The injury is called decompression illness (DCI), an umbrella term covering two distinct problems: arterial gas embolism (AGE), where expanding gas tears lung tissue and bubbles enter the bloodstream, and decompression sickness (DCS), where dissolved nitrogen comes out of solution in tissues and blood as ambient pressure falls. Divers, instructors, and hyperbaric physicians use "DCI" because the two conditions can occur together and are treated identically in the field.
The two injuries behind the name
Decompression illness is not one disease. It is a pair of injuries with different mechanisms, different timelines, and overlapping symptoms. Knowing which one is likelier changes what you tell the emergency team.
Arterial gas embolism is the more immediately dangerous of the two. During a rapid ascent, gas in the lungs expands according to Boyle's law: at constant temperature, pressure and volume are inversely proportional. A diver breathing compressed air at 33 feet of seawater (fsw) has lungs holding gas at twice surface pressure. Ascend that last 33 feet while holding your breath, and that gas wants to double in volume. A closed glottis or a region of lung blocked by mucus, scarring, or bronchospasm traps the expanding gas. When alveolar pressure exceeds the tissue's breaking strength, gas forces through the alveolar-capillary membrane into pulmonary veins. From there it travels to the left heart and is pumped to the brain and coronary arteries. This is why AGE can produce stroke-like symptoms within seconds to minutes of surfacing, not hours.
Decompression sickness works on a different clock. At depth, breathing gas at elevated partial pressure, nitrogen dissolves into blood and tissue in proportion to ambient pressure. The tissues load slowly, governed by perfusion and solubility; fatty tissue and poorly perfused joints take up gas more sluggishly than blood. On ascent, ambient pressure drops and the dissolved gas becomes supersaturated. If the pressure gradient exceeds what the tissue can tolerate, nitrogen forms bubbles in situ, in the joint spaces, spinal cord, inner ear, or skin. DCS symptoms typically appear within 15 minutes to several hours of surfacing, though delays beyond 24 hours are reported.
Plain-language version: AGE is gas that was never dissolved, forced into the bloodstream by a torn lung; DCS is gas that was dissolved, coming back out as bubbles inside tissue.
Quantified context: pressures, depths, and time windows
The pressure arithmetic is simple and worth memorizing. Seawater pressure increases by one atmosphere (atm) for every 33 feet of depth, so 33 fsw equals 2 atm absolute (ata), 66 fsw equals 3 ata, and 99 fsw equals 4 ata. Lung gas volume at 33 fsw is one-half the surface volume for the same breath; at 99 fsw it is one-quarter.
The ascent rate most agencies teach is 30 feet per minute, with a safety stop of 3 minutes at 15 feet after dives that approach no-decompression limits. A direct, uncontrolled ascent from 60 feet at 60 feet per minute takes one minute; the same ascent at 120 feet per minute takes 30 seconds, and that is where breath-holding injury risk climbs sharply.
Breath-holding injury is not limited to deep dives. A lung overpressure injury can occur in as little as 4 feet of water if a diver ascends while holding a full breath. Pool-depth incidents are documented in the medical literature, which is why "never hold your breath on ascent" is taught from the first confined-water session.
The no-decompression limits themselves are tissue-loading estimates, not hard walls. At 60 fsw, the commonly taught no-decompression limit is around 55-60 minutes; at 100 fsw, around 20-25 minutes. These figures come from dive tables and computer algorithms, not from a single regulatory standard, and they vary by table, algorithm, and conservatism setting. Treat them as planning tools, not thresholds that guarantee safety on either side.
The time window for treatment matters. Hyperbaric recompression within roughly 4-6 hours of symptom onset is associated with better outcomes in the DCS literature, though delayed treatment still helps and is not futile. This is why the practical instruction is to treat symptom onset as an emergency, not as something to sleep off.
Edge cases that change the picture
Several scenarios make the standard AGE-versus-DCS split blurry or misleading.
A diver who surfaces rapidly and develops symptoms within minutes is usually assumed to have AGE, but rapid ascent also causes DCS, and the two can coexist. Field management does not distinguish them: both get the same oxygen, positioning, and evacuation.
Inner-ear DCS (vestibular DCS) can mimic AGE with vertigo, hearing loss, and nausea, but it is a tissue-bubble problem and may respond differently to positioning. A diver with vertigo after a dive should not be assumed to have AGE based on timing alone.
Patent foramen ovale (PFO), a persistent opening between the heart's upper chambers present in roughly a quarter of adults, allows venous bubbles to cross into arterial circulation without any lung injury. This is a proposed mechanism for some unexplained neurological DCS and for cutaneous-to-neurological symptom progression. PFO screening is a clinical decision, not a routine dive requirement, and closure is not universally recommended.
Breath-hold divers and divers using rebreathers face different loading profiles. Rebreather divers control partial pressure of oxygen and diluent gas, so their inert gas loading can be lower or higher than open-circuit air divers at the same depth, depending on setpoint and diluent. Freedivers who breathe up aggressively before a deep breath-hold can load nitrogen and then ascend rapidly, a combination that produces DCS in a population many assume is exempt.
Counterintuitive professional caveat: the absence of symptoms during the dive means little. Bubbles can form and remain silent, and a diver who feels fine at the surface can develop spinal DCS an hour later. Conversely, many divers with mild joint pain after a dive have musculoskeletal strain, not DCS, which is why hyperbaric physicians weigh symptom character, timing, and dive profile together rather than treating every ache as an emergency.
Misconceptions worth correcting
The most persistent misconception is that DCS only happens to deep divers. Shallow repetitive dives, multiple dives in a day, and dives followed by strenuous activity or flying can produce DCS at modest depths. The tissue loading is cumulative across the day, not reset by surfacing.
Another is that "the bends" refers only to joint pain. The bends is a historical term for DCS, and joint pain is one presentation; neurological, pulmonary, and cutaneous forms exist and can be more serious than a painful knee. Cutaneous DCS (marbling or itching) can precede neurological symptoms and should not be dismissed as a rash.
A third is that breathing oxygen after surfacing cures the problem. Surface oxygen is first aid, not treatment. It reduces the nitrogen gradient and improves oxygenation of ischemic tissue, but it does not recompress bubbles. Only a hyperbaric chamber does that.
A fourth is that ascent rate alone determines risk. Depth-time profile, repetitive dives, hydration, alcohol, fatigue, age, and individual physiology all contribute. Two divers on identical profiles can have different outcomes.
A fifth is that a negative dive computer alert means you are safe. Computers model a limited set of tissue compartments. They are decision aids, not physiological measurements.
Comparison of pressure-related injuries and their signatures
| Condition | Core mechanism | Typical onset | Signature symptoms | First aid identical? |
|---|---|---|---|---|
| Arterial gas embolism | Lung overpressure tears alveolar-capillary membrane; gas enters arterial circulation | Seconds to minutes after surfacing | Sudden confusion, weakness, vision loss, chest pain, seizure | Yes: oxygen, supine, urgent evacuation |
| Decompression sickness (joints) | Dissolved nitrogen bubbles in tissue | 15 minutes to hours | Deep boring pain in shoulder, elbow, knee | Yes |
| Decompression sickness (spinal) | Bubbles in spinal cord vasculature or tissue | 30 minutes to hours | Numbness, weakness, gait instability, bladder changes | Yes |
| Vestibular DCS | Bubbles in inner ear | Minutes to hours | Vertigo, hearing loss, tinnitus, nausea | Yes, but avoid head-down positioning |
| Pulmonary barotrauma without AGE | Alveolar rupture without systemic gas | At ascent | Chest pain, cough, blood-tinged sputum, hoarseness | Yes, plus monitor for pneumothorax |
| Nitrogen narcosis | Inert gas effect on neurotransmission at depth | At depth, resolves on ascent | Euphoria, impaired judgment, fixation | Not applicable; resolves with ascent |
| Oxygen toxicity (CNS) | Elevated oxygen partial pressure | At depth on high PO2 | Visual disturbance, twitching, seizure | Not applicable; reduce PO2 |
Positioning deserves a note in the table's shadow. The traditional advice to lay a suspected AGE patient flat or slightly head-down is intended to limit cerebral gas embolization, but current guidance for DCS generally favors lying flat and avoiding head-down Trendelenburg position because it can worsen cerebral edema and aspiration risk. Field practice is to keep the diver horizontal, on oxygen, and still.
Timeline from ascent to treatment
| Time since surfacing | What is happening | What to do |
|---|---|---|
| 0-1 minute | Gas expansion peaks; lung tear and arterial embolization possible | Stop ascent, breathe normally, do not hold breath; if symptoms, surface and get on oxygen |
| 1-10 minutes | AGE symptoms typically manifest; DCS usually silent | Recognize stroke-like signs; call emergency services; 100% oxygen |
| 10 minutes-2 hours | Early DCS symptoms (joint pain, skin, mild neuro) appear | Oxygen, hydration by mouth if alert, contact dive medical resource |
| 2-6 hours | Spinal and vestibular DCS most likely window; treatment outcome best | Evacuate to hyperbaric facility; do not wait for symptoms to resolve |
| 6-24 hours | Delayed presentation possible; still treat | Hyperbaric referral still indicated; document dive profile and symptom timeline |
| 24-72 hours | Late treatment still considered; recovery may be slower | Hyperbaric physician assessment; no diving until cleared |
| Beyond 72 hours | Residual symptoms may persist; rehabilitation role grows | Specialist follow-up; diving fitness evaluation |
The table's practical message is that the first 10 minutes separate AGE from most DCS, and the first 6 hours separate good outcomes from harder ones. It does not mean later treatment is pointless.
Practical actions and red-flag warnings
The steps below are first aid and evacuation guidance, not a treatment plan.
Ascend slowly and never hold your breath. Target 30 feet per minute, and do a 3-minute safety stop at 15 feet on dives that approach no-decompression limits. If symptoms appear after a dive, breathe 100% oxygen if available, lie the diver flat, keep them still, and call local emergency services or a dive medical hotline immediately. Do not re-enter the water to "test" symptoms. In-water recompression is a specialized, high-risk procedure with specific prerequisites and is not a self-treatment. Do not delay evacuation to see if symptoms improve on their own, and do not let a diver sleep off neurological symptoms. Record the dive profile: maximum depth, bottom time, ascent rate, surface intervals, gas mixes, and symptom onset time. This information changes the hyperbaric assessment. Avoid flying after diving. The commonly taught surface intervals are 12 hours after a single no-decompression dive and 18 hours after repetitive or decompression-stop dives, though these are consensus recommendations, not a regulatory threshold.
Red flags that should trigger immediate emergency evaluation:
Sudden weakness, numbness, or paralysis on one side, or facial droop Loss of vision, double vision, or new hearing loss with vertigo Confusion, personality change, seizure, or loss of consciousness Chest pain, coughing blood, or difficulty breathing after ascent Gait instability, bladder or bowel dysfunction, or band-like torso pain Skin marbling or itching that spreads, especially with any neurological symptom
Plain-language version of the list: any new neurological symptom after a dive is an emergency until proven otherwise; joint pain alone is less urgent but still merits a call.
Limitations of what this explains
This article explains mechanisms, not individual risk. It cannot tell you whether a specific dive profile was safe, whether a specific symptom is DCS, or whether a specific person should dive. Dive tables and computers are population-level models; individual susceptibility varies with physiology, hydration, illness, medication, and factors that are not fully characterized.
The epidemiological numbers are also softer than they look. DCS incidence estimates vary widely by population and reporting, and mild cases are likely underreported. The treatment window figures are associations from observational series, not randomized thresholds. What is well established is the mechanism: expanding gas tears lungs, dissolved gas bubbles in tissue, and both improve with oxygen and recompression.
This article is general educational information, not a diagnosis or treatment plan. If you or another diver has symptoms after a dive, contact local emergency services or a qualified dive medicine professional immediately; do not rely on this article to decide whether to seek care.