Decompression sickness: signs, prevention and first aid

Two hours after your last dive, your shoulder aches. You hauled tanks all morning, so you shrug it off. DAN warns divers about this kind of denial. This page explains DCS the way recreational divers learn it. It is study material, not medical advice: if you suspect DCS, call emergency services.

What is decompression sickness?

Decompression sickness is an injury caused by gas bubbles that form in your tissues and blood when the pressure around you drops after a dive. Divers also call it “the bends”. With arterial gas embolism, a lung injury explained in barotrauma and equalizing, it belongs to a wider group called decompression illness (DCI). DAN notes that DCS can follow uneventful dives within accepted limits, but is more likely after a rapid ascent.

What causes decompression sickness?

Nitrogen causes it. Air is about 78% nitrogen, and under pressure your body absorbs more of it: the deeper and longer the dive, the more you take on (Henry’s law). On the way up, you breathe it off. Come up too fast, or after too long at depth, and it can form bubbles, like a soda bottle opened in one go.

DAN lists the known risk factors as deep or long dives, cold water, heavy exercise at depth and rapid ascents. Dehydration, heavy exercise after the dive, obesity and lung disease may add to the risk, with weaker evidence. Flying or driving over a mountain too soon after diving lowers the pressure again and can bring on DCS.

What are the signs of decompression sickness?

The most common signs are joint pain and numbness or tingling. Watch for:

• Common signs: pain in a joint or limb, unusual tiredness, numbness or tingling, itchy skin or a blotchy rash.
• Serious signs: weakness or paralysis, dizziness, trouble breathing, confusion, trouble urinating, collapse.
• Timing: mostly within 6 hours of surfacing, sometimes 24 hours or more, as detailed in the questions below.

DAN calls denial “arguably the worst symptom”: divers blame the ache on effort and wait. When in doubt, treat it as DCS.

Try a question from the app

when signs appear

When do most signs of DCS appear?

1. ADuring the dive itself
2. BWithin 6 hours of surfacing
3. COnly after 24 hours have passed
4. DOnly on the flight home

Show the answer

Answer B. Most signs show up within 6 hours, though some appear up to 24 hours or more after the dive.

How do you prevent decompression sickness?

You prevent DCS by limiting the nitrogen you take on and letting it out slowly.

1. Stay well inside your no-decompression limit. End the dive or move shallower long before your computer’s limit runs out. Writing for DAN, hyperbaric physician Nick Bird notes that staying within 50% of the allowed bottom time carries negligible risk. See dive computers and NDL.
2. Ascend slowly. The US Navy and NOAA use 9 m (30 ft) per minute, according to DAN. Follow your computer’s ascent alarm.
3. Make a safety stop: 3 minutes at 5 m (15 ft), on every dive.
4. Wear your own computer on every dive of the day and of the trip.
5. Wait before flying, as explained in the questions below.

DCS can happen within the limits, so keep a margin, especially when you’re cold, tired or working hard.

What should you do if you suspect DCS?

Act at once; don’t wait to see if it passes. As taught to recreational divers:

1. Have the diver lie down and keep them still and comfortable.
2. If you’re trained, give emergency oxygen at the highest concentration available, ideally 100%, as early as possible.
3. Call local emergency services, then a diving emergency hotline such as DAN’s. Numbers vary by country: put the right ones in your emergency plan before the trip.
4. Watch their breathing and be ready to start CPR.
5. Write down the dives and when each sign appeared, and keep the diver’s computer with them.

Never put the diver back in the water to recompress: it risks cold, drowning and worse symptoms. Treatment happens in a recompression chamber, and the UHMS calls hyperbaric oxygen the mainstay of treatment. DAN warns that signs can fade on oxygen and come back, so the diver still needs a medical evaluation.

Try a question from the app

first aid for DCS

An hour after a dive, a diver has numbness in one arm. What do you do?

1. AWait a few hours to see if it goes away by itself
2. BTake them back down to 5 m (15 ft) to recompress
3. CGive oxygen and call emergency services and a diving hotline
4. DGive painkillers and let them sleep it off

Show the answer

Answer C. Treat it as possible DCS: oxygen at the highest concentration available, the diver lying down, emergency services and a diving hotline such as DAN’s. Never recompress in the water.

Which dive training levels cover decompression sickness?

All four. Open Water Diver teaches the signs, first aid and flying after diving. Advanced Open Water covers deep dives and overstaying your limit. Rescue Diver goes further into first aid, and Divemaster explains the theory behind your computer. Check yourself with the Open Water practice questions, then the Rescue Diver ones.

Frequently asked questions

Can you get decompression sickness within the no-decompression limits?

Yes. DAN notes that DCS can follow uneventful dives within accepted no-decompression limits, and the MSD Manual reports a small number of cases after no-stop dives. Your computer runs a mathematical model; it doesn’t measure your body. A margin, a slow ascent and a safety stop lower the risk, but nothing removes it.

How long after diving can DCS symptoms appear?

Most appear within 6 hours of surfacing: about half within an hour and 90% by 6 hours, according to the MSD Manual. DAN says signs can show up to 24 hours later, and the UHMS notes that a flight or other altitude exposure can bring them on later still. Take any suspicious sign within a day of diving seriously.

Is DCS the same as an arterial gas embolism?

No, but the first aid is the same. DCS comes from nitrogen bubbles and its signs are often gradual. An arterial gas embolism comes from a lung injury, often after holding your breath on the way up, and its signs are sudden, within minutes. You don’t need to tell them apart: oxygen, emergency services and a diving hotline.

How long should you wait to fly after diving?

DAN’s guidelines call for at least 12 hours after a single no-decompression dive, at least 18 hours after several dives a day or several days of diving, and substantially longer after dives with required decompression stops. Treat them as minimums. Your computer shows a no-fly time too: follow the longer of the two.

Sources

1. DAN, Treating Decompression Sickness (The Bends)
2. DAN, Decompression Illness: What Is It and What Is the Treatment?
3. UHMS, Hyperbaric Oxygen Therapy Indications, Decompression Sickness
4. DAN, Guidelines for Flying After Diving (2002 workshop)
5. Nick Bird, A Critical Look at No-Decompression Limits, DAN Alert Diver (2025)
6. DAN Alert Diver, Ascent Rates (2012)
7. DAN Alert Diver, Defensive Dive Profile Planning (2016)
8. MSD Manual Consumer Version, Decompression Sickness
9. MSD Manual Consumer Version, Diving Safety Precautions and Prevention of Diving Injuries
10. DAN, Medical Services and emergency hotline