What Causes Decompression Sickness (the Bends)?

DAN's Project Dive Exploration recorded two to four cases of decompression sickness for every 10,000 dives. Each one starts the same way. Nitrogen dissolves into your tissues at depth. If the pressure then drops faster than your blood can carry that gas to the lungs, it comes out of solution as bubbles. Depth and bottom time decide how much nitrogen you load, and the ascent decides how it leaves.

Decompression illness (DCI) is the umbrella term for two injuries. Decompression sickness (DCS) comes from bubbles forming out of dissolved gas. Arterial gas embolism (AGE) comes from gas forced into the arteries by a lung overexpansion injury, the reason behind the rule about holding your breath on ascent. The first aid is identical, so nobody on the boat has to tell them apart. In two recent case series totalling 3,018 injured divers, 93% had DCS.

How does bottom time load your tissues with nitrogen?

Air is about 78% nitrogen, and at the surface your body sits in balance with it. At 10 metres the pressure doubles, so each breath carries twice the nitrogen, and the extra gas dissolves into blood and then tissue. Not evenly, though. Blood and brain catch up within minutes, while poorly perfused ligament, cartilage and fat take hours. That's why nitrogen from this morning's dive is still in the sums this afternoon.

Decompression models track this with "half-time compartments". A 5-minute compartment closes half the pressure gap in 5 minutes, then half of what's left in the next 5, and is saturated after about six half-times. DAN notes that some algorithms model slow compartments out to almost 500 minutes. The idea dates to 1908, when physiologist John Scott Haldane and two colleagues published rules for preventing "compressed-air illness" using five notional tissues, informed partly by experiments on goats. Many dive computers still run on his logic through Albert Bühlmann's 16-compartment version.

What is supersaturation, and when does it turn into bubbles?

On the way up, the pressure around you falls before the nitrogen in your tissues can follow it out. For a while those tissues carry more dissolved gas than the lower pressure can keep in solution. That's supersaturation, and DAN says most of a diver's tissues pass through it during and after surfacing. A modest gradient is normal: the gas diffuses into the blood and leaves through the lungs. Push it too far with a fast ascent or a skipped stop, and the gas forms bubbles instead.

Bubbles aren't the whole injury. Doppler ultrasound picks up venous bubbles after dives that end with no symptoms, so DAN treats them as a marker of decompression stress rather than a diagnosis. Under a heavy load they distort tissue, block small vessels and trigger inflammation, and DAN's reference calls them the gateway to a far wider cascade of effects. About a quarter of adults have a patent foramen ovale, a small opening between the heart's upper chambers. The large ones, found in under 2% of people, let venous bubbles bypass the lung filter and are strongly linked to brain, spinal, inner-ear and skin DCS.

Can you get the bends inside your computer's limits?

Yes, and DAN is blunt about it. Its treatment guidance says DCS can follow uneventful dives within accepted no-decompression limits, and its medical reference rejects the old label "undeserved hit". An algorithm reads depth and time and applies them to an average body. The no-decompression limit on the screen is a margin built for that average, while your own margin moves from day to day. How that margin gets set is covered in what a dive computer's algorithm changes.

Heat shifts it more than most divers expect. In US Navy experiments, being warm at depth and cooler on the ascent carried at least ten times the DCS risk of the reverse. Warm tissue soaks up gas, and cooled tissue releases it slowly. After the dive, DAN warns that a hot shower or heavy lifting can trigger bubbles. Dehydration gets more blame than DAN's reference considers warranted. DCS is fickle, and a missed water bottle makes a tidier culprit than chance.

What are the symptoms of decompression sickness, and how soon do they start?

Pain and numbness, mostly. A 2011 Lancet review, summarised by DAN, found them among the first complaints in nearly two-thirds of cases. The pain tends to be a deep ache near a joint that doesn't worsen when you move it. Symptoms often start small and build, so they're easy to write off as a strained shoulder. DAN calls denial the worst symptom of DCS.

Pain and numbness: an ache in the shoulders, elbows, hips or knees, or pins and needles.
General symptoms: headache, unusual fatigue or nausea, in about 40% of cases.
Dizziness or weakness: roughly one case in five.
Skin changes: itching, a rash, or marbled skin that DAN warns can come before worse symptoms.
Breathing trouble: dry cough, chest pain and breathlessness, known as "the chokes" and life-threatening.
Nervous system signs: confusion, an unsteady walk or paralysis.

The old split into Type 1 (pain, skin and lymph swelling) and Type 2 (neurological, inner-ear and cardiopulmonary) still turns up, though specialists now name the organ system instead. DAN says symptoms can appear immediately or up to 24 hours after surfacing, most often between 15 minutes and 12 hours. In one series of more than 5,000 cases, 99% had symptoms within six hours. Flying or driving over high ground can bring them on later, which is where the waiting times for flying after diving come from.

What's the first aid for suspected decompression sickness?

Oxygen, as early and as close to 100% as you can deliver. DAN calls it the standard of care for dive injuries, and you don't need to tell DCS from an embolism before starting. A demand valve suits a diver who's breathing comfortably. If they can't trigger it, DAN's guidance is a non-rebreather mask at 10 to 15 litres a minute. A simple mask with no reservoir bag won't reach 100% at any flow, and a 2024 review names that shortfall as a common error.

Speed shows up in the data. In a DAN injury database study of 2,231 cases, divers given oxygen within four hours of surfacing had half the odds of needing multiple chamber treatments. Lay the diver flat, or in the recovery position if they're breathing but not fully conscious. Call emergency services first, then DAN. Don't give painkillers unless medical staff advise it, and don't send the diver back underwater. Symptoms that fade on oxygen can return once it stops, so a medical evaluation still follows.

Definitive treatment happens in a recompression chamber. The most common regime, US Navy Treatment Table 6, takes the patient to the pressure found at 18 metres. They breathe pure oxygen with air breaks for a little under five hours. DAN advises against turning up unannounced at a chamber, because a hospital should rule out other causes first. Take your dive computer along. Its profile is the debrief the treating doctor needs.

When should you call DAN, and what training covers this?

Call as soon as a diver has symptoms after a dive, even mild ones that seem to be easing. DAN Southern Africa runs a 24-hour emergency hotline on +27 82 810 6010, and its medical staff help coordinate evacuation and chamber care. Save it in your phone. Before a Sodwana weekend, not during one. The PADI Rescue Diver course includes an exercise in giving oxygen to a diver with suspected DCI. First Aid Primary & Secondary Care covers the CPR and first aid that Rescue Diver requires beforehand.

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Frequently Asked Questions

Is decompression sickness the same as barotrauma?
No. Barotrauma is damage to an air space, such as a middle-ear squeeze or a lung overexpansion injury, caused by gas volume changing with pressure. DCS comes from dissolved gas forming bubbles on ascent. Both are pressure injuries with different causes.

Does diving on nitrox prevent the bends?
Nitrox cuts nitrogen uptake, but DAN notes that diving it to its own longer limits carries about the same DCS risk as air dived to air limits. The extra margin comes from breathing nitrox while keeping to air limits. Its higher oxygen content also sets a shallower maximum depth.

Is a deep stop safer than a normal safety stop?
There's no good evidence that adding deep stops to recreational no-decompression dives helps, and a 2024 review calls their benefit uncertain. PADI's recommended safety stop is three minutes at 5 metres, and a five-minute stop at 3 to 5 metres is widely used.

How long after the bends can you dive again?
No evidence-based rule exists. A common recommendation is at least a month after treatment ends and symptoms fully resolve, and longer after serious neurological DCS. A doctor trained in dive medicine should make that call.

Should all divers be tested for a PFO?
Two major consensus statements say routine screening isn't appropriate, because PFOs are common and shunt-related DCS is rare. Testing is usually advised after one or two episodes of DCS that look shunt-related. One consensus also suggests it for candidates with migraine with aura or a parent, sibling or child with a PFO.

This is general information, not a substitute for a dive medical or advice from a doctor trained in dive medicine. In an emergency, contact your local emergency services and DAN Southern Africa on +27 82 810 6010.

Reviewed September 2026. Devon Ford runs Scuba Evolution, a PADI 5 Star Dive Centre in Glenvista, Johannesburg.