Decompression sickness (DCS) happens when dissolved gas, primarily nitrogen, forms bubbles in your blood and tissues after a rapid drop in surrounding pressure. Most people associate it with scuba diving, where it’s often called “the bends,” but it can also occur during high-altitude flight in unpressurized aircraft, work inside pressurized construction chambers (historically called caissons), and even spacewalks. Among recreational divers, the estimated rate is about 3 cases per 10,000 dives.
How Nitrogen Bubbles Form
The air you breathe is roughly 78% nitrogen. At the surface, your body absorbs and releases nitrogen in a steady balance. When you descend underwater, the increased pressure forces more nitrogen to dissolve into your blood and tissues, the same way carbonation stays dissolved in a sealed bottle of soda. This follows a basic physics principle: the higher the pressure, the more gas a liquid can hold.
As you ascend, that pressure drops. If the drop is gradual, nitrogen leaves your tissues slowly through normal breathing and never causes problems. But if you rise too fast, the nitrogen can’t escape through your lungs quickly enough. It comes out of solution inside your body, forming bubbles, just like twisting the cap off a soda bottle causes a rush of fizz. Those bubbles are the root cause of every symptom associated with DCS.
Mild vs. Serious Symptoms
DCS is traditionally divided into two categories based on severity.
Type I involves the joints, skin, and lymphatic system. The hallmark symptom is deep, aching joint pain, most commonly in the shoulders, elbows, and knees. This is the origin of the nickname “the bends,” because affected people often hunch or bend to relieve the discomfort. You may also notice a blotchy, marbled rash on the skin or localized swelling.
Type II is more dangerous. It affects the central nervous system, the inner ear, or the lungs. Neurological symptoms can include numbness, tingling, muscle weakness, difficulty walking, problems with coordination, and in severe cases, paralysis or bladder dysfunction. When DCS hits the inner ear, it causes vertigo, hearing loss, and ringing. The spinal cord is the most commonly affected part of the nervous system, with the mid-back (thoracic) region particularly vulnerable because of its blood vessel layout and higher fat content, which absorbs more nitrogen.
A rare but serious form called “the chokes” targets the lungs. Bubbles form aggressively in the veins returning blood to the heart and lodge in the lung tissue. Symptoms include chest pain behind the breastbone, a dry persistent cough, and shortness of breath. These can develop within minutes to several hours after surfacing and closely mimic a blood clot in the lungs.
How Bubbles Damage the Nervous System
When nitrogen bubbles reach the brain or spinal cord, they can cause harm in several ways. Bubbles may physically block blood vessels, starving nearby tissue of oxygen in much the same way a stroke does. In the spinal cord specifically, bubbles are thought to obstruct and congest the veins that drain blood from the cord, while also forming directly within the cord tissue itself. Brain involvement tends to affect the arterial side of circulation more.
Beyond simple blockage, the bubbles trigger inflammation. They damage the inner lining of blood vessels, setting off a cascade of oxidative stress and inflammatory signals that compound the initial injury. MRI scans of affected divers sometimes show areas of swelling and reduced blood flow in the brain or spinal cord, though a normal-looking scan doesn’t guarantee a good outcome. Several factors are linked to poorer neurological recovery: older age, dives deeper than about 39 meters (128 feet), bladder problems at the time of diagnosis, pre-existing spinal degenerative disease, and symptoms that worsen before treatment begins.
Who Is at Higher Risk
Some people are more susceptible to DCS than others, even on identical dive profiles. One of the most significant risk factors is a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that roughly 25% of adults have without knowing it. In most situations a PFO is harmless, but in divers it allows venous blood carrying nitrogen bubbles to bypass the lungs (which normally filter out small bubbles) and pass directly into the arterial system. The overall risk of decompression illness doubles for divers with a PFO. For neurological DCS specifically, the risk increases fourfold, and for those with a large PFO, it jumps sixfold.
Other factors that raise your risk include dehydration, fatigue, heavy exertion during or right after a dive, cold water exposure, obesity (fat tissue absorbs more nitrogen), and older age. Repeated dives over multiple days allow nitrogen to build up in slow-absorbing tissues like cartilage and fat, increasing cumulative risk even when each individual dive stays within standard limits.
Treatment in a Hyperbaric Chamber
The definitive treatment for DCS is recompression therapy inside a hyperbaric chamber. The basic idea is to put you back under pressure so the bubbles shrink and redissolve, then bring you back to normal pressure slowly enough for the gas to exit safely through your lungs.
The most widely used protocol compresses you to the equivalent of about 18 meters (60 feet) of seawater depth. At that pressure, you breathe pure oxygen in 20-minute cycles, with short air breaks in between to prevent oxygen toxicity. The pressure is then reduced to the equivalent of 9 meters (30 feet) for additional oxygen-breathing periods before a slow ascent back to surface pressure. The entire session takes about 4 hours and 48 minutes. Some patients need multiple sessions depending on symptom severity.
If you suspect DCS after a dive, the most important first aid step is breathing high-flow oxygen immediately and staying well hydrated. Lying flat helps prevent bubbles from traveling toward the brain. Getting to the nearest hyperbaric facility quickly matters: delays in treatment are consistently associated with worse outcomes, particularly for neurological symptoms.
How to Reduce Your Risk
Most cases of DCS are preventable with conservative dive planning. The core principle is controlling your ascent rate. Slow ascents and safety stops in shallow water (typically 3 to 5 minutes at about 5 meters/15 feet) give nitrogen time to leave your tissues gradually. Extended time in shallow water has been shown to effectively reduce bubble formation even when deeper portions of the dive were aggressive.
Modern dive computers calculate nitrogen loading in real time and warn you if your ascent is too fast or if you’re approaching no-decompression limits. Following their guidance substantially lowers your risk, though no algorithm eliminates it entirely.
Flying after diving is a common trigger because cabin pressure in commercial aircraft is lower than sea-level pressure, equivalent to roughly 1,800 to 2,400 meters (6,000 to 8,000 feet) of altitude. The CDC recommends these minimum surface intervals before boarding a plane:
- Single no-decompression dive: at least 12 hours
- Multiple dives or multiple days of diving: at least 18 hours
- Dives requiring decompression stops: 24 to 48 hours
These intervals reduce but don’t eliminate risk. Waiting longer is always safer, especially after intensive dive vacations where you’ve been in the water daily.