Decompression sickness explained: real risk, real symptoms, real treatment

What it actually is

Air is 78% nitrogen. Your body does nothing with nitrogen — it just sits inert in your tissues at surface pressure. When you breathe compressed air underwater, nitrogen at higher partial pressure dissolves into your blood and tissues. The deeper you go and the longer you stay, the more nitrogen accumulates.

When you ascend, ambient pressure drops, and the dissolved nitrogen has to come out. If you ascend slowly, it comes out gradually through your lungs as you exhale — no problem. If you ascend too fast, or if you have too much nitrogen loaded for the depth profile, the gas comes out of solution inside your body as bubbles.

Those bubbles cause decompression sickness. Where they form determines what symptoms you get:

• Joints (mostly shoulders, elbows, knees) — Type 1 DCS, “the bends.” Pain, usually 1–24 hours after the dive.
• Skin (chest, abdomen) — Type 1 cutaneous DCS, “skin bend.” Mottled rash, itching.
• Spinal cord — Type 2 neurological DCS. Tingling, numbness, weakness, paralysis.
• Brain — Type 2 neurological DCS. Confusion, dizziness, vision changes.
• Lungs — “chokes,” rare. Chest pain, difficulty breathing.
• Inner ear — inner-ear DCS. Vertigo, hearing loss. Tricky to distinguish from middle-ear barotrauma.

Type 1 cases hurt but rarely cause permanent damage. Type 2 cases are medical emergencies — minutes matter.

How often it happens

The 1–2 per 10,000 figure comes from DAN’s annual diving safety reports and matches independent studies from the Royal Navy, US Navy, and various commercial diving operations. It is a population average across all kinds of dives — shallow easy ones and deep repetitive ones.

For a recreational diver doing 2 dives a day to 18 metres on a week-long trip (about 14 dives), the calculated risk is well below 0.1%. For a diver doing 5 dives a day to 30 metres on a liveaboard for a week (35 dives), the risk creeps higher, especially without a day off and a 24-hour pre-flight gap.

Type 2 cases get more medical attention because they can be permanently disabling. Roughly 30% of treated DCS cases are Type 2. The remaining 70% are joint pain or skin manifestations.

Full recovery — defined as no lasting symptoms after treatment — runs around 95% when treatment is prompt (within 6 hours). Recovery drops to around 75–80% when treatment is delayed beyond 24 hours.

What raises your risk

Not all dives are equal. Several factors push the calculated risk well above the population average:

Notes on each:

Deep repetitive diving is the single biggest driver. Each dive loads nitrogen; insufficient surface interval doesn’t fully off-gas before the next dive. Three deep dives in a day starting at 8 a.m., 11 a.m., and 2 p.m. carries much more risk than the same three dives across 24 hours.

Dehydration thickens blood and slows off-gassing. Tropical dive trips combine sun, alcohol, AC, and salty food — most divers are mildly dehydrated by day three. Drink water aggressively.

Flying within 18–24 hours of diving means a second pressure drop while nitrogen is still elevated. DAN’s recommendation: 18 hours after a single no-stop dive, 24 hours after multiple dives or any deco diving. Liveaboards typically end with a full no-dive day.

Alcohol the night before raises risk in two ways: dehydration and impaired sleep. A few drinks during the day after diving is also bad, but the night-before pattern is what most studies have measured.

Cold water triggers vasoconstriction — blood vessels narrow, nitrogen takes longer to leave. Conservative profiles in cold water.

Age over 40 correlates with reduced cardiovascular fitness, slower micro-circulation, and (in many divers) a few extra kilos. The age effect is real but small; a fit 55-year-old has less risk than an unfit 30-year-old.

PFO (patent foramen ovale) — a small hole between the heart’s atria that 25–30% of adults have without knowing. Most never notice. For divers, PFO can let venous nitrogen bubbles bypass the lungs and enter arterial circulation, which raises Type 2 DCS risk. The discussion is nuanced: most PFO carriers dive safely for decades; the elevated risk is real but the absolute number is still small. Tests (bubble echo) exist if you have unexplained DCS.

High body fat percentage — nitrogen dissolves slightly more in fat than lean tissue, and loads/unloads slower. The effect is small in most studies.

Strenuous exertion at depth — pumps gas into tissues faster. Don’t fight currents at 30m if you can avoid it.

Symptoms and timing

A common misconception: DCS symptoms appear immediately on surfacing. They don’t. Median onset is 4–6 hours, with cases ranging from immediate to 24 hours later.

Type 1 symptoms:

• Dull aching joint pain (shoulders most common, then knees, elbows)
• Pain worsens with movement, sometimes localised, sometimes vague
• Skin: blotchy rash, marbled appearance on chest or abdomen, itching
• Fatigue disproportionate to the dive

Type 2 symptoms (medical emergency):

• Numbness or tingling, often starting in extremities
• Weakness, especially asymmetric (one side of body)
• Difficulty walking, balance problems
• Vertigo, severe dizziness
• Visual disturbances
• Confusion, difficulty speaking
• Loss of bladder or bowel control
• Difficulty breathing
• Chest pain

The honest difficulty: many Type 1 symptoms (vague joint pain, fatigue) overlap with normal post-dive sensations and with banged-up gym soreness. If something feels wrong after a dive and the symptoms are new since you got out of the water, treat it as DCS until proven otherwise.

What to do if you suspect DCS

1. Get the diver onto 100% oxygen immediately. Demand-valve or non-rebreather mask. Most dive boats carry oxygen. Keep them breathing it continuously.
2. Lay them flat. Not Trendelenburg (head-down), just supine. Knees can be slightly bent for comfort.
3. Keep them hydrated — water orally if they can drink, or IV if a paramedic is involved.
4. Call DAN’s hotline: +1-919-684-9111 worldwide, 24/7. They route you to local hyperbaric facilities and medical advice.
5. Do not re-enter the water. “In-water recompression” is dangerous without specialised equipment and trained crew. Surface oxygen and a chamber are the path.
6. Get to a chamber. Treatment can be 4–6 hours in a multi-place hyperbaric chamber under US Navy Table 6 or equivalent protocol.

Symptoms can resolve in transit, especially with surface oxygen. Don’t be reassured by improvement — go to the chamber anyway. Bubbles can re-form and damage can compound silently.

Where the chambers are in Asia

A practical map for tropical recreational diving:

• Thailand — Phuket: Vachira Phuket Hospital chamber. Bangkok Hospital Phuket and Mission Hospital also have hyperbaric. About 30 minutes from most Phuket dive sites.
• Thailand — Koh Samui & Koh Tao: SSS Network chamber on Koh Tao (the diving capital). On-island, well-equipped for recreational dives.
• Thailand — Bangkok: Royal Thai Navy hyperbaric facility, plus several private hospitals.
• Indonesia — Bali: Sanglah Hospital in Denpasar has a multi-place chamber. Most Bali dive operators have established transit routes.
• Indonesia — Manado / Bunaken: chamber in Manado.
• Indonesia — Komodo / Labuan Bajo: no on-site chamber — air evacuation to Bali required. Plan accordingly.
• Philippines — Manila and Cebu: multiple chambers in metro Manila, AFP medical centre. Cebu has a chamber serving the Visayas dive areas.
• Malaysia — Kuala Lumpur: several hospitals with hyperbaric.
• Maldives: Bandos Island Resort chamber (the main facility), plus chamber on Kuredu. Reachable from most atolls within hours by speedboat.
• Egypt — Sharm El-Sheikh and Hurghada: multiple chambers on the Red Sea coast.

For each destination, knowing your closest chamber and the evacuation route before you dive matters. Liveaboard operators should brief this. If they don’t, ask. A chamber 6 hours away in a remote area is a different risk profile than one 30 minutes away on Koh Tao.

DAN insurance — why every diver should have it

Divers Alert Network insurance covers the financial side of a DCS event. The chamber treatment alone can run $5,000–$25,000. Evacuation flights from remote locations can add $20,000–$100,000.

DAN membership and dive accident insurance:

• DAN Member (annual): ~$40 USD. Includes the emergency hotline access and basic services.
• DAN Preferred (covers chamber, evac, hospital): ~$70 USD/year.
• DAN Master: ~$95 USD/year. Highest coverage tier.

Regular travel insurance often excludes scuba diving beyond shallow depths. Read the fine print. Many policies cover snorkelling but not scuba, or scuba to 18m but not below.

DAN is non-profit, dive-specific, and the hotline staff are trained in dive medicine. The annual fee is less than one dive on most trips. Treat it like motorcycle insurance — you don’t need it until you do, and then you really do.

Modern dive computers and why they matter

A dive computer continuously calculates how much nitrogen is dissolved in your tissues, models how much you can take, and tells you when to ascend. Compared to old-school dive tables (which assume a square profile to the deepest point of your dive), a computer:

• Tracks your actual depth profile second by second
• Adjusts no-stop time as your depth changes
• Calculates safe ascent rates in real time
• Warns on ascent speed violations
• Tracks repetitive nitrogen loading across multiple dives in a day
• Logs the dive automatically

A computer-equipped diver doing typical recreational profiles has substantially lower DCS risk than the same diver using paper tables — the computer is less conservative when you’re shallow, more conservative when you’re deep, and it tracks reality rather than worst-case assumptions.

Algorithms and conservatism

Different computer brands use different mathematical models for how nitrogen moves through tissues:

• Bühlmann ZH-L16C with Gradient Factors — Garmin, Shearwater, Mares, some Suunto models. The current scientific standard. Adjustable via “gradient factor” settings — GF Low / GF High — that let you choose how conservative.
• Suunto Fused RGBM — Suunto’s variation of a bubble model. Conservative-leaning, especially on repetitive dives. Some divers experience shorter bottom times than Bühlmann.
• VPM-B (Varying Permeability Model) — used in some tech computers and planning software. Bubble-based, considered conservative in deco scenarios.
• DSAT / older PADI tables — derived from US Navy tables, less conservative than modern models.

The honest reality: for recreational diving within no-stop limits, all modern algorithms keep DCS rates near the same low population baseline. The differences matter at deeper, longer, more repetitive profiles — exactly where tech divers care.

For new divers, default settings on any modern computer are safe. Don’t aggressive-tune your gradient factors until you understand them.

Conservative recreational habits that drop your risk

Beyond the computer itself, habits that lower DCS risk to well below the 1–2 per 10,000 baseline:

• Stay shallow for the last few minutes. A 3-minute safety stop at 5 metres is not legally required for recreational dives but is universally recommended. It off-gases a significant fraction of dissolved nitrogen.
• Ascend slowly. Modern guidance is 9 metres per minute or slower. Many computers warn at 10 m/min. The old “60 feet per minute” rule is outdated.
• Surface intervals matter. An hour between dives is the practical minimum. Two hours is better for deep repetitives.
• Skip the deep dive when tired. Fatigue isn’t quantified in any algorithm but it correlates strongly with DCS in incident reviews.
• Don’t dive when sick. Congestion, a chest infection, recent surgery — all reasons to skip.
• Hydrate aggressively. Water and electrolytes. Skip pre-dive alcohol entirely.
• Plan the deepest dive of the day first. Standard recreational practice — repetitive profile is conservative if depth decreases through the day.
• Build in a “no dive” day at the end of a multi-day trip before flying.

What treatment looks like

If you arrive at a hyperbaric chamber with confirmed or suspected DCS:

1. Initial assessment — neurological exam, history of the dive profile, symptoms, oxygen status.
2. Recompression — chamber pressurised to 18 metres equivalent (US Navy Table 6 standard).
3. Oxygen breathing periods — alternating 20-minute pure-O₂ periods with 5-minute air breaks, for several hours.
4. Slow decompression — controlled depressurisation back to surface over additional hours.
5. Total chamber time — 4–6 hours typically, longer for Type 2.

After treatment, divers are advised not to fly for 24–72 hours, and not to dive again for several weeks. Type 2 cases often have follow-up evaluations to confirm no neurological deficit.

Most divers who get treated promptly return to diving within 4–8 weeks with their doctor’s clearance.

Bottom line

Decompression sickness is the dive injury that gets the most attention because it can be permanently disabling. The numbers, in honest perspective:

• Risk for typical recreational diving: 1–2 cases per 10,000 dives.
• Risk for conservative profiles with safety stops, dive computer, hydration, no alcohol: lower still, probably under 1 in 20,000.
• Risk reduction from DAN insurance: zero (it doesn’t prevent DCS), but financial protection is meaningful.
• Recovery rate with prompt treatment: ~95%.

The actions that matter most:

1. Dive a computer and respect what it tells you.
2. Ascend slowly with a safety stop on every dive.
3. Hydrate and skip alcohol around dives.
4. Wait 18–24 hours before flying.
5. Carry DAN insurance.
6. Know where the nearest chamber is at every destination.

This is a manageable risk. Tens of millions of recreational dives happen every year. Most divers will never have a DCS event. Of those who do, the vast majority recover fully. None of that is comfort if you’re the unlucky one — which is why the basics above are non-negotiable.