Understanding Decompression Sickness
Decompression sickness — the bends, DCS, or in older literature caisson disease — is the injury most closely associated with diving in the public mind, and it is the one that generates the most anxiety in new divers. Understanding what actually causes DCS, rather than treating it as a vague hazard attached to going deep, is the first step toward managing the risk sensibly. The physics are not complicated, and the preventive measures are practical and well-established.
The Physics: What Nitrogen Does Under Pressure
Air is approximately 78 percent nitrogen. When you breathe compressed air underwater, the increased pressure forces nitrogen to dissolve into your blood and tissues at a rate proportional to the depth and duration of the dive. This is Henry's Law operating in your body: at higher pressure, more gas dissolves into liquid. The nitrogen that enters your tissues during a dive is not dangerous while you remain at depth under the pressure that keeps it dissolved. The problem arises during ascent.
As pressure decreases on the way up, the dissolved nitrogen must come back out of solution. If the ascent is slow enough, nitrogen diffuses back into the bloodstream and is transported to the lungs, where it exits the body as exhaled gas. This is the normal, safe process that dive tables and dive computers manage by limiting how much nitrogen can be loaded before an ascent and by requiring ascent rates slow enough to allow the off-gassing to occur without forming bubbles.
If ascent is too fast, or if the nitrogen load in the tissues is too high for the rate of ascent, dissolved nitrogen forms bubbles inside the tissues and bloodstream rather than diffusing out through the lungs. These bubbles are the direct cause of decompression sickness. Where they form determines the symptoms: bubbles in the joints cause the characteristic joint pain that gave the condition its historical name. Bubbles in the spinal cord cause neurological DCS, which may present as tingling, weakness, or paralysis. Bubbles in the lungs cause pulmonary DCS, or 'chokes'. Bubbles in the brain cause cerebral DCS with stroke-like symptoms.
No-Decompression Limits
Recreational diving is structured around staying within no-decompression limits (NDLs) — the maximum time at a given depth after which an ascent directly to the surface remains safe, provided the ascent rate is controlled. NDLs shorten dramatically with increasing depth: at 18 metres the no-decompression limit on air is in the range of 56 minutes according to standard tables; at 30 metres it falls to about 20 minutes; at 40 metres to approximately 9 minutes. These are general figures that vary between dive table systems and dive computer algorithms, but the principle is consistent.
A dive computer tracks your actual depth profile continuously rather than assuming a square profile (constant depth for the entire dive), which gives a more generous NDL for dives that begin shallower and go deeper only briefly. Most recreational divers today rely on their dive computer's real-time NDL display rather than manual table calculations, but understanding where these numbers come from and their limitations remains important. Dive computers are models based on mathematical algorithms — they do not directly measure nitrogen in your blood, they estimate it. Different computers use different algorithms and can give different NDLs for identical dives.
Factors That Increase DCS Risk
Several individual factors increase the likelihood of nitrogen bubble formation for a given dive profile. Dehydration is one of the best-documented: when blood volume decreases, tissues are less efficiently perfused and nitrogen is removed more slowly. Diving while even mildly dehydrated — a common state after a flight or a night of moderate alcohol consumption — measurably increases DCS risk. Heavy physical exertion immediately before or during the ascent phase also increases risk by raising perfusion demands and creating micronuclei (tiny bubble seeds) in tissues.
Patent foramen ovale (PFO), a small opening between the right and left chambers of the heart that failed to close after birth, is present in approximately 25 percent of the population. A PFO allows venous blood (which carries more dissolved nitrogen) to pass into the arterial circulation without going through the lungs, where nitrogen would otherwise be exhaled. Divers with large or symptomatic PFOs have a significantly higher risk of neurological and cerebral DCS for a given dive profile. Many serious diving injuries in people whose profiles appeared safe are associated with PFO. Some divers choose to have their PFO assessed and, if indicated, closed. The Divers Alert Network (DAN) has published detailed guidance on PFO and diving.
Repetitive diving across multiple days, as on a liveaboard or dive holiday, creates residual nitrogen in the tissues that successive dives add to. No-decompression limits shorten on repetitive dives, and adequate surface intervals between dives allow partial nitrogen elimination. The standard minimum surface interval recommended by most tables and computers for multi-dive days is one hour, though longer intervals provide more complete off-gassing. After a multi-day dive schedule, a full 24-hour surface interval before flying is commonly recommended; many authorities advise 18 hours at minimum.
Recognising Symptoms
DCS symptoms can appear within minutes of surfacing or take up to 24 hours to develop, though the majority of cases present within six hours. The classic symptom is joint pain — most commonly in the large joints of the shoulders, elbows, hips or knees — that cannot be attributed to an injury. The joint pain of DCS is dull, aching and persistent, and it does not improve with rest. A rash of marbled or mottled skin (cutaneous DCS or 'skin bends') across the trunk is another presentation, generally indicating that bubbles are present in the venous blood.
Neurological symptoms are a more serious presentation and require immediate action: tingling, numbness or 'electric' sensations in the limbs or trunk, weakness or paralysis, bladder or bowel dysfunction, or any visual disturbances. Pulmonary DCS produces chest pain, difficulty breathing, and a dry persistent cough. Cerebral DCS presents with confusion, headache, visual disturbance, or stroke-like unilateral symptoms.
Any symptom that develops after diving should be treated as potentially related to DCS until proven otherwise. The presence of a 'perfect' dive profile does not exclude DCS — individual variation in susceptibility is real and significant.
Treatment: Recompression and Oxygen
The definitive treatment for DCS is recompression in a hyperbaric chamber. Breathing 100 percent oxygen at elevated pressure shrinks existing bubbles (Boyle's Law working in reverse), accelerates nitrogen elimination, and reduces the tissue damage associated with bubble occlusion. Most significant DCS cases require between 1 and 5 hyperbaric treatments, and mild cases often resolve completely when treated promptly. Delay in treatment worsens outcomes, particularly for neurological DCS where spinal cord or brain tissue is affected.
Before reaching a hyperbaric facility, the primary first aid intervention is breathing 100 percent oxygen from a demand-valve oxygen system. Surface oxygen at atmospheric pressure accelerates nitrogen washout from the blood and reduces bubble size modestly. This is why DAN and PADI both recommend that boats and dive operations carry oxygen first aid kits. Fluid rehydration and rest help maintain perfusion while evacuation is arranged.
The key number to know is your nearest hyperbaric facility. Before diving in a remote location, identifying the nearest recompression chamber and the evacuation route to it is part of responsible trip planning. DAN provides a chamber locator as part of its member services, and pre-trip research into this single resource can be the most important preparation you make.
Keeping the Risk Manageable
DCS in recreational diving is uncommon when dive profiles are managed conservatively. Staying well within no-decompression limits, ascending at no faster than 9 metres per minute, and performing a three-to-five-minute safety stop at 5 metres on every dive reduces the probability of bubble formation significantly. Staying hydrated, avoiding alcohol the evening before diving, and not flying within the recommended interval after the last dive complete the practical prevention toolkit.
Open the map to locate dive sites and, just as usefully, plan the logistics between dive locations and the nearest emergency medical services — information that is worth having long before you need it.