What Is Decompression Sickness? Gas Coming Out of Solution in the Wrong Place
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A diver breathing air at depth absorbs extra nitrogen into their tissues because pressure forces more of it into solution. Ascend too quickly and that nitrogen comes out of solution as bubbles inside the body rather than being carried away in the blood, which is the same thing that happens when a bottle of fizzy drink is opened suddenly.
The physics
Henry's law states that the quantity of a gas dissolving in a liquid is proportional to the pressure of that gas above the liquid. Pressure underwater rises by about one atmosphere for every ten metres of depth, so at thirty metres a diver breathes air at four times sea level pressure and the nitrogen in it dissolves into blood and tissues at four times the rate. Nitrogen is physiologically inert, meaning the body does not consume it as it does oxygen, so it simply accumulates until the tissues reach equilibrium with the breathing pressure, a process taking different times in different tissues: blood and well-perfused organs saturate within minutes while fat, cartilage and bone take hours. On ascent the process reverses, and if pressure falls slowly the dissolved nitrogen diffuses back into the blood and is exhaled harmlessly. If pressure falls faster than that transport allows, the tissue becomes supersaturated and bubbles form directly, which is the injury.
What bubbles do
The damage depends entirely on where the bubbles appear, and the classification reflects that:
- •Joint and limb pain, historically the commonest presentation, which gave the condition the name the bends because affected workers adopted a stooped posture
- •Skin symptoms including itching and a mottled rash, caused by bubbles in the skin's small vessels
- •Neurological involvement, with bubbles in the spinal cord or brain producing numbness, weakness, paralysis and confusion, which is the most serious form
- •Pulmonary involvement, sometimes called the chokes, where bubbles reach the lung circulation and cause chest pain and breathlessness
- •Arterial gas embolism, a related and separate injury caused by lung overexpansion during a rapid ascent while holding the breath, which forces gas directly into the arterial circulation and can be immediately fatal
- •Onset is usually within an hour of surfacing and almost always within a day, and symptoms can be vague enough that divers dismiss them
How divers avoid it
Management rests on controlling the ascent. Decompression tables, originally calculated by John Scott Haldane in 1908 for the British Admiralty, model the body as a set of theoretical tissue compartments each absorbing and releasing gas at its own rate, and prescribe how long a diver may stay at a depth before staged stops become necessary on the way up. Dive computers now perform the same calculation continuously using the actual depth profile, which is both safer and more permissive than a fixed table. A slow final ascent and a safety stop of a few minutes at around five metres are standard practice even within no-decompression limits. Breathing gas mixtures alter the problem: enriched air with more oxygen and less nitrogen reduces the nitrogen load at the cost of an oxygen toxicity limit, and technical divers use helium mixtures at depth, which introduces its own considerations since helium diffuses faster. Flying after diving is restricted for a period because cabin pressure is lower than sea level and can provoke symptoms from residual gas.
Treatment and the other contexts
The treatment is recompression in a chamber, which raises pressure to shrink the bubbles and drive the gas back into solution, followed by a controlled decompression over hours while the diver breathes oxygen, which accelerates nitrogen elimination by removing it from the breathing mixture entirely. Delay worsens outcomes, particularly for neurological cases, so administering high-flow oxygen and arranging transport immediately is the correct first response. The same physics applies well beyond recreational diving. Caisson workers building bridge foundations under pressurised air suffered severely in the nineteenth century, with the Brooklyn Bridge project a well-documented case, and the condition was named from that work before diving made it familiar. Aviators and astronauts face it in reverse, since ascending to low pressure without pre-breathing oxygen can produce bubbles, which is why spacewalk preparation includes hours of breathing pure oxygen to purge nitrogen. Whales and some diving mammals show anatomical features interpreted as managing the same problem.
The takeaway
Pressure forces extra nitrogen into a diver's tissues, and if ascent outpaces the rate at which that nitrogen can be carried away in the blood, it forms bubbles directly in the body. Damage depends on location, from joint pain to spinal and brain involvement. Decompression tables and computers model tissues as compartments absorbing gas at different rates and prescribe staged stops. Treatment is recompression with oxygen, and the same physics affects caisson workers and astronauts preparing for spacewalks.