Safety

What Is Decompression Sickness?

What the bends actually is, why it happens, how divers avoid it, and what to do if you ever suspect it.

By Mat Mora · Updated 17 June 2026 · ~6 min read

Decompression sickness sounds frightening, and severe cases are serious, but understanding it makes it far less mysterious, and shows you exactly how divers keep it from happening.

Why the bends happens

The air you breathe is mostly nitrogen. Underwater, the higher pressure makes your body absorb more nitrogen than usual, and it dissolves harmlessly into your tissues. As you ascend, pressure drops and that nitrogen has to come back out. Ascend slowly and it leaves gently through your lungs. Ascend too fast, and it can come out of solution as bubbles inside your body, the same way a shaken fizzy drink foams when you open it. Those bubbles cause decompression sickness.

The symptoms

DCS is usually split into two types. Type I is milder: joint and limb pain (the classic 'bends'), unusual fatigue, and skin rashes or itching. Type II is more serious and affects the nervous system: numbness or tingling, weakness, dizziness, difficulty breathing, confusion, and in severe cases paralysis. Symptoms often appear within an hour of surfacing but can be delayed, so stay alert after every dive.

• Joint or muscle pain, often in the shoulders, elbows or knees
• Unusual tiredness out of proportion to the dive
• Tingling, numbness or weakness in the limbs
• Dizziness, confusion or trouble breathing (a medical emergency)

How divers prevent it

1. Ascend slowly: Rise no faster than about 9 to 10 metres per minute, slower than your smallest bubbles. Your dive computer guides this in real time.
2. Stay within your no-decompression limits: Every depth has a time limit you can stay without needing mandatory stops. Your computer tracks it; respect it with margin to spare.
3. Always do a safety stop: Pause for three minutes at five metres at the end of the dive to let extra nitrogen off-gas gently. See our guide to the safety stop.
4. Dive conservatively and wait before flying: Stay hydrated, avoid back-to-back deep dives, and don't fly too soon after diving. See how long to wait before flying.

What to do if you suspect DCS

Treat it as urgent. Get the diver out of the water, lay them down, give 100% oxygen if available, keep them hydrated, and contact emergency services and a diving emergency hotline such as DAN. Definitive treatment is recompression in a hyperbaric chamber, which shrinks the bubbles and lets the nitrogen dissolve away safely. Do not wait to 'see if it passes', and never put the diver back in the water to recompress.

Modern recreational diving is remarkably safe precisely because dive computers, slow ascents and safety stops keep nitrogen loading well within limits. Follow your training and DCS is very unlikely.