When a diver descends beneath the surface, the surrounding water pressure increases significantly, affecting the gases they breathe. Decompression diving is the controlled management of a diver’s ascent back to the surface to safely reduce this pressure. This process is necessary to prevent a potentially severe injury known as Decompression Sickness, commonly referred to as “the bends.” A controlled ascent is required because the body absorbs gases under pressure, and releasing that pressure too quickly causes a dangerous physical reaction.
The Physics of Gas Absorption
The mechanism behind gas absorption and release in the body is governed by a fundamental principle of physics. This principle states that the amount of gas that will dissolve into a liquid is directly proportional to the partial pressure of that gas above the liquid. Since the human body is largely liquid, the deeper a diver goes, the higher the ambient pressure, and the more gas from the breathing mixture dissolves into the body’s tissues and bloodstream.
The primary inert gas of concern in standard compressed air is nitrogen, which makes up about 79% of the air we breathe. Different body tissues, such as blood, muscle, and fat, absorb and release this nitrogen at varying speeds. Tissues with high blood supply absorb and release gas quickly, while fatty tissues and those with less blood flow take much longer to off-gas.
As a dive progresses, the diver’s tissues become progressively saturated with dissolved nitrogen. If the pressure is maintained, this dissolved gas remains harmlessly in solution, but the problem arises when the diver begins their ascent and the ambient pressure rapidly drops, reversing the process of gas absorption.
Decompression Sickness The Bends
If the pressure reduction during ascent is too fast, the dissolved nitrogen cannot be safely eliminated through the lungs quickly enough. This rapid pressure change causes the gas to come out of solution, forming bubbles within the blood and body tissues. These bubbles are the direct cause of Decompression Sickness (DCS) and cause physical damage by blocking blood flow in small vessels, a phenomenon called an embolism.
The bubbles can also mechanically compress or irritate nerves and joints, leading to the condition’s classic symptoms. The historical nickname “the bends” comes from the painful joint and muscle aches that cause sufferers to hunch over. Symptoms range widely depending on where the bubbles lodge. Mild cases (Type I DCS) involve skin rashes, itching, or minor joint pain, while severe Type II DCS involves neurological symptoms like numbness, paralysis, or inner ear disturbances.
In the most serious cases, bubbles can overwhelm the pulmonary circulation, leading to difficulty breathing and chest pain, known as “the chokes.” The definitive medical treatment involves immediate recompression in a hyperbaric chamber, which increases the ambient pressure again to re-dissolve the bubbles, followed by a slow, medically supervised decompression schedule.
Managing Ascent and Decompression Stops
To prevent the formation of bubbles, the diver must control the rate at which the ambient pressure is reduced. This means maintaining a slow, controlled ascent rate, typically not exceeding 9 to 18 meters (30 to 60 feet) per minute, to give the tissues time to off-gas the nitrogen through respiration. When a diver exceeds the no-decompression limit (NDL), they must perform mandatory decompression stops, pausing their ascent at specific, shallow depths for calculated periods.
This pause provides the necessary time for the excess nitrogen to diffuse out of the tissues and be exhaled safely. In addition to mandatory stops, divers perform a precautionary safety stop near the end of every dive, typically at 5 meters (15 feet) for three to five minutes, providing a final, conservative buffer before surfacing. Dive computers and dive tables are used to calculate these requirements precisely, ensuring the diver’s ascent profile allows for a safe and controlled release of absorbed gas.