Decompression sickness occurs when dissolved gas, primarily nitrogen, forms bubbles inside the body after a rapid drop in surrounding pressure. The condition earned its nickname “the bends” because workers who developed it in the nineteenth century often doubled over in joint pain. While most people associate it with scuba diving, it can also strike pilots, astronauts, and even breath-hold divers under the right conditions. The underlying physics are straightforward, but the ways those bubbles damage the body, who is most vulnerable, and what happens if treatment comes late paint a more complicated picture.
How Bubbles Form in the Body
When you breathe compressed air at depth, the elevated pressure pushes more nitrogen into your blood and tissues than would dissolve at the surface. The deeper you go and the longer you stay, the more nitrogen accumulates. This is simply gas dissolving under pressure, the same reason a sealed bottle of sparkling water holds its fizz until you twist the cap off.
During ascent, the surrounding pressure drops. If it drops slowly enough, dissolved nitrogen travels back through the bloodstream to the lungs and gets exhaled harmlessly. If the pressure falls too fast, the gas comes out of solution faster than the body can clear it, and small bubbles form in the extravascular space or in blood vessels, eventually passing into the venous circulation.1PubMed Central. Decompression illness: a comprehensive overview Those bubbles are the root of the problem. They physically obstruct blood flow, stretch tissues, and trigger a cascade of inflammatory and clotting responses that can damage organs far from where the bubbles first appeared.
What Bubbles Actually Do to Blood Vessels
The damage from decompression sickness goes beyond simple physical blockage. Bubbles scrape against the inner lining of blood vessels, called the endothelium. That contact sets off inflammation and promotes clotting. In animal studies, markers of endothelial injury peak roughly twelve to twenty-four hours after surfacing and can remain elevated for more than seventy-two hours, and the severity of the damage tracks closely with how many bubbles were detected.2PubMed Central. Time Course of Endothelial Dysfunction Induced by Decompression Bubbles in Rats This means the injury is not a one-time hit at the moment of surfacing; it unfolds over days.
Bubbles also shed tiny cell fragments called microparticles into the bloodstream, and these fragments carry inflammatory signals such as interleukin-1β that cause diffuse vascular injuries throughout the body.3PubMed. Microparticle-induced vascular injury in mice following decompression is inhibited by hyperbaric oxygen: effects on microparticles and interleukin-1β The result is that decompression sickness is not purely a mechanical problem of bubbles plugging vessels. It is also an inflammatory disease, which explains why symptoms can worsen or shift in character hours after a dive even without further pressure changes.
Recognizing the Symptoms
Symptoms range from a dull ache in a joint to life-threatening neurological emergencies, and they usually appear within the first few hours of surfacing, though onset can sometimes be delayed up to twenty-four hours. Doctors loosely categorize cases by severity.
Milder presentations typically involve joint and muscle pain, skin mottling or itching, and unusual fatigue. Pain in the shoulders, elbows, and knees is especially common and is the symptom most responsible for the “bends” nickname. The skin may develop a marbled, reddish-blue rash sometimes called cutis marmorata.
More serious cases involve the nervous system. Spinal cord involvement can cause numbness, tingling, weakness in the legs, and difficulty urinating. Cerebral symptoms include confusion, visual disturbances, difficulty speaking, and in rare cases, stroke-like presentations. Inner ear decompression sickness produces vertigo, hearing loss, and ringing in the ears, and it is particularly tricky because it mimics inner ear barotrauma, a different diving injury with a very different treatment.4PubMed Central. Inner ear barotrauma and inner ear decompression sickness: a systematic review on differential diagnostics Distinguishing the two matters because inner ear barotrauma tends to affect hearing more than balance, typically starts during descent, and occurs more often on compressed-air dives at moderate depths. Inner ear DCS tends to hit balance harder, begins during or after ascent, is more common on mixed-gas dives at greater depths, and often accompanies other neurological signs of DCS.
Why Some Divers Are More Vulnerable
Two divers can do the exact same dive profile and one walks away fine while the other ends up in a recompression chamber. Several factors tilt the odds.
Body fat plays a role because nitrogen is much more soluble in fat than in water-based tissue, so a diver with more body fat absorbs more nitrogen at depth and takes longer to off-gas it. Age, dehydration, heavy exertion during or after a dive, cold water exposure, and even strong currents have all been identified as contributors.5Journal of Education, Health and Sport. Risk factors of decompression sickness in scuba diving The practical takeaway is that risk is not determined by the dive tables alone. Your physical state on the day of the dive matters considerably.
The Patent Foramen Ovale Problem
Roughly one in four adults has a patent foramen ovale, a small flap-like opening between the left and right sides of the heart that never fully closed after birth. In everyday life it is harmless for most people. In diving, it can be a serious vulnerability. The opening allows venous blood to bypass the lungs under certain conditions, and with it any nitrogen bubbles that would normally be filtered out by the lung capillaries. Those bubbles can then travel directly to the brain, spinal cord, or inner ear.6Journal of Education, Health and Sport. Patent Foramen Ovale as a Risk Factor for Neurological Decompression Sickness in Recreational Divers: A Narrative Review
The numbers are striking. In one study of recreational divers, a PFO was found in over 97% of those with unexplained DCS compared to about 36% of control divers, and the highest-grade PFOs carried the greatest risk.7PubMed. High-grade patent foramen ovale is a risk factor of unprovoked decompression sickness in recreational divers Earlier research had already shown that PFO was tied specifically to neurological DCS affecting the brain, cerebellum, and inner ear, but not to the lower spinal cord forms of DCS.8PubMed. Patent foramen ovale and decompression sickness in sports divers This pattern makes anatomical sense: bubbles that cross through the heart via a PFO enter the arterial system and tend to lodge wherever arterial blood flows first, which is the brain.
If you are a diver who has had an unexplained neurological DCS hit, especially one involving vertigo, visual symptoms, or confusion after a dive that should have been safe according to the tables, screening for a PFO is worth discussing with a dive medicine physician. Some divers with large PFOs choose surgical closure; others adopt more conservative dive profiles.
First Aid and Treatment
The single most important first-aid step is breathing pure oxygen as soon as symptoms appear. Normobaric oxygen, meaning regular surface-pressure oxygen through a mask, speeds the elimination of nitrogen from tissues and shrinks existing bubbles by creating a steeper gradient for nitrogen to diffuse out. Studies on decompression illness have shown that early first-aid oxygen inhalation significantly improved or stabilized clinical conditions before patients reached a chamber.9PubMed. What is the Best Therapeutic Strategy for Decompression Illness? First Aid Oxygen Inhalation and Hyperbaric Oxygen Therapy Divers should also lie flat, stay hydrated, and avoid any further pressure changes, including flying.
Definitive treatment is hyperbaric oxygen therapy. The patient is placed in a pressurized chamber, typically at two to three times surface pressure, and breathes pure oxygen. The pressure physically compresses the bubbles back to a smaller volume while the oxygen accelerates nitrogen washout and helps oxygen reach tissues that the bubbles had starved. Most treatment protocols involve several hours in the chamber and may be repeated over multiple sessions depending on the severity of the case. Research in mice has demonstrated that hyperbaric oxygen not only resolves bubble-related blockages but also reduces the inflammatory microparticles and interleukin-1β that drive ongoing vascular damage.3PubMed. Microparticle-induced vascular injury in mice following decompression is inhibited by hyperbaric oxygen: effects on microparticles and interleukin-1β
Time matters. Some divers dismiss early symptoms as fatigue or minor joint soreness from hauling gear and delay seeking help. Because the inflammatory injury continues to develop for hours after surfacing, waiting can allow more tissue damage to accumulate. A general rule in dive medicine is that any unusual symptom appearing within twenty-four hours of a dive should be treated as possible DCS until proven otherwise.
Long-Term Consequences
Most cases of DCS resolve fully with prompt treatment. But repeated or severe episodes can leave lasting marks on the body.
Neurological Effects
A study of over two hundred retired professional divers found that those who reported a history of DCS had significantly more symptoms and clinical signs on neurological examination than those who had never been hit. Divers who had experienced neurological DCS showed measurable deficits on motor, sensory, and coordination tests, and their cognitive reaction times were significantly slower.10PubMed Central / Elsevier. Long-term neurological sequelae after decompression sickness in retired professional divers The slower reaction times appeared in divers with any history of DCS, not only those whose episodes were classified as neurological. This suggests that even cases that seem to resolve can leave subtle traces in the nervous system over a career of diving.
Bone Damage
Dysbaric osteonecrosis is a form of bone death linked to repeated or severe pressure exposure. Nitrogen bubbles that enter the fatty marrow inside long bones, particularly the femur and humerus, can reduce blood flow enough to kill sections of bone tissue.11Clinical Journal of Sport Medicine. Dysbaric Osteonecrosis: A Literature Review of Pathophysiology, Clinical Presentation, and Management It predominantly affects divers exposed to prolonged or repeated pressure changes.12PubMed Central. Dysbaric Osteonecrosis in Divers: A Narrative Review With a Systematic Literature Search of Pathophysiology, Prevalence, Clinical Features, and Screening The condition may not cause symptoms for years and is often discovered only when imaging is done for another reason, or when the weakened bone collapses near a joint. Commercial divers and those who work at depth for extended periods are at the highest risk. Recreational divers who follow standard no-decompression limits face far lower odds, but the condition has been documented in sport divers who have had multiple DCS episodes.
Flying After Diving
Climbing to cabin altitude in an aircraft exposes you to lower pressure than at sea level, typically equivalent to sitting at around 1,800 to 2,400 meters above sea level. If you still have excess nitrogen dissolved in your tissues from a dive, that pressure drop can be enough to trigger bubble formation. Guidelines from the Undersea and Hyperbaric Medical Society recommend that after a single no-decompression dive with less than two hours of total dive time in the previous forty-eight hours, divers wait at least twelve hours before flying. After multiday or unlimited diving, the minimum wait is twenty-four hours. If a dive required decompression stops, waiting at least twenty-four hours is recommended, with forty-eight hours preferred.13PubMed. Flying after diving guidelines: a review
These are minimums. Divers who pushed the tables close to their limits, did particularly deep or long dives, or feel unusually tired after diving are wise to add extra surface time. Driving to high altitude, such as crossing a mountain pass on the way home from a coastal dive site, carries the same risk and should be treated with the same caution.
Decompression Sickness Without Scuba Gear
Scuba divers are the group most commonly affected, but they are not the only ones at risk. Anyone experiencing a significant drop in ambient pressure can develop DCS.
Altitude DCS
Pilots and passengers in unpressurized or poorly pressurized aircraft, as well as astronauts performing spacewalks, face the same physics in reverse. Instead of descending under water and absorbing extra nitrogen, they start at sea level and climb into thinner atmosphere. The pressure drop can be enough to pull nitrogen out of solution, especially above about 5,500 meters. Traditional decompression models originally built for diving have had to be adapted for altitude exposures because the physiology changes: breathing rate, blood flow distribution, and the starting amount of dissolved gas are all different at altitude compared with underwater.14PubMed. Adaptive Inert Gas Exchange Model for Improved Hypobaric Decompression Sickness Risk Estimation NASA and military aviation programs use prebreathe protocols, having crew members breathe pure oxygen for extended periods before exposure, to wash out nitrogen and reduce risk.
Breath-Hold Divers and Taravana Syndrome
For a long time, the assumption was that breath-hold divers could not get decompression sickness because they never breathe compressed gas at depth. They carry only one lungful of air from the surface. But competitive and traditional freedivers who do repeated deep dives with short surface intervals can accumulate enough dissolved nitrogen to form bubbles. Polynesian pearl divers recognized a pattern of neurological symptoms after heavy diving days and called it taravana. The most accepted explanation today is that taravana is a specific form of decompression sickness, with microbubbles forming directly in brain tissue.15PubMed Central. Taravana syndrome and posterior reversible encephalopathy syndrome: a microbubble hypothesis for neurological accidents in breath-hold divers A modern case report involving a freediver using an underwater scooter, which allowed faster, deeper, and more repetitive dives than unaided swimming, helped illustrate how the condition develops when surface intervals are too short for adequate nitrogen clearance.
How Dive Computers Try to Keep You Safe
Early recreational diving relied on printed tables that assigned maximum bottom times for a given depth and prescribed surface intervals between dives. These tables were conservative for some profiles and dangerously liberal for others, since they could not account for multileveled descents or individual variation. Modern dive computers sample the ambient pressure every few seconds and run real-time decompression algorithms to estimate how much nitrogen your tissues have absorbed and how much time you can safely spend at depth.
One widely used family of algorithms, the reduced gradient bubble model, was validated against a large database of decompression exposure data and is used in tables, software, and dive computers across recreational, technical, and commercial diving.16PubMed. Computer validation and statistical correlations of a modern decompression diving algorithm Despite these advances, no algorithm eliminates risk. Every model makes assumptions about tissue perfusion rates and bubble behavior that may not match your body on a particular day. Dive computers reduce the probability of DCS, but they are not guarantees. Staying well within the computer’s limits, making slow ascents, and performing safety stops are still the bedrock of prevention.
Decompression Sickness in Marine Mammals
Humans are not the only species to suffer from bubble-related injuries after pressure changes. Beaked whales that strand following exposure to naval sonar have been found to have gas bubble lesions and fat emboli in vital organs that closely resemble human DCS pathology. Researchers concluded that in-vivo bubble formation, possibly triggered or worsened by abnormal diving behavior caused by the sonar, led to nitrogen supersaturation beyond what the animals’ tissues normally tolerate.17PubMed. “Gas and fat embolic syndrome” involving a mass stranding of beaked whales (family Ziphiidae) exposed to anthropogenic sonar signals
Controlled experiments have added supporting evidence. When rats were subjected to simulated dives with simultaneous exposure to intense underwater sound at high pressure levels, they developed significantly higher rates of DCS than animals that dove without the sound exposure.18PubMed Central. Evidence for the initiation of decompression sickness by exposure to intense underwater sound The mechanism is not entirely settled, but one hypothesis is that intense sound energy nucleates bubble formation in tissues that are already supersaturated with nitrogen, essentially giving the gas a nudge to come out of solution faster than it otherwise would. This line of research has fueled ongoing debate about the impact of military and industrial sonar on deep-diving marine species, since animals that routinely dive to great depths and manage nitrogen loading through behavioral strategies may be uniquely vulnerable when those strategies are disrupted.
The Nineteenth-Century Origins of “The Bends”
The condition was first widely recognized not in divers but in laborers building bridge foundations and tunnel shafts using pressurized chambers called caissons. Workers entered these high-pressure environments to excavate underwater, and when they returned to normal atmospheric pressure at the end of a shift, many developed crippling joint pain, paralysis, or worse. Physicians in the nineteenth century studied thousands of casualties among caisson workers and gradually identified the cause: gas bubbles in the blood and tissues caused by too-rapid decompression.19PubMed Central. Discovery of caisson disease: a dive into the history of decompression sickness Their work led to the first treatment plans and prevention procedures, including slower, staged returns to surface pressure, the conceptual ancestor of every decompression schedule used today. The Brooklyn Bridge, completed in 1883, was one of the most famous projects plagued by caisson disease, and its chief engineer Washington Roebling was left partially paralyzed by the condition.