Decompression Sickness: What Every Diver Should Know

Decompression sickness — “the bends” — is the condition every diver trains to avoid. Understanding what it is, how it happens, and what to do about it makes you a safer diver.

What it is

During a dive, inert gas (primarily nitrogen, or nitrogen and helium in trimix) dissolves into your blood and tissues under pressure, as described by Henry’s Law. On ascent, the ambient pressure drops and this dissolved gas needs to come back out of solution.

If you ascend slowly enough, the gas is carried by your blood to your lungs, where you exhale it. If you ascend too quickly or have too much dissolved gas, the gas comes out of solution faster than your body can eliminate it — forming bubbles in your blood and tissues. These bubbles cause decompression sickness.

Types of DCS

Type I — musculoskeletal and skin

Joint pain (“the bends”): Deep, aching pain in joints — most commonly shoulders, elbows, knees, and hips. The pain may start mild and increase over hours
Skin symptoms: Itching, rash, mottled or marbled skin (cutis marmorata)
Lymphatic involvement: Localized swelling

Type I DCS is painful and concerning but generally not immediately life-threatening.

Type II — neurological and cardiopulmonary

Spinal cord: Weakness, numbness, tingling, paralysis — often in the legs. Bladder dysfunction. This is the most common form of serious DCS
Brain: Confusion, visual disturbances, difficulty speaking, headache, loss of consciousness
Inner ear: Vertigo, hearing loss, tinnitus (can be confused with inner ear barotrauma)
Pulmonary (“the chokes”): Chest pain, cough, shortness of breath — caused by massive venous bubble loads

Type II DCS can cause permanent neurological damage if not treated promptly.

Symptoms and timeline

DCS symptoms typically appear within 1-6 hours after surfacing, though they can appear during ascent or as late as 24-48 hours after diving. The distribution:

50% of cases: Symptoms within 1 hour
90% of cases: Symptoms within 6 hours
Rare cases: Symptoms up to 24-48 hours later

Common early signs include unusual fatigue, joint pain, and “pins and needles” sensations. These should never be dismissed as normal post-dive tiredness.

Risk factors

Rapid ascent: The primary risk factor. Exceeding recommended ascent rates (9-10 m/min) reduces your body’s ability to off-gas safely
Exceeding no-deco limits: Staying longer or deeper than your tables or computer allow
Missing decompression stops: If you have a deco obligation, skipping stops dramatically increases risk
Repetitive diving: Multiple dives per day accumulate residual nitrogen. Inadequate surface intervals increase risk
Deep dives followed by shallow dives: “Reverse profiles” may increase risk, though this is debated
Cold water: Cold reduces perfusion to peripheral tissues, impairing off-gassing
Exercise at depth: Increases gas loading in well-perfused tissues

Personal factors

Dehydration: Reduces blood volume and may impair gas elimination. Dehydration is common in diving (breathing dry gas, heat, seasickness)
Age: Older divers may be at slightly higher risk
Body fat: Nitrogen is more soluble in fat tissue, which saturates and desaturates slowly
Fatigue: Physical and mental fatigue may increase susceptibility
Patent foramen ovale (PFO): A hole between the right and left atria of the heart, present in about 25% of adults. Allows venous bubbles to cross to the arterial side, potentially causing neurological DCS from otherwise benign venous bubbles
Previous DCS: A history of DCS may indicate increased susceptibility

What to do if you suspect DCS

Immediate actions

Surface the diver safely if still in the water — do not skip remaining deco stops if possible, but a convulsing or unconscious diver must be surfaced
Administer 100% oxygen — high-flow oxygen via a non-rebreather mask. This reduces bubble size and accelerates nitrogen washout
Keep the diver lying down — supine (flat on back) is recommended. Avoid Trendelenburg (head-down) position
Hydrate — oral fluids if the diver is conscious and not vomiting. Water or electrolyte drinks, not alcohol
Contact emergency services — call DAN (Divers Alert Network) at +1-919-684-9111 or your local diving emergency number
Do not re-enter the water — “in-water recompression” is a last resort for remote locations with specific protocols. It is not a substitute for chamber treatment

Definitive treatment

The definitive treatment for DCS is recompression in a hyperbaric chamber. The increased pressure reduces bubble size, and breathing oxygen under pressure accelerates gas washout. Treatment should begin as soon as possible — delays worsen outcomes.

Do not dismiss mild symptoms. Many divers rationalize joint pain or tingling as “just a strain” or “I slept funny.” Any new symptom appearing within 24 hours of diving should be evaluated.

Prevention

Dive conservatively: Stay well within no-deco limits, especially on deeper dives
Ascend slowly: 9-10 meters per minute maximum. Slower is better
Do safety stops: 3-5 minutes at 5 meters on every dive
Stay hydrated: Drink water before, between, and after dives
Manage surface intervals: Allow adequate time between repetitive dives
Use nitrox conservatively: Nitrox reduces nitrogen loading. Some divers use nitrox with air tables for an added safety margin
Wait before flying: DAN recommends a minimum 12-hour surface interval before flying after single no-deco dives, and 18+ hours after repetitive or deco dives
Know your limits: Do not push depth or time limits to impress anyone

Sources

DAN (Divers Alert Network) Emergency Hotline and DCS Resources
Vann, R.D. et al. (2011). “Decompression illness.” The Lancet
NOAA Diving Manual, 6th Edition
US Navy Diving Manual, Revision 7