Decompression Sickness: Symptoms, Prevention, and What to Do
Every diver learns about decompression sickness in their open water course. Most treat it as a distant, theoretical risk — something that happens to other people who make mistakes. Then it happens to someone they know. Or to them.
DCS is not rare. It's not random. And it is largely preventable once you understand what's actually happening inside your body at depth.
What Is Decompression Sickness?
When you breathe compressed air underwater, nitrogen — which makes up about 78% of that air — dissolves into your blood and tissues under pressure. This is normal and harmless as long as you ascend slowly enough to allow that nitrogen to exit your body the way it came in: gradually, through your lungs.
Ascend too quickly, and the dissolved nitrogen can't escape fast enough. It comes out of solution and forms bubbles inside your tissues and bloodstream. Those bubbles are the problem. Depending on where they form, they can compress nerves, block blood vessels, cause joint inflammation, and damage organ tissue.
This is the mechanism behind every DCS case — from a minor skin rash to a life-threatening spinal cord injury.
Types of DCS
Divers and dive medicine physicians classify DCS into two types based on severity and symptom location.
Type I (Mild) involves musculoskeletal and skin symptoms:
- Joint and limb pain ("the bends") — typically the shoulders, elbows, or knees. Often described as a deep, aching pain that doesn't improve with movement.
- Cutaneous (skin) DCS — mottled, marbled, or blotchy discoloration of the skin, sometimes accompanied by itching. Called "skin bends."
- Lymphatic symptoms — swelling, fatigue, malaise.
- Neurological DCS — numbness, tingling, weakness or paralysis (especially in the legs), loss of bladder or bowel control, confusion, vision problems.
- Pulmonary DCS ("the chokes") — chest pain, coughing, shortness of breath. Caused by bubbles in the pulmonary circulation.
- Inner ear DCS — vertigo, nausea, hearing loss, tinnitus. Often confused with nitrogen narcosis.
- Cerebral DCS — confusion, altered consciousness, seizures.
Symptoms Checklist
Symptoms typically appear within 6 hours of surfacing, with most cases presenting within 1 hour. A small percentage present up to 24 hours later, particularly after flying.
Watch for:
- [ ] Joint or limb pain, especially in large joints
- [ ] Unusual fatigue or heaviness in the limbs
- [ ] Numbness, tingling, or "pins and needles" anywhere
- [ ] Skin mottling or marbling
- [ ] Weakness in arms or legs
- [ ] Difficulty walking or coordinating movement
- [ ] Confusion, difficulty concentrating, or altered behavior
- [ ] Visual disturbances
- [ ] Chest pain or shortness of breath
- [ ] Vertigo or nausea after surfacing
Prevention: The Four Pillars
DCS is not entirely avoidable — the physics guarantee some residual nitrogen at the end of almost any dive. But the risk is manageable with disciplined habits.
1. Ascent rate
The standard recreational ascent rate is 9–18 meters per minute (30–60 feet per minute), with 9 m/min being safer. Most dive computers will alert you if you ascend faster. Slow down, especially in the final 10 meters. The last portion of the ascent is where a significant fraction of off-gassing happens.
2. Safety stops
A 3-minute stop at 5 meters (15 feet) at the end of every dive is standard practice. This is not a regulatory requirement for recreational dives within NDL limits, but it provides meaningful additional off-gassing time, especially for repetitive or deep dives. Think of it as cheap insurance.
3. NDL management
No-decompression limits exist for a reason. Staying within your computer's NDL, with some buffer — not surfacing at 2 minutes of NDL remaining on a repetitive third dive — gives your body time to off-gas properly between dives and within each dive.
4. Hydration
Dehydrated tissue absorbs nitrogen less efficiently and off-gasses less efficiently. A dehydrated diver on the same dive profile as a well-hydrated diver faces meaningfully higher DCS risk. Drink water before and after every dive. Avoid heavy alcohol the night before diving.
Additional risk factors worth knowing: cold water (reduces peripheral circulation and off-gassing), heavy exercise at depth (increases nitrogen uptake), patent foramen ovale (PFO, a heart defect that allows bubbles to bypass the lungs), and altitude travel after diving (reduces ambient pressure before nitrogen is fully eliminated).
If You Suspect DCS: What to Do
Time matters. Every minute of delay before treatment can affect recovery outcomes.
Immediate steps:
1. Administer 100% oxygen continuously. This is the single most important first-aid intervention. Breathing pure oxygen creates a steep partial pressure gradient that drives nitrogen out of bubbles and into the bloodstream for elimination. Every dive boat should carry oxygen; every diver should know where it is. 2. Keep the diver horizontal. Do not have them sit or stand. Lying flat prevents bubbles from pooling in the brain or lungs. 3. Do not re-immerse. Despite the "in-water recompression" protocols that exist in remote locations, re-immersion without a structured protocol and supervision carries real risk of making things worse. 4. Hydrate if conscious. Oral fluids, not alcohol or caffeine. 5. Call DAN (Divers Alert Network) immediately. In the US: +1-919-684-9111 (24-hour emergency line). They will help coordinate evacuation to the nearest recompression chamber. 6. Transport to hyperbaric treatment. Do not delay for a "wait and see" period.
How Hyperbaric Treatment Works
Recompression therapy takes place inside a hyperbaric chamber — a sealed vessel that can be pressurized to simulate depth. By breathing 100% oxygen at elevated pressure (typically equivalent to 18 meters / 60 feet), the increased ambient pressure mechanically compresses the nitrogen bubbles back into solution, and the high oxygen concentration accelerates tissue oxygenation and bubble elimination.
Standard treatment protocols (US Navy Treatment Table 6, Table 5) take 4.5 to 5 hours. Severe cases may require multiple treatments over several days. The sooner treatment begins, the better the outcome — within hours is strongly preferable to the next day.
DCS that resolves spontaneously on its own does not mean hyperbaric treatment wasn't needed. Some symptoms can appear to resolve and return worse. Get evaluated.
FAQ
Can I get DCS on a shallow reef dive?
Yes, though it's much less common. Most recreational DCS cases involve dives deeper than 18 meters (60 feet), rapid ascents, or multiple repetitive dives in a single day. But divers have gotten DCS on dives as shallow as 10 meters under the right (wrong) combination of circumstances.
How long do I need to wait before flying after diving?
DAN and major training agencies recommend at least 12 hours after a single no-decompression dive, and 24 hours after repetitive dives or multiday diving. The cabin pressure in commercial aircraft (typically equivalent to 2,400 meters / 8,000 feet altitude) reduces ambient pressure enough to precipitate bubbles from residual nitrogen. Don't rush it.
Is there a way to test for PFO before diving?
Yes — a bubble contrast echocardiogram ("bubble study") can detect PFO. PFO is present in roughly 25–30% of the population. Most people with PFO dive without incident, but the subset of divers who experience DCS at unusual frequencies or from seemingly minor dives are often found to have PFO. Discuss it with a dive medicine physician if you've had unexplained DCS.
What's the difference between DCS and arterial gas embolism (AGE)?
AGE occurs when lung tissue ruptures (usually from breath-holding on ascent) and air bubbles enter the arterial bloodstream directly. It presents immediately on surfacing with severe symptoms including loss of consciousness and is even more immediately life-threatening than DCS. Both require 100% oxygen and urgent evacuation to a hyperbaric chamber — the treatment approach is the same.