Decompression Sickness (DCS): Signs, Causes and Prevention

Decompression sickness (DCS), also called "the bends", is a term divers hear on their very first course. This guide explains it in plain words: what it is, why it happens, the signs, what raises the risk, how divers lower it, and what to do if you think you or your buddy has it.

All facts here come from DAN (Divers Alert Network) and PADI (a dive training organisation). The Thai emergency number comes from Thailand's National Institute for Emergency Medicine (NIEMS). This is not medical advice, and you cannot use it to diagnose yourself. If you have symptoms after a dive, contact emergency services and a doctor straight away.

For the wider picture of diving risks, read Is scuba diving dangerous?

What is decompression sickness?

DAN explains that during a dive, your body tissues take up nitrogen from the air you breathe, in proportion to the pressure around you. The deeper the dive, the faster this uptake. While you stay at pressure, the nitrogen causes no problem. But if the pressure drops too quickly, the nitrogen may form bubbles in the tissues and the bloodstream. These bubbles cause DCS.

PADI compares it to opening a bottle of soda: the sudden drop in pressure makes bubbles appear.

DAN says some cases are mild and not an immediate threat, while others are serious injuries. The sooner treatment starts, the better the chance of a full recovery.

DAN groups DCS with arterial gas embolism (AGE) under one name: decompression illness (DCI). AGE happens when a diver ascends without breathing out: air trapped in the lungs expands and may tear lung tissue. Early first aid is the same for both. That is why PADI's golden rule for the whole dive, and especially the ascent, is "never hold your breath".

What causes decompression sickness?

DAN says the most important risk factor is the dive itself: its time, its depth and how fast you ascend. PADI says the deeper you dive, the more nitrogen you take up and the greater the risk, and that the biggest pressure change is from 10 metres to the surface, so ascending too quickly at the end of a dive can lead to DCS.

Worth knowing: DAN says bubbles can form when divers break their limits, and also when they follow accepted guidelines. A diver may make the same kind of dive many times without a problem, then get DCS on the same profile. Some divers get it more often than others who dive the same way.

Signs and symptoms of DCS

DAN says the most common signs are joint pain and numbness or tingling, followed by muscle weakness and not being able to empty a full bladder. The signs DAN lists include:

Unusual tiredness
Itchy skin or a blotchy rash
Pain in the joints, or in the muscles of the arms, legs or body
Dizziness or a spinning feeling (vertigo)
Ringing in the ears
Numbness, tingling, muscle weakness or paralysis
Shortness of breath
Difficulty urinating
Confusion or changes in behaviour
Staggering
Unconsciousness or collapse

When signs start: DAN says they usually appear within 15 minutes to 12 hours after surfacing. In severe cases they may start before surfacing or right after it. Later onset is rare but can happen, especially when a flight follows diving. PADI says onset can take up to 36 hours.

Don't brush it off: DAN says these signs are often blamed on something else, such as overexertion, heavy lifting or a tight wetsuit. PADI says some divers keep quiet out of embarrassment or to avoid worrying others, and put the signs down to sunburn, dehydration or seasickness. PADI warns that delaying treatment can lead to permanent injury.

What raises the risk of DCS?

DAN lists these known risk factors for divers:

Deep or long dives
Cold water
Heavy exercise at depth
Rapid ascents

DAN says other factors may raise the risk but the evidence is not conclusive: obesity, dehydration, heavy exercise right after surfacing, and lung disease.

Flying or going up to altitude too soon after diving also raises the risk (DAN). DAN also says that warming up fast after a dive, such as a hot shower or a hot tub, makes bubbles form more easily in the tissues.

How divers lower the risk

Ascend slowly: PADI says no faster than 18 metres per minute, and slower still near the surface, where the pressure changes most.
Make a safety stop: PADI recommends a three-minute stop at about 5 metres near the end of a dive, to give your body extra time to release nitrogen. DAN also says to add safety stops to every dive.
Stick to the plan with a dive computer or dive tables: the PADI Open Water course teaches you to use dive tables or a dive computer, which tell you how deep and how long you can dive. PADI says going deeper or longer than planned, or cutting your surface interval short, raises the risk. Compare dive computers
Don't follow your computer blindly: DAN says computer models reflect an average diver, not you. A computer knows nothing about your health, fitness, how cold you are or how hard you work, and divers do get DCS within its limits. DAN advises conservative dive profiles and more conservative settings on the computer. More in What is a dive computer and how to choose one
Practise buoyancy control: PADI says good buoyancy helps you ascend slowly and steadily, which lowers the risk of decompression illness.
Work less, and don't warm up fast after the dive: DAN says keep your effort as low as possible at depth, avoid exercise after a dive for as long as you can, and put off the hot shower or hot tub. PADI adds: avoid alcohol after diving.
Rest and drink enough: DAN counts staying well rested and well hydrated among the small steps that make each dive safer.
Wait before you fly: DAN suggests at least 12 hours after a single no-decompression dive, and at least 18 hours after several dives in a day or several days of diving. Details in When can you fly after diving?

PADI says DCS isn't 100% preventable, but it is extremely rare when divers follow safe diving practices, at around three cases per 10,000 dives on average. PADI also says learning to avoid it is an essential part of dive training at every level. Start with the first course: Learn to dive: PADI Open Water

What to do if you suspect DCS

If you think a diver has a diving injury, DAN says:

Check their airway, breathing and circulation.
Give 100% oxygen if you are a trained oxygen provider.
Call local emergency medical services to get the diver to medical care. In Thailand, NIEMS gives 1669 for medical emergencies.
Call the DAN Emergency Hotline on +1-919-684-9111. DAN says its medics are on call 24 hours a day, 365 days a year, and the line accepts collect calls. From outside the US, dial the country code +1.

If the situation is life-threatening, DAN says to call emergency services or get the diver to the nearest medical facility first, then call DAN.

What DAN and PADI add:

Oxygen is not the end of treatment: DAN says early oxygen may ease the signs, but they can come back later. Always contact DAN or a doctor trained in diving medicine, even if the signs seem to have gone.
Don't wait: DAN says delays raise the risk of lasting symptoms, and after 24 hours or more, treatment may work less well.
Don't go back in the water: DAN says a diver with signs of DCS should, as a general rule, be taken out of the water and given first aid on the surface. PADI says another dive is not the answer.
Hospital first, not straight to a chamber: the treatment for DCS is recompression, with oxygen at high pressure in a recompression chamber. But DAN says an injured diver should be checked at a hospital or emergency clinic first. The nearest chamber may not be the right one, or may not be open at that time.
Ask about chambers before you travel: DAN says your dive operator should be able to give you this information before you arrive. Divers travelling outside the US and Canada can contact DAN's information line about chambers and an emergency plan for their destination.
Keep the facts for the doctor: DAN suggests writing down all dives in the 48 hours before the symptoms, when the symptoms started and what first aid was given, and taking your dive computer with you to the medical check. Some facilities can read your dive profile from it.

To be better prepared, PADI offers the Emergency Oxygen Provider course, which teaches emergency oxygen, and points to a simple neurological check developed by DAN that people with no medical training can do.

Only a doctor can diagnose it

PADI says only a medical professional can diagnose DCS. A diver's job is to know the signs, so the person gets treatment quickly.

DAN says not every diving problem is DCS. Similar signs can come from other conditions, such as inner-ear injury from pressure, stings from marine life, or a neurological problem that happens at the same time by chance, such as a stroke. A doctor needs to rule these out and check that the diver can safely have recompression treatment. Before you dive again, DAN says to always ask a doctor first.

FAQ

What is decompression sickness?

Decompression sickness (DCS) happens when nitrogen your body took up during a dive forms bubbles in the tissues and blood because the pressure around you dropped too quickly. DAN says it ranges from mild cases to serious injuries.

What are the signs of DCS?

DAN says the most common signs are joint pain and numbness or tingling. Others include unusual tiredness, itching or a rash, dizziness, muscle weakness, difficulty urinating and shortness of breath. Most start within 15 minutes to 12 hours after surfacing.

What causes DCS?

Pressure dropping too quickly after a dive. DAN says the most important risk factors are dive time, depth and ascent rate. Other known factors are cold water and heavy exercise at depth, and flying too soon after diving also raises the risk.

What should I do if I suspect DCS?

Give 100% oxygen if you are trained, call emergency services (1669 in Thailand), then call the DAN Emergency Hotline on +1-919-684-9111. Go to the nearest hospital first, and don't go back in the water.

Can I get DCS if I follow my dive computer?

Yes. DAN says dive computers and tables predict, but they cannot guarantee. Dive conservatively, ascend slowly and make a safety stop on every dive.