Decompression Sickness (DCS), commonly known as “the bends,” is a medical condition caused by a rapid pressure change that results in gas bubbles forming within the body’s tissues and bloodstream. The risk of DCS depends not only on depth but also on the duration of exposure and the rate of ascent. DCS can affect anyone exposed to significant pressure reduction, including scuba divers, caisson workers, and high-altitude aviators. Understanding the physical laws governing gas behavior explains why the onset of DCS is not tied to a single depth.
The Underlying Cause: Nitrogen Saturation
The mechanism behind Decompression Sickness involves the absorption and subsequent release of inert gas, primarily nitrogen, from the breathing air. As a diver descends, the ambient water pressure increases, forcing more nitrogen to dissolve into the body’s tissues and blood. This process is governed by Henry’s Law, which states that the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas. Tissues high in fat, such as the brain and spinal cord, absorb nitrogen readily. This absorption, or “on-gassing,” continues throughout the dive.
The danger occurs during ascent when the surrounding pressure decreases. If the pressure reduction is too rapid, the dissolved nitrogen comes out of solution too quickly, forming bubbles. Boyle’s Law also applies, dictating that as pressure decreases, the volume of a gas increases. These expanding bubbles obstruct blood flow and cause direct tissue damage, leading to DCS symptoms.
Depth and Time Thresholds for Risk
DCS onset is not tied to a single depth but rather to the combination of depth and time spent under pressure. Nitrogen absorption increases exponentially with both factors, meaning a slightly deeper dive or a longer duration raises the risk. Recreational diving guidelines use “No-Decompression Limits” (NDLs) to define the maximum time a diver can remain at a depth and still ascend directly without mandatory decompression stops. For example, a diver can spend up to 55 minutes at 60 feet (18 meters) without a stop, but this time drops sharply to about 10 minutes at 130 feet (40 meters).
Even shallow dives, such as 30 feet (9 meters), can cause DCS if the exposure time is extremely long. Dive computers and tables track these time-and-depth relationships to calculate residual nitrogen. Exceeding the NDL requires specific stops during ascent to allow the body to off-gas excess nitrogen slowly and safely. These required decompression stops manage the pressure gradient between dissolved gas in the tissues and the surrounding water pressure. Many divers also make an optional safety stop for a few minutes at a shallow depth near the end of every dive.
Recognizing the Signs of Decompression Sickness
The signs of Decompression Sickness range from mild discomfort to severe, life-threatening injury, typically appearing within minutes to a few hours after surfacing. Symptoms are categorized into two types based on their severity and location.
Type I DCS
Type I DCS is the less severe form, often affecting the musculoskeletal system and skin. The most common symptom is joint pain, particularly in the elbows and shoulders, which is the origin of the term “the bends.” This pain may be accompanied by swelling, skin mottling, or a rash.
Type II DCS
Type II DCS involves more serious organ systems, including the neurological system. Neurological symptoms may include numbness, tingling, muscle weakness, partial paralysis, or difficulty with bladder control, often indicating spinal cord involvement. Other severe manifestations can involve the inner ear, causing vertigo and hearing loss, or the respiratory system, leading to shortness of breath. Immediate medical attention and treatment in a hyperbaric chamber are necessary for any suspected case of DCS.
Factors That Increase Risk Beyond Depth
Although depth and time are the primary variables, several individual and environmental factors increase a diver’s susceptibility to DCS, even when safe limits are followed. These additional factors explain why two divers following the exact same depth and time profile might experience different outcomes.
A rapid ascent rate is a significant risk factor, as it creates a larger pressure differential and reduces the time for nitrogen to exit the body safely. Cold water also contributes to increased risk because it can restrict blood flow to the extremities, affecting the rate at which nitrogen is absorbed and released. Heavy exertion during the deepest part of the dive increases blood flow, causing the body to absorb more nitrogen than it would at rest. Dehydration and underlying health issues, such as a Patent Foramen Ovale (PFO)—an opening between the heart’s upper chambers—can also compromise the body’s ability to filter nitrogen bubbles.