If you surface too quickly from a dive without proper decompression, dissolved nitrogen in your blood and tissues can form bubbles as the surrounding pressure drops. These bubbles trigger a condition called decompression sickness (DCS), which ranges from joint pain and skin rashes to spinal cord damage, paralysis, and in rare cases, death. About 50% of people who develop DCS notice symptoms within an hour of surfacing, though onset can be delayed up to 24 hours.
Why Nitrogen Bubbles Form
At depth, the increased water pressure forces extra nitrogen from your breathing gas to dissolve into your blood and tissues. The deeper and longer you dive, the more nitrogen saturates your body. When you ascend, that pressure decreases and the nitrogen needs to come back out of solution gradually, much like carbonation fizzing out of a soda bottle when you open the cap.
A controlled ascent with proper stops gives your lungs time to filter out the excess nitrogen naturally. Skip those stops or rise too fast, and the nitrogen comes out of solution faster than your body can handle. It forms bubbles, some as small as one micrometer, that grow from tiny preexisting gas pockets in your tissues. These bubbles are the root cause of everything that follows.
Joint Pain and Skin Symptoms
The most common form of decompression sickness, sometimes called “the bends,” affects your muscles, joints, skin, and lymphatic system. The shoulder is the joint most frequently involved, though bubbles can settle in any joint. The pain typically builds gradually after surfacing and can range from a dull ache to severe, debilitating pressure deep in the joint.
Skin involvement often appears as a mottled, marbled pattern called cutis marmorata, which can be localized to one area or spread across large sections of the body. Some divers also develop swollen, painful lymph nodes. These symptoms are uncomfortable and disruptive, but they’re generally the least dangerous form of DCS.
Spinal Cord and Brain Involvement
The more serious form of DCS targets the central nervous system. The spinal cord is the most commonly affected site, particularly the mid-back (thoracic) region, which is vulnerable because of its blood supply and relatively high fat content. Nitrogen is especially soluble in fat, so fatty tissues accumulate more dissolved gas and are more prone to bubble formation.
In one documented case, an experienced diver stepped onto his boat after a dive and immediately felt burning, tingling sensations radiating from his mid-back into both arms, around his chest in a band-like pattern, and down into his legs. Within minutes he had bilateral leg weakness and couldn’t stand without help. He also lost bladder control. This type of spinal cord injury from DCS can happen even to divers with years of experience.
When bubbles reach the brain, the effects can include headache, visual and hearing problems, poor coordination, nausea, ringing in the ears, confusion, and altered consciousness. In the most extreme cases, divers lose consciousness, develop seizures, or fall into a coma. Any diver who loses consciousness shortly after surfacing should be treated as having a gas embolism until proven otherwise.
Gas Embolism: The Fastest Emergency
Decompression sickness isn’t the only risk of a bad ascent. If a diver holds their breath or ascends rapidly from even shallow depths, expanding air can rupture lung tissue and force gas bubbles directly into the arterial bloodstream. This is an arterial gas embolism, and it’s distinct from DCS in one critical way: it strikes almost immediately, often during or within minutes of surfacing, from depths as shallow as one meter (about three feet).
The most common sign is sudden unconsciousness, frequently accompanied by seizures. While DCS tends to build gradually over minutes to hours, a gas embolism hits like a switch. Both conditions require the same emergency treatment, but a gas embolism is more likely to be fatal without rapid intervention.
How Recompression Treatment Works
The primary treatment for decompression sickness is hyperbaric oxygen therapy, which places the diver in a sealed chamber where pressure is increased to simulate being back at depth. This shrinks the nitrogen bubbles and forces the gas back into solution so it can be eliminated safely through the lungs. The diver breathes pure oxygen throughout.
Standard treatment sessions last between about two and five hours depending on severity. Recovery rates are generally good when treatment starts promptly. In a study of more than 5,000 patients, roughly 90% achieved complete recovery at discharge. Another analysis found that 92% of patients with joint and skin symptoms reported full recovery within a month, even if they didn’t feel completely better right after their first session. When treatment is delayed, the outcomes are less favorable, particularly for neurological symptoms. Persistent or worsening symptoms before reaching a chamber are associated with poorer results.
Long-Term Bone Damage
Repeated or poorly managed decompression episodes can cause a chronic condition called dysbaric osteonecrosis, where gas bubbles disrupt blood flow inside bones. The bubbles damage the lining of small blood vessels within the bone, triggering clotting that cuts off circulation. Without blood supply, the bone tissue dies.
The hip joint is the most commonly affected area, and bilateral involvement (both hips) is not uncommon. Shoulders are also frequently hit. Over time, the dead bone weakens. The cartilage surface flattens, subchondral fractures develop beneath the joint surface, and the bone can eventually collapse entirely. In advanced cases, divers develop painful arthritis and may need a total joint replacement. Small lesions away from major joints sometimes cause no symptoms at all, but large lesions near the hip or shoulder carry a poor prognosis without surgical intervention.
Factors That Raise Your Risk
Not every diver who ascends too quickly develops symptoms, and certain factors make bubble formation more likely. Dehydration is one of them. In animal studies, pre-dive hydration dramatically reduced the rate of severe DCS, from 47% in a control group to 0% in a well-hydrated group. Human studies have confirmed that drinking an adequate amount of fluid before diving significantly reduces the number of venous bubbles detected afterward. Nitrogen dissolves more readily in fat than in other tissues, so higher body fat increases the total nitrogen load your body absorbs at depth.
Flying too soon after diving is another common trigger. Cabin pressure in commercial aircraft is equivalent to an altitude of 2,000 to 8,000 feet, which further reduces the pressure on your body and can pull dissolved nitrogen out of solution hours after a dive. 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 dives per day or multiple days of diving. For dives that required decompression stops, the wait should be substantially longer than 18 hours.