How common is DCS?

Decompression sickness—skin mottling after a dive, pain, or even neurological deficits—is frightening. But how common is decompression sickness, actually?

At first glance, the figures circulating seem reassuring. For recreational diving, incidence is usually quoted somewhere between about 0.5 and 3 cases per 10,000 dives. For more demanding profiles—such as in technical diving—a higher risk is assumed. But these numbers share something that’s rarely considered: they are based almost exclusively on cases that show up anywhere at all. In other words, dives after which someone sought medical help, called a hotline, or ended up in a hyperbaric chamber.

Data collection on this question usually fails for two reasons: neither the number of DCS cases is known, nor the number of dives… Still, there are a number of datasets that have tried to close both gaps.

An overview of various datasets worldwide between 1970 and 2007 was provided by Dunford et al. at the start of their analysis of data collected in the DAN “Project Dive Exploration.” Here, values range between 0.5 and 15 per 10,000; averaged across the more than 4 million dives collected in 14 different projects, it’s about 2 per 10,000. We included two of these datasets in our overview below. In the Abacus Project (Ladd 2002), researchers estimated how many cylinders were filled by dive centers in the region and compared that number with cases reported in the press and by insurers. And in an overview of dives on a very large liveaboard with its own hyperbaric chamber, the number of dives can also be determined precisely—and so can the number of people who had to be treated on board (Gilliam 1991). In both examples, however, the definition of what counts as DCS is set very high: requiring treatment, hyperbaric chamber, publicly known—something like “a few spots on the belly” isn’t included.

In Project Dive Exploration, more than 120,000 complete dive profiles were uploaded from dive computers and analyzed over many years (1995 to 2008). This study found a risk of about 0.7 per 10,000 for average recreational dives, and 3 per 10,000 for the full dataset, which includes riskier profiles. Here, DCS was defined as what scientists later recognized as DCS based on clearly traceable criteria. It didn’t always have to involve hyperbaric treatment, but symptoms first had to be reported and had to clearly indicate DCS; unclear symptoms were not included. From this dataset, we show the three strands with clearly different incidences separately.

In addition to the large datasets, we also include a few studies that we’ll discuss in more detail in the next section: survey results that can look very different from the official figures.

In the overview, we show four key data points: the incidence in this study—1 per 10,000 (10⁴) is used as a reference. Many are around this value, but some break out upward.
So we can judge the relevance of the study, we also provide the number of dives that were analyzed.
The next columns then become important: what exactly is counted as DCS in this study, and what type of dives are we talking about?

You can see very clearly here that the incidence varies widely. What stands out is that studies with self-reported cases show far more DCS cases. It’s hard to avoid the suspicion that many symptoms stay “under the radar” and therefore never make it into the statistics.
Of course, it’s possible that some of the cases reported here weren’t actually DCS—but that’s even harder to verify retrospectively than right after the incident. Perhaps the higher incidence in self-reported cases isn’t so alarming, but rather a sign that there’s still something to uncover—more on that later.

And then it matters what kind of dives we’re talking about. The groups with extremely high incidences were tech divers doing dives sometimes beyond 100 m in icy Finnish waters, and divers in Scapa Flow (cold water with square profiles to mid-tissue depths)—followed by dive guides who often drop back down after the actual dive to free an anchor, who do heavy lifting after diving, and who do many dives back-to-back. Normal recreational dives, by contrast, show a very low incidence.

But first, let’s take a look at self-reported symptoms: what can they tell us about whether there’s an unreported number beyond the official figures? And what did our own survey find on this point?

Systematic underreporting—how big is the unreported number?

The list of incidences makes one thing very clear: when self-perceived symptoms are counted as DCS, incidence is massively higher than when validation (doctor, insurer, researcher) is required. A realistic figure will lie somewhere between the “official” numbers and self-assessment.
To estimate how large the unreported number might be, it’s worth looking at the few studies that asked about symptoms that may never have been reported. The three interesting examples we included in the overview are: a survey of Swedish dive professionals; a questionnaire from the British association BSAC to its members; and, as a current and perhaps most interesting source, a study of Finnish tech divers’ “self-treatment.”

An analysis of reports from Swedish dive professionals (Hagberg and Örnhagen 2003) found an extremely high risk of more than 1 in 1,000 dives. In a questionnaire, more than 1,500 active divemasters and instructors were asked about their diving behavior and the symptoms they perceived over one year. While in 1999—the year surveyed here—only 23 DCS cases in Sweden were treated in a hyperbaric chamber, a total of 190 respondents reported symptoms. This self-report yields a risk of 1.52 (men) and 1.27 (women) per 1,000 dives—a figure that clearly differs from what is officially known. The numbers suggest that hyperbaric chamber data are far from capturing all cases.

This questionnaire explicitly focused on people who (a) dive a lot and (b) as a group may have a somewhat more relaxed approach to mild DCS cases. That’s obviously not the average diver. You might find that in another study that also relies on self-reports, from the UK (Leger Dowse 2002). Here, questionnaires were sent to BSAC members over four years; the primary aim wasn’t DCS incidence, but rather differences in incidence between men and women. Here too, the result is a somewhat higher DCS incidence than 1 per 10,000, but at 1.9 per 10,000 dives it remains far below the Swedish cases. What becomes visible, however, is that even here a relevant share—especially of very mild symptoms—never becomes officially known and appears only as self-diagnosed.

The ratio of treated to never officially recorded DCS becomes even more extreme when you look at a study of technical divers in Finland. A group of hyperbaric physicians led by Laura Tuominen launched a survey among technical divers to clarify questions that arose from a previous analysis of more than 20 years of hyperbaric treatments (Tuominen 2022).

In this small group of just 55 divers, 17 (!) reported DCS symptoms over the course of a year, some of them multiple times—a total of 27 cases. Extrapolated to dives, this yields an incidence of almost 1 in 100, an entirely different risk than generally assumed. These are extremely demanding conditions—very cold water and long, deep technical dives undeniably carry higher risk—but the number is still unexpectedly high. As a bias, the authors themselves note that these divers knew they were supposed to report symptoms and therefore observed themselves much more closely than usual. Their way of dealing with symptoms at least doesn’t suggest they were particularly worried: rest, fluids, wait, and sometimes oxygen.
Only sometimes oxygen? Yes, oxygen was used in only 7 of the 27 cases. When asked why, the study quotes some interesting answers:

“The symptoms were so mild or unclear,” “minor pain is part of technical diving,” “there are too many things you have to do after a dive—no time for oxygen,” “a kind of shame about having symptoms,” “don’t know why I didn’t use it even though I teach other divers to use it” (from Tuominen 2022)

What’s already emerging here is a central issue for symptom reporting—and, above all, for how the community deals with DCS: Why is it so hard to talk about? Why is this condition associated with shame and guilt? And what can we do better?

What does our own survey say about this?

In early 2026, we launched a small survey to hear from affected divers about what it was like when they had DCS symptoms. In just a few days, we received surprisingly many responses, and we’ve already analyzed the first 78.

We also asked whether, at any point, the insurance hotline was called, which is available for advice. This question can indicate whether the case was recorded anywhere or not. And a very clear picture emerges: in 58% of reports, there was no call at all. These are at least the cases that don’t show up in any statistics.

Have we solved the question of the unreported number with that? Definitely not. For one thing, our reports come from a group of particularly active, particularly well-informed divers who often assume they can respond appropriately themselves and don’t need further help.
With less experienced divers, the result could be different—without us being able to say in which direction. On the one hand, vacation-only divers are likely less familiar with the symptoms and may simply not recognize one sign or another. One experience report, for example, hints at this: “I do think so [that I had DCS], but as an AOWD diver I didn’t understand it. Only when I got into technical diving did I understand that I’d had DCS.” (ID 1)
On top of that, divers who dive only occasionally often don’t have their own insurance—and therefore don’t know that you can call a hotline. Whether something gets reported then depends on the dive center.
At the same time, it’s reasonable to assume that occasional divers are less inclined to quietly self-treat. If they notice something is wrong, it would be more likely that the case also makes it into official reports.
We won’t be able to resolve which way this goes due to lack of data. And our goal isn’t to determine the unreported number precisely. But we can take these data as an indication that there is a relevant unreported number of relatively mild DCS cases.

Why is the question of the unreported number so important?

In general, diving is a very safe sport—and whether mild symptoms occur a bit less or a bit more often, is that really so important?
Yes, it is—and for several reasons:
It would be great to be able to estimate the risk of a dive as accurately as possible beforehand. Even though that’s currently hard to do reliably, we already know that not all types of dives carry the same risk. Even from the few studies available, we can see that the discrepancy between the risk on a relaxed no-decompression warm-water dive and a 100 m dive in icy conditions is enormous. And to estimate risk, we need data: if someone had symptoms after a specific dive, that’s valuable information that can help improve risk estimation. Data on DCS are therefore extremely valuable for research.
And then the unreported number indicates how DCS is viewed in the community: as something better kept hidden. DCS isn’t treated like a normal sports injury; instead, people keep looking for explanations—even though by far most cases occur after completely normal dives within all recognized limits. A high unreported number is a sign that too little is being said about it, and too quietly. And as committed divers, we urgently need to change that.