The Bends in Scuba Diving: Causes, Symptoms & Prevention
This is the article I was born to write. Not because I've had decompression sickness — I haven't, and I'd like to keep it that way. But because DCS is fundamentally a chemistry problem, and I'm a chemist who dives. The gas laws that govern DCS are the same ones I used to calibrate instruments in the lab. Except here, the instrument is your body, and a miscalculation doesn't just ruin a data set — it can paralyze you.
Let me walk you through what actually happens.
What Is Decompression Sickness?
Decompression sickness (DCS) — colloquially "the bends" — occurs when dissolved nitrogen in your body tissues forms bubbles during or after ascent. Those bubbles can lodge in joints, skin, spinal cord, brain, or lungs. The location of the bubbles determines the symptoms. None of them are pleasant.
The name "the bends" comes from the joint pain that characterized early cases among bridge construction workers (caisson workers) in the 1800s. The pain caused them to bend over. The name stuck.
The Chemistry: Henry's Law
Here's where my day job becomes useful. Henry's Law states:
The amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid.
In plain language: the deeper you dive, the more nitrogen dissolves into your blood and tissues. At the surface (1 ATA), your tissues are saturated with nitrogen at 0.79 ATA partial pressure (air is 79% nitrogen). At 30 meters (4 ATA), that partial pressure climbs to 3.16 ATA. Your tissues absorb nitrogen until they reach equilibrium with this new pressure.
The absorption isn't instant. Different tissues absorb nitrogen at different rates — a concept called "tissue compartments" in decompression theory. Your blood saturates quickly (5-minute half-time). Your cartilage and bone? Hours. This is why repetitive dives and long bottom times are higher risk — you're loading slow tissues that take a long time to off-gas.
What Happens When You Ascend
As you ascend, ambient pressure drops. The nitrogen dissolved in your tissues is now at a higher partial pressure than the surrounding environment. It wants out. If the pressure differential is gradual, the nitrogen comes out of solution slowly, returns to your lungs via blood circulation, and you exhale it. No problem.
If the pressure differential is too rapid — you ascended too fast, skipped safety stops, or had too much nitrogen loaded in the first place — the nitrogen comes out of solution as bubbles. Think of opening a soda bottle. The CO2 is dissolved under pressure. Release the pressure suddenly and you get fizz.
You do not want to fizz.
Symptoms by Type
Type I DCS (Mild)
- Joint pain ("the bends"): Deep, aching pain in shoulders, elbows, knees, or hips. Usually develops within 1-6 hours after surfacing. Doesn't improve with aspirin.
- Skin bends: Itching, mottling (marbled red/blue skin), or rash. Often on the torso. Sometimes called "cutis marmorata."
- Lymphatic DCS: Swelling in a limb due to bubble blockage of lymph drainage. Rare.
Type II DCS (Serious)
- Neurological DCS: Numbness, tingling, weakness, paralysis (especially lower limbs — spinal cord DCS is the most common neurological presentation), confusion, difficulty speaking, visual disturbances, bladder dysfunction.
- Inner ear DCS: Vertigo, hearing loss, ringing in ears. Can be disorienting and dangerous, especially underwater.
- Pulmonary DCS ("the chokes"): Bubbles in the pulmonary vasculature causing chest pain, shortness of breath, and cough. Rare but serious.
- Arterial gas embolism (AGE): Technically a separate condition from DCS, but often grouped as "decompression illness." Bubbles in arterial circulation can cause stroke-like symptoms within minutes of surfacing. Medical emergency.
Risk Factors
Not all dives carry equal DCS risk. The variables that increase your risk:
- Rapid ascent rate: The single biggest controllable factor. Slower is always safer. 9 meters (30 feet) per minute maximum. Your dive computer will scream at you if you exceed this.
- Skipping safety stops: The 3-5 minute stop at 5 meters is not optional. It gives your fastest tissues time to start off-gassing before the final pressure change to 1 ATA.
- Dehydration: Dehydrated blood is thicker, circulates more slowly, and is less efficient at transporting dissolved nitrogen to the lungs for elimination. Drink water. Not beer. Water.
- Repetitive dives: Residual nitrogen from your first dive loads onto the second. Surface intervals matter. Your dive computer tracks this.
- Cold water: Vasoconstriction reduces blood flow to extremities, slowing nitrogen elimination during ascent. You loaded nitrogen in warm peripheral tissues during the dive; now cold is preventing efficient off-gassing.
- Exercise after diving: Increased cardiac output can mobilize stable microbubbles. Don't hit the gym after a dive day.
- Flying after diving: Cabin altitude is typically 6,000-8,000 feet (1,800-2,400m equivalent). That's a significant additional pressure reduction. DAN recommends waiting at least 18 hours after repetitive dives, 12 hours after a single no-deco dive.
- Age and fitness: Older divers and those with poor cardiovascular fitness show higher DCS incidence in epidemiological data.
- Patent Foramen Ovale (PFO): A hole between the heart's atria present in ~25% of the population. Allows venous bubbles (normally filtered by the lungs) to cross into arterial circulation. Significantly increases neurological DCS risk.
Prevention
Prevention is straightforward if you're disciplined:
1. Ascend slowly. 9m/min max. I aim for 6m/min on my dives. Patience is a survival skill. 2. Make safety stops. 3-5 minutes at 5 meters. Every dive. No exceptions. 3. Follow your dive computer. It's running decompression algorithms based on your actual dive profile. Don't override it because you "feel fine." 4. Stay hydrated. Start hydrating the day before dive day. 500ml of water per hour while diving. 5. Respect surface intervals. Your computer calculates residual nitrogen. Trust it. 6. Don't fly too soon. 18+ hours after repetitive dives. 24 hours is better. 7. Know your limits. Dive within your [certification depth](/blog/how-deep-can-you-scuba-dive). The limits exist for this exact reason. 8. Maintain cardiovascular fitness. A healthy circulatory system eliminates nitrogen more efficiently.
Treatment
If you suspect DCS:
1. Administer 100% oxygen immediately. High-flow, non-rebreather mask. This creates a massive gradient favoring nitrogen elimination and shrinks existing bubbles. 2. Lay the diver flat. Keep them horizontal. Hydrate with oral fluids if conscious. 3. Call DAN (Divers Alert Network). Emergency hotline: +1-919-684-9111. They will coordinate evacuation to a hyperbaric facility. Save this number in your phone. Now. 4. Hyperbaric treatment. Recompression in a hyperbaric chamber re-dissolves bubbles and allows controlled, gradual off-gassing. The standard protocol (US Navy Treatment Table 6) takes ~5 hours. Some cases require multiple sessions.
Do not "wait and see" with suspected DCS. Early treatment dramatically improves outcomes. Delayed treatment — especially for neurological symptoms — can result in permanent damage.
The Statistics
DAN's annual diving report shows:
- DCS incidence: roughly 2-4 cases per 10,000 recreational dives
- That's approximately 1,000 treated cases per year in the US
- Fatality from DCS specifically is rare when treated promptly
- Most cases are Type I (joint pain) and resolve fully with hyperbaric treatment
- Neurological DCS can leave residual deficits if treatment is delayed
The Bottom Line
DCS is a gas physics problem with a human biology variable. Henry's Law is indifferent to how experienced you are. Your tissues absorb nitrogen based on pressure and time, and your ascent rate determines whether that nitrogen comes out as dissolved gas (good) or bubbles (bad).
Every dive is a decompression obligation. Your computer manages it. Your behavior determines whether the management works. Ascend slowly. Stop at 5 meters. Stay hydrated. And carry [DAN insurance](https://www.diversalertnetwork.org/). It's the cheapest important thing in diving.
I'm Chad. I think in partial pressures. And I always, always do my safety stop.