The bends is a condition caused by gas bubbles forming in your blood and tissues when you move too quickly from a high-pressure environment to a lower one. Its medical name is decompression sickness (DCS), and it most commonly affects scuba divers who surface too fast. The nickname comes from the way affected divers double over in pain, unable to straighten their joints. While relatively rare, occurring in roughly 2 to 4 out of every 10,000 recreational dives, the bends can cause serious injury if not treated promptly.
How Nitrogen Bubbles Form in Your Body
The physics behind the bends follows a simple principle: the amount of gas that dissolves in a liquid depends on the pressure surrounding it. At the surface, your blood holds a normal amount of dissolved nitrogen, the gas that makes up about 78% of the air you breathe. When you dive underwater, the increased water pressure pushes more nitrogen into your blood and tissues. The deeper you go and the longer you stay, the more nitrogen dissolves.
If you ascend slowly, that extra nitrogen seeps out gradually through your lungs as you exhale. But if you rise too fast, the pressure drops before your body can offload the gas. The nitrogen comes out of solution and forms bubbles, the same way carbon dioxide fizzes out of a soda bottle when you twist off the cap. These bubbles can lodge in joints, block blood vessels, and damage tissues throughout the body.
Types and Symptoms
Decompression sickness is classified into two types based on severity.
Type I is the more common form and what most people mean by “the bends.” It involves the skin, joints, muscles, and lymph nodes. The hallmark symptom is deep, aching joint pain, most often in the shoulders, though elbows, knees, and other joints can be affected. Some people develop a mottled, bluish-red rash on the skin. Swollen, tender lymph nodes can also appear.
Type II is more dangerous because it affects the brain and spinal cord. Symptoms include headache, dizziness, blurred vision, hearing loss, ringing in the ears, nausea, poor coordination, and confusion. In severe cases, divers can experience numbness, paralysis, or loss of bladder control. Type II DCS happens when gas bubbles cross into the arterial blood supply and reach the central nervous system.
Symptoms typically begin within hours of surfacing, though they can sometimes be delayed. Joint pain often starts as a dull ache that gradually worsens. Neurological symptoms tend to appear faster and escalate more quickly.
Who Is Most at Risk
Any diver can get the bends, but certain factors raise the odds. Diving deep, staying down long, making multiple dives in a day, and ascending without proper decompression stops all increase the amount of nitrogen your body absorbs and reduce the time it has to release it safely.
Dehydration, fatigue, obesity, and older age also increase susceptibility, likely because they slow the rate at which nitrogen clears from tissues. Cold water is another contributor, since it constricts blood vessels and reduces gas exchange.
One significant risk factor is a heart defect called a patent foramen ovale (PFO), a small opening between the upper chambers of the heart that roughly 25% of people have without knowing it. This opening allows nitrogen bubbles that would normally be filtered out by the lungs to pass directly into arterial blood and reach the brain or spinal cord. Divers with a PFO have about 2.5 times the odds of developing the more serious Type II form of the bends.
What Treatment Looks Like
The single most important first aid step is breathing pure oxygen as soon as possible. Surface-level oxygen doesn’t eliminate the bubbles, but it dramatically slows their growth by flushing nitrogen out of the blood and replacing it with oxygen the body can use. For this to work properly, the diver needs a tight-fitting mask with a reservoir bag that delivers close to 100% oxygen. Standard masks without a reservoir only deliver about 40%. Oxygen should be started immediately, ideally at the dive site, and continued during transport to a treatment facility.
The definitive treatment is recompression in a hyperbaric chamber. The diver is placed inside a sealed chamber where the air pressure is raised to about 2.8 times normal atmospheric pressure, simulating the conditions of a dive. This physically shrinks the gas bubbles while the diver breathes pure oxygen, which accelerates nitrogen removal from the blood and delivers high concentrations of oxygen to damaged tissues. A standard treatment session involves cycling between oxygen breathing and short air breaks over several hours to manage the risk of oxygen toxicity. Some patients need multiple sessions depending on the severity of their symptoms.
Long-Term Effects
Most divers who receive prompt treatment recover fully. But repeated episodes, or a single severe case left untreated, can cause lasting damage. One recognized long-term complication is a form of bone death called dysbaric osteonecrosis. Over time, subclinical bubble formation disrupts the blood supply inside bones, particularly in the hip. The bone tissue dies, weakens, and can eventually collapse, leading to chronic pain and the potential need for joint replacement. Both hips are frequently affected. This condition is most common in commercial divers and compressed-air workers who face repeated pressure exposure over years.
Neurological damage from Type II DCS can also persist. Some divers report lasting problems with memory, concentration, balance, or sensation in their limbs even after treatment, particularly if recompression was delayed.
How Divers Prevent It
Prevention centers on controlling your ascent. Dive tables and dive computers calculate safe ascent rates and decompression stops based on depth, time, and the number of dives you’ve done. The general rule is to ascend no faster than about 30 feet per minute and to make a safety stop at 15 feet for three to five minutes on every dive.
Flying after diving is a well-known trigger because cabin pressure at altitude is lower than sea-level pressure, creating the same conditions as a too-fast ascent. The Divers Alert Network recommends waiting at least 12 hours after a single no-decompression dive before flying, and at least 18 hours after multiple days of repetitive diving. Dives that required mandatory decompression stops call for waiting substantially longer than 18 hours. In testing, no cases of DCS occurred when divers waited at least 11 hours after a single dive to 60 feet or deeper, or at least 17 hours after repetitive no-stop dives.
Staying well hydrated, avoiding alcohol before and after diving, and being conservative with depth and bottom time on successive dive days all reduce risk. If you feel any unusual joint pain, skin changes, or neurological symptoms after a dive, the safest approach is to treat it as possible DCS until proven otherwise.