She stepped off the flight at Suvarnabhumi Airport in Bangkok feeling fine. Six hours earlier she had completed two fun dives in Ko Tao — relaxed profiles, no decompression obligations, standard ascents. Now, in the arrival hall, she reached for her luggage and felt nothing below her waist. The sensation returned, then vanished. By the time she cleared customs, her right leg had stopped responding entirely.
This is not a hypothetical. It is a textbook Type II decompression sickness presentation: delayed neurological onset following an apparently uneventful dive, triggered by cabin pressure equivalent to 2,400 metres altitude. The hyperbaric chamber that ultimately treated her was three hours away. Every minute of that journey mattered.
Understanding decompression sickness — not just its name, but its mechanisms, its disguises, and the minutes that determine outcomes — is the most important medical knowledge any diver can carry into the water.
1. What Is Decompression Sickness?
Compressed air is roughly 78% nitrogen. At depth, Henry's Law forces that nitrogen to dissolve into your blood and tissues at concentrations proportional to ambient pressure. A controlled ascent — slow, with safety stops — allows nitrogen to migrate back to the lungs and be exhaled harmlessly.
When ascent is too fast, or surface pressure drops further (as in a pressurized aircraft cabin), dissolved nitrogen can no longer stay in solution. It forms physical gas bubbles inside blood vessels and tissues. Those bubbles block circulation, compress nerves, trigger inflammatory cascades, and — in the worst cases — rupture capillaries.
(For a detailed treatment of Henry's Law and the nitrogen off-gassing math, see the Flying After Diving guide on this site.)
The result is decompression sickness: a spectrum of injury ranging from a mild skin rash to full paralysis, depending on where the bubbles form and how large they grow.
2. Two Types of DCS
The dive medicine community classifies DCS into two types based on which body systems are affected. Both are emergencies. Neither can be diagnosed by feel alone.
Type I — Musculoskeletal & Lymphatic
Joint pain — commonly the shoulders, elbows, wrists, hips, and knees — that is dull, deep, and unrelated to exercise. Skin marbling or mottling (cutis marmorata): a blotchy, lace-like discoloration that spreads across the trunk. Lymphatic swelling in the armpits or groin. These symptoms feel manageable. That is the danger: a diver who rests and waits can slide into Type II while convincing themselves the pain is post-dive soreness.
Type II — Neurological, Pulmonary & Inner Ear
Neurological: Numbness, tingling, or burning sensations — often described as "pins and needles" that don't resolve. Muscle weakness or partial paralysis. Bladder dysfunction. In severe cases, loss of consciousness.
Pulmonary ("the chokes"): Chest pain behind the sternum. A dry, persistent cough. Breathing difficulty that worsens with deep inhalation. Caused by nitrogen emboli in the pulmonary circulation.
Inner ear (vestibular DCS): Sudden vertigo, nausea, and vomiting. Hearing loss — often in one ear. Tinnitus. Easily confused with "ear barotrauma" from equalization issues; the distinction matters because treatments differ.
3. Symptom Timeline: When They Appear
One of the most dangerous misconceptions about DCS is the belief that symptoms appear immediately. They often don't. A diver who feels fine at the surface can deteriorate hours later — especially if they board an aircraft, ascend to altitude, or exercise vigorously post-dive.
DCS Symptom Onset Timeline
Source: DAN Diving Fatalities & Injuries Database. Onset window distribution based on reported incident data.
The critical implication: a diver who boards a flight four hours after a dive — feeling fine — is still within the highest-risk window for symptom onset. The cabin's reduced atmospheric pressure (equivalent to 1,800–2,400 m altitude) further expands any silent microbubbles already present. This is exactly the mechanism behind the Ko Tao scenario that opened this article.
4. Risk Factors That Multiply Your Odds
DCS is not random. Certain physiological and behavioral factors significantly raise the probability that nitrogen bubbles will form and cause injury. Recognizing them before a dive is prevention. Recognizing them after is context for faster diagnosis.
5. DAN First Aid Protocol
Decompression sickness is not treatable in the field. But what happens in the first 30 minutes after symptom onset — before the hyperbaric chamber — determines how much neurological damage is reversible. The following protocol is endorsed by DAN (Divers Alert Network) and is the global standard for in-water and boat-side response.
Stop Diving Immediately
Any symptom that could be DCS — joint pain, skin changes, numbness, vertigo — ends the diving day. There is no "let's do one more dive and see if it improves." Further pressure exposure does not treat DCS; it loads more nitrogen and worsens bubble formation. Cancel all planned dives for the entire party if a DCS case is suspected.
Administer 100% Oxygen Continuously
Use a non-rebreather mask at ≥10 L/min flow rate. Breathing pure oxygen creates a maximum pressure gradient between lungs and tissues, accelerating nitrogen wash-out from the bloodstream. Do not use a demand valve (scuba regulator) as a substitute — oxygen should flow continuously. Most quality dive boats carry an oxygen first aid kit; ask at the dive briefing if yours does not.
Hydrate — Orally, Not IV
If the diver is conscious and not vomiting, encourage 500–1000 mL of water or sports drink over 30 minutes. This supports blood volume and peripheral circulation. IV fluids are a hospital intervention — do not attempt field IV access unless you are trained and equipped. Alcohol and caffeine are contraindicated (see Section 6).
Keep the Diver Horizontal
Lay the diver flat, not seated. A horizontal position prevents venous bubbles from migrating upward to the cerebral circulation under gravity. If the diver is vomiting, roll them to the recovery position (left lateral). Do not elevate the legs (Trendelenburg position) — this is outdated guidance that increases intracranial bubble migration risk.
Call DAN Emergency Line — Do It Now, Not Later
DAN emergency line: +1-919-684-9111 (24/7, worldwide). The physician on duty will assess the case, advise on evacuation priority, and identify the nearest hyperbaric facility. In Southeast Asia, the nearest chambers are in Ko Samui (from Ko Tao), Pattaya, and Kuala Lumpur. In Bali, the closest chamber is at BIMC Hospital Nusa Dua. Do not wait to see "if it gets better." Call while the diver is still symptomatic — it changes the treatment protocol.
6. What Not to Do
The following actions are commonly taken by well-meaning divers and bystanders — and all of them make DCS outcomes worse.
7. Your Dive Center Matters More Than You Think
Decompression sickness does not strike randomly. Incident data consistently shows a correlation between dive operation practices and DCS rates: overloaded groups that rush ascents, instructors who skip safety stops to stay on schedule, operators who send underequipped beginners into currents they cannot manage.
A skipped 3-minute safety stop at 5 metres reduces nitrogen off-gassing efficiency by an estimated 30–40%. A rushed ascent at double the recommended rate can double the bubble load in critical tissue compartments. These are operational decisions — made by the dive center, not the diver.
When you search for a dive center on ScubaProof, the Safety score reflects aggregated reviewer data on exactly these behaviors: ascent discipline, safety stop compliance, group size management, and emergency oxygen availability. Dive centers with Safety scores below 3.5 have a statistically elevated frequency of phrases like "rushed ascent," "no safety stop," and "guide was in a hurry" in their verified reviews.
Choosing a dive center with a high Safety score is not a luxury. It is the most direct action you can take to reduce your personal DCS risk before entering the water.