Decompression sickness, commonly called DCS or “the bends,” occurs when dissolved gas in a diver’s tissues forms bubbles during or after ascent from a dive. Inert gas, typically nitrogen, accumulates in the body under pressure; when a diver rises too quickly, that gas can come out of solution the same way carbon dioxide fizzes from a freshly opened bottle. More than a thousand cases of decompression illness are reported annually in the United States alone, and the condition can range from a mild skin itch to life-threatening neurological damage.1Postgraduate Medical Journal. Bubble trouble: a review of diving physiology and disease Understanding how DCS works, who is most vulnerable, and what to do about it can make the difference between an uneventful dive day and a medical emergency.
How Bubbles Form Inside the Body
When you descend underwater breathing compressed gas, the elevated ambient pressure pushes nitrogen (or whatever inert gas is in your breathing mix) into your blood and tissues. The deeper you go and the longer you stay, the more gas dissolves. On the way back up, ambient pressure drops and that dissolved gas wants to return to its gaseous state. If the pressure of the dissolved gas in your tissues exceeds the surrounding pressure, tiny bubbles can form in the spaces between cells or inside small blood vessels, eventually traveling into the veins.2PubMed Central. Decompression illness: a comprehensive overview
A question researchers have wrestled with for decades is where these bubbles actually start. Bubbles do not simply appear from nothing in a uniform liquid; they need a “seed,” a microscopic starting point called a gas nucleus. Laboratory experiments suggest that spontaneous bubble formation in pure liquid requires far greater pressure differences than divers ever encounter. Instead, the best candidates for nucleation sites are tiny gas pockets trapped on rough or water-repelling surfaces inside blood vessels and tissues. Nanobubbles on hydrophobic surfaces have been observed under atomic force microscopy and remain a possible seed source, though their ability to grow is debated because they are extremely stable.3Advances in Colloid and Interface Science. A critical review of physiological bubble formation in hyperbaric decompression For practical purposes, this means bubble formation is not perfectly predictable. Two divers doing the exact same profile can end up with very different bubble loads.
What DCS Feels Like
DCS symptoms can show up within minutes of surfacing or creep in over several hours. Clinicians traditionally divide them into two broad categories, though plenty of cases fall somewhere in between.
- Musculoskeletal DCS: Deep, aching joint pain, most often in the shoulders, elbows, or knees. This is the classic “bends.” The pain may be dull at first and worsen over time.
- Neurological DCS: Numbness, tingling, muscle weakness, difficulty walking, dizziness, or changes in vision. Spinal cord hits are particularly common in divers, sometimes causing bladder dysfunction or leg weakness that can become permanent if untreated.
- Cutaneous DCS: A marbled, purplish rash sometimes called “cutis marmorata,” often on the trunk. Skin-only symptoms are considered less severe, but they can signal deeper problems developing.
- Inner ear DCS: Vertigo, hearing loss, and ringing in the ears. This variant is more common in divers breathing helium-based gas mixes.
- Pulmonary DCS: Rarely, a massive bubble load overwhelms the lungs’ ability to filter venous gas, causing chest tightness, coughing, and breathing difficulty, historically called “the chokes.”
Because DCS can mimic many other conditions, from a pinched nerve to a stroke, it should always be on the radar for anyone who develops new symptoms after a dive. The formal term “decompression illness” lumps DCS together with arterial gas embolism, a related but distinct problem where gas enters the arterial circulation, usually from lung over-expansion during ascent. The two can overlap and are sometimes impossible to tell apart in the field.4PubMed Central. Decompression illness: a comprehensive overview
Who Gets Hit and Why
DCS is not equally likely for everyone, even on the same dive. Research on recreational divers has shown that age, body weight, and aerobic fitness all correlate with how many venous bubbles a person generates after a dive. Older, heavier divers with lower aerobic capacity produced significantly more post-dive bubbles, while body fat percentage alone was a weaker predictor than those other factors.5PubMed. Circulating venous bubbles in recreational diving: relationships with age, weight, maximal oxygen uptake and body fat percentage Dehydration, fatigue, cold water, and exertion during the dive also appear to increase risk, though isolating any single variable in human studies is difficult.
One anatomical factor that has received a lot of attention is a patent foramen ovale, a small flap-like opening between the left and right chambers of the heart that persists from fetal development. Roughly a quarter of the general population has one. In a case-control study of Belgian sport divers, those who suffered neurological DCS affecting the brain, inner ear, or upper spinal cord had a significantly higher prevalence of large PFOs compared to matched controls who had never had DCS. The same association did not hold for DCS localized to the lower spinal cord.6PubMed. Patent foramen ovale and decompression sickness in sports divers The mechanism makes intuitive sense: venous bubbles that would normally be filtered by the lungs can cross through the PFO into arterial blood and travel directly to the brain or other organs. Whether all divers should be screened for a PFO remains debated, but the data are strong enough that divers with a history of unexplained neurological DCS are often advised to get tested.
First Aid That Actually Matters
If you suspect DCS after a dive, two interventions stand out above everything else: breathing high-flow oxygen and drinking fluids. Surface-level oxygen helps for a simple reason. Breathing pure oxygen creates a steep gradient that pulls dissolved nitrogen out of tissues faster while also delivering more oxygen to any tissues starved by bubble-blocked blood flow. Studies have found that early first-aid oxygen improved or stabilized clinical symptoms of decompression illness in a meaningful number of cases.7PubMed. What is the Best Therapeutic Strategy for Decompression Illness? First Aid Oxygen Inhalation and Hyperbaric Oxygen Therapy Some divers improve enough with surface oxygen alone that they ultimately avoid recompression treatment entirely.8The Lancet. Field management of decompression sickness
Hydration matters because bubbles make blood stickier and promote clotting in small vessels. Animal research has demonstrated dramatic benefits from pre-hydration: in the most hydrated group, severe DCI dropped from nearly half to zero. Oral fluids are usually fine unless the diver is vomiting or unconscious; intravenous fluids are an option when medical personnel are available. Beyond oxygen and fluids, the definitive treatment is hyperbaric oxygen therapy in a recompression chamber, which both shrinks bubbles mechanically and drives inert gas out of tissues under controlled conditions.9The Lancet. Field management of decompression sickness
Recompression Treatment and Oxygen Toxicity
The standard treatment protocol used by many facilities worldwide is based on the U.S. Navy Treatment Table 6, which involves breathing oxygen at increased pressure inside a hyperbaric chamber over several hours. The treatment works well, but it is not risk-free. Breathing oxygen at elevated pressures for extended periods can trigger seizures, a phenomenon called central nervous system oxygen toxicity. A review of a thousand treatments with the Navy protocol found an overall seizure rate of about 0.6% in multiplace chambers and no seizures in monoplace chambers, with no statistically significant difference between the two chamber types.10PubMed Central. Oxygen toxicity seizures during United States Navy Treatment Table 6: An acceptable risk in monoplace chambers? That rate is low enough to be considered acceptable given the stakes involved, but it is one reason chamber treatments include periodic “air breaks” where the patient temporarily switches back to breathing regular air.
Dive Computers and Decompression Models
Modern dive computers use mathematical models to estimate your tissue gas loading in real time and tell you when to stop, slow down, or make decompression stops. These models trace their lineage back more than a century, but they are not infallible. When four widely used commercial algorithms were tested against U.S. Navy experimental dive data with known DCS outcomes, none of the four passed all validation tests at their factory-default conservatism settings. Two of the four, the Bühlmann ZHL-16C and Suunto RGBM models, could be made to pass by adjusting user-defined conservatism settings upward.11PubMed Central. Validation of algorithms used in commercial off-the-shelf dive computer
The practical takeaway is that your dive computer gives you an estimate, not a guarantee. Divers who push right up against their computer’s no-decompression limits on every dive are operating closer to the edge than they might realize. Adding personal conservatism, whether by increasing your computer’s conservatism setting, ascending more slowly than required, or adding a safety stop, provides a buffer against the model’s inherent limitations and your own individual susceptibility.
Exercise, Conditioning, and Pre-Dive Habits
There is a curious timing effect with exercise and bubble formation. Vigorous exercise during or immediately after a dive increases DCS risk, presumably by generating more nucleation sites and increasing blood flow to already-supersaturated tissues. But exercise before a dive appears to do the opposite. In rat experiments, exercise performed about 20 hours before a simulated dive dramatically suppressed bubble formation, and most of the exercised animals survived conditions that killed unexercised controls. Exercise at other time windows had no such effect.12PubMed Central. Exercise and nitric oxide prevent bubble formation: a novel approach to the prevention of decompression sickness? The leading explanation involves nitric oxide: exercise boosts nitric oxide production in blood vessel walls, which may reduce the number or stability of gas nuclei that can later grow into problematic bubbles.
Human studies have confirmed this general pattern. A single sub-maximal exercise session performed shortly before immersion significantly reduced post-dive bubble grades in scuba divers. Interestingly, the researchers found that moderate dehydration from sweating at the start of the dive seemed beneficial, while rehydrating during the decompression phase was important. This suggests hydration has a two-phase relationship with bubble formation: slightly concentrated blood at depth may be protective, but once you are off-gassing on the way up, being well-hydrated helps clear nitrogen faster.13PubMed. Endurance exercise immediately before sea diving reduces bubble formation in scuba divers
DCS in Breath-Hold Divers
Most divers assume DCS only affects people breathing compressed gas underwater, but that is not strictly true. Breath-hold divers who perform repeated deep dives with short surface intervals can accumulate enough nitrogen to develop decompression sickness. The condition has been recognized for decades in the Pacific islands, where it is known as “Taravana syndrome,” after neurological symptoms observed in pearl divers. Taravana is characterized by confusion, visual disturbances, or other neurological problems after intensive repetitive freediving, and the most widely accepted explanation is that it represents a form of DCS specific to breath-hold diving.14PubMed Central. Taravana syndrome and posterior reversible encephalopathy syndrome: a microbubble hypothesis for neurological accidents in breath-hold divers
A systematic review found that both DCS and arterial gas embolism should be considered potential risks for breath-hold divers, just as they are for compressed-gas divers.15PubMed Central. The risk of decompression illness in breath-hold divers: a systematic review Brain involvement from breath-hold DCS remains rare but is well-documented, and repeated dives with short surface intervals are the common thread in nearly every reported case.16PubMed Central. Breath-Hold Diving-Related Decompression Sickness with Brain Involvement: From Neuroimaging to Pathophysiology Competitive freedivers doing back-to-back training dives should be aware that they are not immune to bubble-related injury simply because they are not carrying a tank.
Isobaric Counterdiffusion in Mixed-Gas Diving
Technical divers who switch between breathing gases containing different inert gases, such as moving from a nitrogen-based bottom mix to a helium-based decompression mix, face an additional DCS mechanism that recreational divers never encounter. When tissues saturated with one inert gas are suddenly exposed to a different inert gas at the same overall pressure, bubbles can form even though no change in depth has occurred. This phenomenon, called isobaric counterdiffusion, was demonstrated in animal experiments where switching from nitrogen to helium at a constant pressure generated venous bubbles detectable by Doppler ultrasound for up to 12 hours.17PubMed. Venous gas bubbles: production by transient, deep isobaric counterdiffusion of helium against nitrogen The practical lesson for technical divers is that gas switches must be planned carefully. Switching to a helium-rich mix at depth when tissues are already loaded with nitrogen can create local supersaturation even without ascending.
Long-Term Effects of DCS
The consequences of DCS do not always end when acute symptoms resolve. One well-recognized long-term complication is dysbaric osteonecrosis, a condition where areas of bone die due to disrupted blood supply, presumably from bubbles blocking the small arteries that feed bone tissue. It has historically been associated with commercial and military divers who accumulate thousands of hours at depth, but case reports have documented it in technical recreational divers as well. Inadequate decompression, repeated DCS events, deep or frequent dives, and advancing age are all common features in affected individuals.18PubMed Central. Dysbaric osteonecrosis in technical divers: The new ‘at-risk’ group? The condition often affects the hip and shoulder joints, and it can progress to joint collapse requiring surgical intervention.
Even divers who have never been formally diagnosed with DCS may carry evidence of subclinical damage. An MRI study comparing military divers to non-diving controls found brain white-matter lesions in about 23% of divers versus 11% of controls, a statistically significant difference.19PubMed. Cerebral white-matter lesions in asymptomatic military divers These bright spots on brain scans do not necessarily cause noticeable symptoms, but they are markers of small-vessel injury. Whether years of recreational diving at modest depths produces similar changes is less clear, but the finding raises questions about what “uneventful” diving does to the brain over a long career.
In-Water Recompression
When a diver develops serious DCS symptoms far from a recompression chamber, the question of whether to attempt in-water recompression comes up. The idea is straightforward: take the symptomatic diver back underwater on oxygen to shrink bubbles and allow off-gassing, essentially performing a makeshift chamber treatment in the ocean. The theoretical benefit is an extremely short delay to recompression, which data from military settings support. In military training dives where treatment began within two hours, more than 90% of cases resolved completely during the first treatment, often within minutes of repressurization.20PubMed Central. In-water recompression
The problem is that in-water recompression carries real dangers. Breathing oxygen at depth increases the risk of oxygen toxicity seizures, which underwater means drowning. Typical IWR protocols limit depth to about 9 meters and keep the treatment to one to three hours, shallower and shorter than standard chamber protocols. Case reports of successful IWR exist, including in competitive freedivers, but they also highlight serious safety and ethical failures in how the procedures are sometimes carried out.21PubMed. In-water recompression for severe decompression illness in competitive freediving: a case report The general consensus is that IWR is not justified for mild symptoms likely to resolve on their own, nor for divers too impaired to be safe in the water. It occupies a narrow window: life- or limb-threatening DCS, properly trained and equipped support personnel, and no realistic chance of reaching a chamber in time.
Detecting Bubbles Before They Cause Trouble
One of the frustrations of DCS research is that venous bubbles are extremely common after dives, even dives that cause no symptoms at all. Doppler ultrasound and echocardiography have been the standard tools for detecting these circulating bubbles post-dive.22PubMed Central. Perspective on ultrasound bioeffects and possible implications for continuous post-dive monitoring safety In a study testing self-monitoring with portable audio Doppler devices, about a third of post-dive measurements detected bubbles. The technique had high specificity at 95%, meaning it rarely said “bubbles” when there were none, but overall sensitivity was only 40%, climbing with higher bubble grades.23PubMed Central. A Doppler ultrasound self-monitoring approach for detection of relevant individual decompression stress in scuba diving In other words, a clean audio Doppler reading does not reliably mean you are bubble-free after a dive.
On the biomarker front, researchers have found that tiny cell fragments called microparticles, shed from blood cells and vessel walls in response to bubble-induced damage, are strongly associated with DCS. In one study, microparticle levels were between 2.4 and nearly 12 times higher in divers who developed DCS compared to asymptomatic divers matched for dive profile and depth. Elevated microparticles and signs of activated white blood cells both showed strong statistical associations with DCS.24PubMed. Association of microparticles and neutrophil activation with decompression sickness These biomarkers are not yet used clinically to diagnose DCS in the field, but they may eventually help distinguish who needs aggressive treatment from who can be safely monitored.
How Marine Mammals Avoid the Bends
Whales, seals, and sea lions perform deep, repetitive dives that would cripple a human diver with DCS, yet they almost never develop bubble-related injury. Part of the answer is anatomical. Whales possess elaborate networks of blood vessels called retia mirabilia, made up of thick-walled arteries and thin-walled sinusoids embedded in fat. Because nitrogen is roughly six times more soluble in fat than in water, researchers have proposed that these retia act as nitrogen traps: as a whale ascends and its blood becomes supersaturated with nitrogen, the gas diffuses into the surrounding fat rather than forming bubbles that could reach the brain.25Journal of Experimental Biology. On how whales avoid decompression sickness and why they sometimes strand
Behavioral adaptations play a role too. Many marine mammals ascend slowly from deep dives, take longer surface intervals after particularly deep excursions, and collapse their lungs at depth to limit gas exchange. Studies on Steller sea lions found that blood microparticle levels, a possible marker of decompression stress, did not correlate with diving depth or time underwater, suggesting that their bodies may acclimate to repeated pressure exposure in ways human divers cannot easily replicate.26PubMed. Dive, food, and exercise effects on blood microparticles in Steller sea lions (Eumetopias jubatus): exploring a biomarker for decompression sickness Interestingly, mass strandings of beaked whales have been linked to sonar exposure, raising the possibility that abnormal behavior triggered by acoustic disturbance might override the animals’ natural anti-DCS strategies. The physiology of diving mammals continues to inform human decompression research, even if humans are unlikely to evolve retia of their own anytime soon.