The important thing about decompression sickness is not the physics. It is that symptoms are often mild at first, often delayed, and easy to explain away as something else.
This is general information for divers, not medical advice. If you suspect decompression sickness, contact emergency services and a dive medicine specialist rather than a website.
What it is
Under pressure, your body absorbs more nitrogen than it does at the surface. On the way up that pressure falls, and the nitrogen has to come back out.
If it comes out gradually it leaves through your lungs and nothing happens. If the pressure drops faster than your body can offload it, the gas comes out of solution where it is — as bubbles in blood and tissue. That is decompression sickness, and the variety of things those bubbles can press on is why the symptom list is so scattered.
The gas was always there. What matters is how quickly you asked it to leave.
What it looks like
Presentations range from vague to unmistakable, and the vague ones are the ones that get missed.
- Unusual fatigue — more than the dive should have cost you.
- Joint or limb pain, often a deep ache in a shoulder, elbow or knee, not obviously connected to an injury.
- Skin changes — itching, or a blotchy marbled rash.
- Numbness, tingling or weakness, particularly if it is on one side.
- Dizziness, difficulty walking normally, or trouble with balance.
- Confusion or altered consciousness, difficulty breathing, or visual disturbance — the serious end, and an immediate emergency.
The timing is the part worth memorizing. Symptoms typically appear between fifteen minutes and twelve hours after surfacing, and can appear as late as twenty-four hours. A diver who feels strange that evening is still inside the window.
What to do
The response is short, and it is the same whether you are confident or merely suspicious.
| Step | What it means |
|---|---|
| 1. Emergency oxygen | At the highest concentration practical, promptly, and kept going — this is the primary first aid measure |
| 2. Call emergency services | Locally that is 911; DAN also runs a 24-hour emergency line on +1-919-684-9111 |
| 3. Keep them still and comfortable | Lying down, warm, and not walking about |
| 4. Fluids if fully alert | Water, not alcohol or caffeine |
| 5. Do not put them back in the water | In-water recompression is not a recreational procedure |
| 6. Keep the dive information | Profiles, times, depths and gas — the treating team will want them |
And the warning that matters most: do not stand the response down because the symptoms cleared. DAN is explicit that symptoms which appear to resolve on oxygen can come back, sometimes worse, and a diver whose symptoms went away still needs evaluating. Hyperbaric oxygen is the only definitive treatment, and that decision is a doctor’s.
Reducing the odds
None of this eliminates risk, and cases do occur on dives that broke no rules. What follows shifts the odds, which is the honest claim.
Ascend slowly and make your safety stop — the reasoning is in ascent rate and safety stops, and it is the single behavior most within your control. Build conservatism into repetitive days rather than diving to the computer’s edge. Stay hydrated, and treat being cold, tired or hungover as a reason to shorten the dive rather than a detail.
Leave a proper interval before flying, which is its own subject in flying after diving, and matters most on the last day of a trip when the temptation to squeeze in one more dive is strongest.
Why divers talk themselves out of it
The reason cases get reported late is rarely ignorance of the symptoms. It is that every early symptom has an innocent explanation available.
Tired after a two-dive day is normal. A sore shoulder after lifting a cylinder is normal. Pins and needles in a hand after an hour in cold water is normal. Each of those is genuinely the likeliest explanation most of the time, and that is exactly what makes the reasoning dangerous — the diver is not being careless, they are being plausible.
The useful question is not “is this decompression sickness?”, which nobody at the dock can answer. It is “did I dive today, and is something not right?” If both are yes, that is enough to start oxygen and make a phone call. Working out which explanation was correct is somebody else’s job, and they would much rather do it early.
Being the person who can help
Everything above assumes somebody present recognizes what is happening and has oxygen to hand. That is a training question rather than a knowledge one.
The Emergency Oxygen Provider course is short, it is not a diving course in the usual sense, and it is the one that makes you useful in exactly this situation. Rescue Diver covers recognizing a diver in difficulty before it becomes an emergency, which is the earlier and better intervention.
It is also worth knowing what is on the boat before you need it. Asking the crew where the oxygen is kept is a completely normal question on a charter, and part of paying attention to the safety briefing described in what to expect on a dive charter. DAN publishes a full account of decompression illness and its treatment if you want the detail behind any of this.
One last point, because it is the one that costs people most: divers delay calling because they feel foolish about a false alarm. Nobody in dive medicine thinks less of a diver who called and turned out to be fine.
Frequently asked questions
How soon do symptoms appear?
Usually between fifteen minutes and twelve hours after surfacing, and occasionally up to twenty-four hours later. That delay is why a diver who feels odd the evening after a dive should take it seriously rather than assume the dive is behind them.
What do I do if I suspect it?
Give emergency oxygen at the highest concentration available, contact emergency services, and keep the diver still and hydrated. Do not put them back in the water. Do not wait to be sure.
The symptoms went away on oxygen. Are we fine?
No. DAN specifically warns that symptoms which appear to resolve on oxygen can return, sometimes worse. A diver whose symptoms cleared still needs to be evaluated.
Can it happen inside the no-decompression limits?
Yes. Dive computers and tables reduce risk rather than eliminate it, and cases do occur on dives that broke no rules. That is a reason to treat symptoms seriously rather than a reason to distrust your computer.
What makes it more likely?
Fast ascents, repeated deep dives, dehydration, being cold and tired, hard work at depth, and flying too soon afterward. Several of those are within your control — see ascent rate and safety stops and flying after diving.
Learn to give emergency oxygen
The Emergency Oxygen Provider course is short and it is the difference between watching and helping. Ask us when the next one runs.