Decompression Sickness Diving: What It Is, What Causes It, and How to Stay Safe
It has a nickname that sounds almost casual: the bends. But decompression sickness is one of the most serious medical emergencies a diver can face — and understanding it isn't just for technical or deep divers. It's essential knowledge for anyone breathing compressed gas underwater.
The good news: decompression sickness diving incidents are largely preventable. The even better news: most of what protects you is straightforward, well-established, and entirely within your control. Here's what you need to know.
What Is Decompression Sickness?
Decompression sickness occurs when inert gas—usually nitrogen absorbed while breathing compressed air—comes out of solution and forms bubbles in tissues or circulation during decompression. The amount of gas absorbed depends on factors including pressure and time. A controlled ascent and a dive profile within your plan and computer limits help manage decompression, but they do not make risk zero.
A carbonated drink is a useful, imperfect analogy: gas stays dissolved under pressure and can form bubbles as pressure falls. Human decompression is more complex, so the analogy should not be used to predict symptoms or treatment.
DCS is uncommon, but estimates vary with dive type, case definition, and reporting method. DAN’s Project Dive Exploration reported an overall estimate of roughly 2–4 cases per 10,000 dives; that figure is not a universal rate for every diver or dive. DCI can also occur after a dive that appeared to follow a computer or table, so symptoms still warrant attention.
The Two Types of Decompression Illness
The umbrella term "decompression illness" (DCI) actually covers two distinct conditions that are often grouped together because their symptoms overlap and field diagnosis can be difficult.
Decompression Sickness (DCS) results from nitrogen bubbles forming in tissues and the bloodstream during or after ascent. Bubbles can form almost anywhere in the body, which is why symptoms vary so widely.
Arterial gas embolism (AGE) usually follows lung overexpansion injury: expanding gas can enter the arterial circulation, including during an ascent in which a diver holds their breath or has another cause of pulmonary barotrauma. AGE can be rapidly life-threatening, including after a shallow dive.
Suspected DCI is an emergency. Do not try to distinguish DCS from AGE in the field before seeking help; activate emergency care and follow trained first-aid guidance.
Recognizing the Symptoms
DCI symptoms vary in type, severity, and timing. Traditional “Type I” and “Type II” labels are shorthand, not a self-triage tool; the boundary between categories is imperfect.
Traditional Type I and Type II labels are shorthand, not a self-triage tool; the boundary between categories is imperfect.
Possible pain, skin, or lymphatic symptoms
Joint and limb pain — the classic "bends," most commonly affecting the shoulders, elbows, and knees
Skin rash or itching (cutaneous DCS) — sometimes presenting as mottled, marbled skin
Fatigue and general malaise that seems disproportionate to the dive
Lymph node swelling
Possible neurological, inner-ear, or cardiopulmonary symptoms
Neurological symptoms: numbness, tingling, weakness, or paralysis — particularly in the legs (spinal DCS) or face
Dizziness, vertigo, tinnitus, or hearing loss (inner ear DCS — its exact prevalence varies across studies but it's considered a less common DCS manifestation)
Breathing difficulty or chest pain (pulmonary DCS — sometimes called "the chokes")
Confusion, vision disturbances, or stroke-like symptoms
In severe cases: cardiac collapse
When Do Symptoms Appear?
Symptoms may begin during ascent, soon after surfacing, or later. Many presentations occur within hours, but there is no single timing window that rules DCI in or out. Do not dismiss symptoms because they seem mild, delayed, or unrelated to the dive.
Do not dismiss symptoms because they seem mild or resemble ordinary post-dive tiredness. Report concerning changes promptly and seek medical advice; only qualified clinicians can assess whether symptoms are dive-related.
When in doubt, seek prompt medical advice. A diver with concerning symptoms should not re-enter the water or fly while awaiting evaluation.
What Can Affect Risk?
DCS risk varies with the dive profile and individual circumstances. No single factor reliably predicts what will happen to one diver.
Dive-profile factors
Ascent rate. Guidance differs among training organizations and computers. DAN reports that agency recommendations vary; follow your training, dive plan, and computer, and make a controlled ascent without exceeding the rate specified for your profile.
Depth and bottom time. Greater depth and longer exposure affect inert-gas uptake. Stay within the limits of your training, plan, and computer, and remember that no-decompression limits are not a guarantee of zero risk.
Safety stops. A three-minute stop around 5 metres (15 feet) is a common recreational precaution when the dive plan, gas supply, conditions, and computer permit it. It is not a substitute for required decompression stops, and it should not delay an emergency ascent or conflict with your plan.
Multiple dives. Residual inert gas from earlier dives matters. Follow your computer and plan across the full sequence of dives, allow appropriate surface intervals, and keep a margin rather than treating each dive as independent.
Flying after diving. DAN recommends at least 12 hours after one no-decompression dive and at least 18 hours after multiple dives or multiple days of diving. After dives requiring decompression stops, DAN advises waiting substantially longer than 18 hours; the CDC Yellow Book describes 24–48 hours as a prudent interval after such dives. These are risk-reduction recommendations, not guarantees. Anyone with possible DCI symptoms should not fly and needs medical advice.
Individual circumstances
Individual circumstances can affect DCS risk, but the evidence varies and no single factor predicts an individual outcome.
Patent foramen ovale (PFO). A PFO is an opening between the heart’s upper chambers that persists after birth; it is present in roughly one in four adults. It is associated with some DCI presentations, but most people with a PFO do not necessarily develop DCI, and finding one does not by itself explain an incident. After unexplained DCI, discuss next steps with a physician experienced in diving medicine.
Hydration. Normal hydration is sensible on a dive day. Available human evidence does not establish dehydration as a direct cause of DCS or show that drinking extra water prevents it.
Fatigue. Fatigue can impair judgment and performance, but it is not a proven DCS-specific mechanism. Do not dive when you are unwell or too tired to dive safely.
Temperature. Temperature changes during a dive can matter; “cold water slows off-gassing” is too simple as a universal rule. Follow your training and use a conservative plan appropriate to conditions.
Body composition. Research findings vary and do not predict an individual outcome. Ask a dive-medicine clinician about personal concerns rather than relying on broad generalizations.
Age. Research findings vary and do not predict an individual outcome. Discuss personal medical history with a dive-medicine clinician.
Prevention: What Helps Manage Risk?
Use your dive computer—and understand it
Learn your computer before relying on it. Track your plan and ascent guidance, and respond calmly to alarms or limits. A computer is an important tool, not a guarantee and not a substitute for training or sound judgment.
Respond calmly to alarms and limits. Use them with your training and plan rather than treating a computer as a guarantee.
Make a controlled ascent and follow your plan
Ascend at the rate specified by your training, plan, and computer. Make a safety stop when appropriate for the dive, and complete any required decompression obligation exactly as planned. Do not turn a safety stop into a rigid rule that overrides an emergency or mandatory decompression instructions.
Leave margin between dives
Plan multiple dives conservatively, allow appropriate surface intervals, and follow the computer’s guidance for the entire day. Follow current no-fly advice for your dive profile.
Take care of yourself on a dive day
Arrive rested, drink normally, and avoid diving when illness, fatigue, or other circumstances make it unsafe. These are sensible general practices; they do not replace decompression planning and should not be presented as proven ways to prevent DCS.
If You Suspect DCI: What to Do
Treat suspected DCI seriously. Do not wait to see if concerning neurological, breathing, or other symptoms disappear.
Step 1: End the dive and activate emergency services. For severe or life-threatening symptoms, contact local EMS first. Do not re-dive or attempt in-water recompression on your own.
Step 2: Give oxygen if trained and equipped. A trained responder should provide the highest practical oxygen concentration using appropriate equipment. Oxygen first aid does not replace emergency assessment.
Step 3: Keep the diver safe while help is arranged. Keep them at rest, monitor their condition, and protect them from becoming too hot or cold. Offer oral fluids only if they are alert, not nauseated, and can swallow safely.
Step 4: Contact DAN for dive-medicine guidance. DAN’s emergency line is +1-919-684-9111; use local emergency services first when immediate danger is present.
Step 5: Coordinate medical transport and further treatment. EMS, DAN, and clinicians can help determine appropriate evaluation and transport, including whether recompression treatment is indicated. Do not delay emergency care to arrange your own chamber trip.
A Surface-Interval Sun-Protection Note
Surface intervals are a chance to rest, hydrate normally, and follow your dive plan. Apparel can be one personal comfort choice, but it is not a DCS risk control.
If full-body sun-protective coverage fits your surface-interval routine, explore Plunge’s Midnight Parrotfish Women’s Dive Suit. The official product page lists UPF 50+ and full-body coverage. That is an apparel feature—not decompression protection, a treatment for DCI, or a substitute for shade and protection on uncovered skin.
What the Fish?!
The Nudibranch: The Ocean's Most Extravagant Creature
A 2023 assessment counted more than 4,700 known nudibranch species, with totals changing as species are described and classified. Many look as if the ocean tried every color at once.
“Nudibranch” joins Latin nudus (“naked”) and Greek branchia (“gills”). These soft-bodied sea slugs occur in oceans from tropical reefs to colder waters. Many lose their shell during development and rely on other defenses as adults.
Some aeolid nudibranchs can sequester stinging cells from prey and store them in specialized structures called cnidosacs in their cerata. This ability is not shared by all nudibranchs. Other species obtain chemical defenses from prey such as sponges. Bright coloration can warn predators, but color alone is not a safe guide to whether an animal can sting or irritate skin.
The blue dragon, Glaucus atlanticus, is a nudibranch that feeds on floating stinging animals, including the Portuguese man o’ war—a siphonophore, not a jellyfish—and can concentrate their stinging cells. Do not handle it. Elysia chlorotica is a sacoglossan sea slug, not a nudibranch; it can retain algal chloroplasts, a phenomenon called kleptoplasty.
Finding a nudibranch is a slow-observation skill. Look carefully at reef surfaces, keep hands and fins clear of marine life, and follow local guidance. A patient dive often reveals small details that are easy to miss at normal swimming speed.
Stay Salty
DCS is serious, and good planning can reduce avoidable risk without eliminating it. Follow your training and dive plan, use your computer thoughtfully, and get help promptly when symptoms are concerning. The ocean is remarkable; careful decisions help keep diving enjoyable.
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