if you dive within recognized limits. But: deeper, longer, and a higher maximum Gradient Factor reached statistically lead to slightly more DCS cases.
The higher the supersaturation, the greater the risk of DCS occurring. This is not just about dives where limits were ignored, but about the normal supersaturation that is accepted when diving. Most DCS cases in the DAN database occur during dives where Gradient Factors between 70 and 90 were reached—i.e., during recreational dives well within no-deco limits. At GFs below 60, DCS occurs only extremely rarely.
There is a separate blog post with more on the topic of Gradient Factors.

The thing about the cold

And then there’s the cold. This is one of the rare cases where the data situation is unusually clear. Studies by the U.S. Navy Experimental Diving Unit have shown that cold during decompression significantly increases the risk of DCS. The classic scenario is particularly problematic: warm at the bottom, cold during the ascent and at the stops.

The mechanism is easy to understand. Warmth at the bottom leads to good blood flow in the periphery and thus to higher inert gas uptake in these tissues. If it gets cold during decompression, the vessels constrict, blood flow to the extremities decreases, and that is exactly where the removal of the gas is hindered. Supersaturations rise, and bubble formation becomes more likely.

Conversely, the effect is also relevant: cold during gas uptake and warm during decompression is significantly more favorable. Cold is thus one of the few risk factors that are really clearly verifiable.

But what to do? Most divers know that it gets a bit chilly at the end of a dive, even in tropical waters. When the dive is more demanding than just looking at fish in shallow water, it makes sense to think about adequate thermal protection. This could be a slightly thicker suit, or a heated vest under the drysuit that you turn on specifically at the end of the dive.

What could be

Exertion

Not quite as clear, but still very likely, is the connection between workload during diving and DCS. The database shows that with current and exertion underwater, the risk of getting DCS increases. Not dramatically, but measurably.
This also has a certain logic, especially if the exertion decreases towards the end of the dive. If you swim with more effort, move more, and increase circulation during the phase in which you absorb inert gases, nitrogen is better distributed in the body. If you then rest a bit more at the end of the dive, the body behaves differently in the desaturation phase than in the saturation phase and tends to eliminate less nitrogen. And that can bring you closer to critical limits.

Nitrox

Many people firmly believe that diving with Nitrox is safer. Fundamentally, one thing is true: if you do the same dive with Nitrox as with air, you come out of the water with lower supersaturation, so the dive is safer. But this only applies in comparison to the exact same dive with air. Since Nitrox is often used to enjoy longer no-deco times, diving with Nitrox does not automatically become safer. Statistically, at least, no difference in DCS risk can be detected between air dives and those with Nitrox.

What is overrated….

Order

“Deepest dive first” is a concept that persists extremely stubbornly and is sometimes enforced in almost absurd ways. On a liveaboard, you MUST go deep on the first dive because the second one is on a wreck at 30m, and the first one just has to be deeper—as if it wouldn’t be safer to keep the total saturation lower throughout the day. Or in the case of DCS, the accusation arises as to why the second dive was made deeper than the first—18m as the second, 6m as the first… well….
This is actually a myth. The order of the dives doesn’t matter—the decompression simply becomes longer, or the no-deco limits shorter, if there is already pre-saturation. If you want to have as much time as possible at depth, it is more efficient to do the deepest dive without pre-saturation. But that’s all.

Behavior around the dive

What we think we know

Especially when it comes to what you should do or avoid before and after diving, there are many strong opinions. Dehydration is said to be so dangerous, you have to drink a lot, and better not do sports, and under no circumstances should you take a hot shower after diving!
The problem with this: we actually know much less about all these factors than the opinions would suggest. Studies are often contradictory, not statistically significant, and it is difficult to find out which factors contributed to the development of DCS symptoms and to what extent. Here, too, the “problem” lies in the fact that DCS is rare, and therefore little data is available.
For some behaviors, it can be well argued that they could represent a risk factor—but nothing more.

Potentially critical

Exertion after diving

Coming out of the water and immediately lugging lots of heavy equipment around? Technical divers in particular are inclined to exercise caution here. If many tanks have to be taken out of the water, perhaps even carried a bit further, this can be a risk factor. During great exertion, gas bubbles could pass through shunts in the heart or lungs and promote the development of DCS symptoms. Even if there are no clear studies on this, the idea is plausible enough to prefer carrying one tank after another very slowly…
Demanding sporting activities after diving should also probably be viewed with some caution. Boosting the circulation leads to increased blood flow, nitrogen is released faster, and this could make the desaturation process a bit too aggressive. So maybe just no more marathons on the same day, but there is really nothing against a walk.

Dehydration

No one knows exactly when this topic became so relevant among divers—but the belief that dehydration is one of the main causes of DCS is extremely widespread.
A few facts on this: Yes, diving dehydrates. You lose an average of nearly one liter of water during a dive; with several dives a day, it becomes a bit less. Therefore, you tend to be slightly dehydrated after diving—whether you get DCS or not.
Studies have shown that people who drink nothing before diving develop more bubbles than those who drink appropriately. But: even if you drink too much, more bubbles develop. So one cannot speak of a clear result here.

Drinking too much water also carries risks, as it can upset the electrolyte balance in the body. So it is certainly sensible to drink enough, especially in regions with a hot, dry climate—but the connection between dehydration and DCS is by far not as clear as is often claimed.

And what do I make of that?

If you take all of this together, a sober core remains. DCS arises from critical supersaturation, but exactly where the limit is exceeded for a specific person on a specific day is not known in advance. Cold during decompression is an unusually clear risk enhancer, workload is relevant, and age and body composition also shift the risk. But a large part of the risk lies in biological factors that we currently cannot reliably measure or control.

Perhaps this is the most important point of all: not to moralize DCS, neither in oneself nor in others. We don’t know the risk factors exactly, so you can’t name a cause if someone happens to get DCS. Let’s just accept that diving—like any serious activity—carries a residual risk, and talk about it without blame or the desperate search for a reason.

Further reading