Decompression Sickness and Decompression Illness: How Bottom Time and Nitrogen Supersaturation Cause the Bends

Decompression sickness, commonly called "the bends," happens when nitrogen dissolved in your body's tissues forms bubbles as you ascend too quickly for that gas to leave safely. It's a mechanical problem before it's anything else: gas coming out of solution faster than your body can clear it, then physically obstructing circulation and damaging tissue on the way through.

Decompression illness (DCI) is the broader umbrella term, covering both decompression sickness and arterial gas embolism together, since the two conditions can look similar on presentation even though the underlying mechanism differs. Here's how nitrogen actually builds up in the body during a dive, what happens when a diver exceeds their planned bottom time, and why the fix isn't as simple as "just come up slower."

How Nitrogen Gets Into Your Tissues in the First Place

Standard scuba air is roughly 79% nitrogen. On the surface, your body is in equilibrium with it; you're not absorbing extra. The moment you descend, increasing pressure changes that. Under higher pressure, more nitrogen dissolves into your blood and tissues than your body holds at the surface, a process that continues throughout the dive. The deeper you go and the longer you stay, the more nitrogen accumulates. This is exactly why dive computers and tables track bottom time and depth together: both variables directly determine how saturated your tissues become.

What "Supersaturation" Actually Means

As you ascend, pressure drops and your tissues need to release that absorbed nitrogen back out, ideally gradually, through normal circulation and respiration. If ascent happens slowly enough, this off-gassing keeps pace and nitrogen leaves the body as dissolved gas, without incident. If ascent happens too quickly relative to how saturated your tissues are, the surrounding pressure drops faster than the nitrogen can diffuse out in dissolved form. At that point, tissues become supersaturated: holding more dissolved gas than the current pressure can keep in solution. Physics resolves that imbalance by forming actual gas bubbles inside the blood and tissue, the same way opening a shaken soda bottle releases dissolved CO2 as visible bubbles rather than keeping it dissolved.

Those bubbles are the direct cause of decompression sickness. They can mechanically block small blood vessels, trigger inflammation, and physically damage the tissue they form in, and the location and extent of bubble formation determines what symptoms appear.

What Happens When You Exceed Your Bottom Time

No-decompression limits exist specifically to keep you within the amount of nitrogen loading your body can safely off-gas during a normal, direct ascent. Exceed that limit and your tissues are carrying more dissolved nitrogen than a standard ascent rate can safely release. Depending on how far over the limit and by how much, the required response ranges from mandatory decompression stops (deliberately pausing at set depths to let nitrogen clear before continuing upward) to a genuinely elevated DCS risk if a direct ascent is made anyway.

This is worth being specific about: exceeding bottom time doesn't guarantee decompression sickness will happen, and staying strictly within limits doesn't guarantee it won't. Research and clinical review both note that a meaningful share of DCS cases occur in divers who followed their computer or tables correctly, so-called "undeserved" hits, while some divers who technically violate limits surface without incident. Bottom time limits are risk management, not a hard safety guarantee, which is exactly why conservative diving habits matter beyond just watching the numbers.

Types and Symptoms

DCS is generally classified by severity and system affected:

• Type I (mild): Joint and muscle pain (the classic "bends" ache, most often in the knees, shoulders, or elbows), skin symptoms like rash or itching, and mild fatigue or swelling in the lymphatic system.
• Type II (severe): Neurological symptoms from bubbles affecting the brain, spinal cord, or inner ear, including numbness, tingling, weakness, dizziness, vertigo, confusion, or vision changes.
• Type III: Pulmonary symptoms sometimes called "the chokes," including chest pain, cough, and breathing difficulty, which represent a genuinely life-threatening presentation.

Symptoms can appear within minutes of surfacing or develop over the following 24 hours, which is part of why divers are advised to stay alert to symptoms well after a dive has ended, not just in the immediate post-surface window.

Treatment: Why It's Not Just "Wait It Out"

The standard first response to suspected DCS is 100% oxygen and immediate evaluation; oxygen helps the body off-gas nitrogen faster and improves tissue oxygenation while bubbles are present. Definitive treatment for confirmed DCS is hyperbaric oxygen therapy in a recompression chamber, which re-pressurises the patient to shrink existing bubbles and then brings them back to surface pressure in a controlled, gradual way that allows proper off-gassing. This isn't something to attempt to self-manage or wait out; DCS is a medical emergency requiring evaluation, and outcomes are generally better the sooner appropriate treatment begins.

Frequently Asked Questions

What's the difference between decompression sickness and decompression illness?
Decompression illness (DCI) is the umbrella term covering both decompression sickness (DCS, from bubbles forming directly in tissue) and arterial gas embolism (AGE, from air entering the bloodstream through lung injury). The two can present similarly, which is part of why DCI is used as a broader clinical term.

Can you get DCS even if you didn't exceed your no-decompression limit?
Yes. A meaningful proportion of documented DCS cases occur in divers who stayed within their computer or table limits. These are sometimes called "undeserved" hits, and they're part of why individual risk factors and conservative diving habits matter beyond simply not exceeding the numbers.

How soon after diving can DCS symptoms appear?
Symptoms can start within minutes of surfacing or develop gradually over the following 24 hours. Any new joint pain, skin symptoms, numbness, dizziness, or breathing difficulty in that window after diving should be taken seriously.

Is decompression sickness the same as an ear or lung barotrauma injury?
No. Barotrauma injuries (ear squeezes, pulmonary barotrauma) result from pressure differences in air-filled spaces like the middle ear or lungs. DCS results from dissolved nitrogen coming out of solution in blood and tissue. They have different mechanisms, though both are pressure-related diving injuries.

What should I do if I suspect decompression sickness?
Administer oxygen if available and seek immediate medical evaluation, ideally with access to hyperbaric treatment. Don't wait to see if symptoms resolve on their own, and don't attempt to treat suspected DCS by re-entering the water.

This article is general information about diving physiology, not a diagnosis or personalised medical assessment. Suspected decompression sickness is a medical emergency requiring prompt evaluation.