The human body is resilient to the hydrostatic pressure of water because it is composed mostly of incompressible fluid. However, the air a diver breathes and the gas contained within body cavities are subject to the physical laws of pressure, which cause nearly all diving-related health risks. As a diver descends, the ambient pressure increases by one atmosphere (ATA) for approximately every 33 feet (10 meters) of depth. This pressure change drastically alters the volume and solubility of gases, affecting the body through two main mechanisms: the physical expansion or compression of gas-filled spaces (Boyle’s Law) and the absorption and release of dissolved gases in tissues (Henry’s Law).
Air-Filled Spaces and Barotrauma
The most immediate effects of pressure changes target the body’s air-filled spaces, a phenomenon known as barotrauma, or pressure trauma. According to Boyle’s Law, the volume of a gas is inversely proportional to the pressure surrounding it. As a diver descends, trapped air shrinks; as they ascend, it expands. These volume changes must be managed to prevent injury to surrounding tissues.
The middle ear and the sinuses are highly susceptible to barotrauma during descent. The middle ear requires constant equalization via the Eustachian tube to match the increasing ambient pressure. Failure to equalize creates a pressure difference that can cause the eardrum to bulge inward, leading to pain and potential rupture (ear squeeze). The paranasal sinuses are affected similarly if their openings are blocked, resulting in sinus squeeze.
The lungs are the largest air-filled organs and are most vulnerable during a rapid ascent. If a diver holds their breath while ascending, the air inhaled at depth expands rapidly as pressure decreases. This expansion can rupture the delicate lung tissue, known as pulmonary barotrauma. Consequences include air escaping into the chest cavity or entering the bloodstream to form an arterial gas embolism (AGE), which can travel to the brain.
Other smaller areas are also affected by these physical forces. A mask squeeze occurs when the air volume inside the diving mask compresses on descent, causing a relative vacuum that can rupture capillaries in the face and eyes. Dental barotrauma, or tooth squeeze, can occur if gas is trapped beneath a filling or in a tooth cavity, causing pressure buildup and intense pain.
The Circulatory System and Decompression Sickness
The circulatory system is the pathway through which inert gases, primarily nitrogen, are absorbed into the body’s tissues under pressure. Henry’s Law dictates that the amount of gas dissolved in a liquid is directly proportional to its partial pressure. As pressure increases during descent, nitrogen dissolves into the blood and subsequently into various body tissues. Tissues with a high lipid content, such as fat and the nervous system, absorb nitrogen more readily.
The problem arises during ascent when the ambient pressure drops, causing the dissolved nitrogen to come out of solution. If the ascent is too fast, the nitrogen outgasses too quickly, forming bubbles within the blood and tissues. This condition is Decompression Sickness (DCS), often called “the bends.”
The resulting gas bubbles can lodge in various parts of the body, causing a spectrum of symptoms. Bubbles forming near the joints are the most common presentation, causing deep, aching pain. More serious forms of DCS affect the central nervous system, where bubbles in the spinal cord can lead to partial paralysis, numbness, and weakness. Bubbles traveling through the bloodstream can also obstruct blood flow, leading to tissue damage or causing skin mottling and itching.
High-Pressure Effects on the Nervous System
High pressure can directly affect the central nervous system through the chemical properties of breathing gases, distinct from physical bubble formation. Nitrogen narcosis, sometimes called “rapture of the deep,” is caused by the increased partial pressure of nitrogen acting as an anesthetic on nerve cell membranes. This effect is noticeable below about 100 feet (30 meters) and worsens with depth.
Symptoms of nitrogen narcosis include impaired judgment, short-term memory loss, disorientation, and reduced motor control, mimicking alcohol intoxication. Although the condition is reversible upon ascending, the cognitive impairment significantly increases the risk of accidents. Nitrogen is highly soluble in lipid-rich neurological tissues, making the brain and spinal cord particularly vulnerable.
Oxygen also becomes toxic at high partial pressures. Central Nervous System (CNS) oxygen toxicity poses an acute threat, causing symptoms like visual disturbances, twitching, and eventually seizures. This risk depends on both the depth and the oxygen concentration in the breathing mix. Pulmonary oxygen toxicity is a separate, chronic issue resulting from prolonged exposure to elevated oxygen partial pressures, which leads to lung damage.
Protecting the Body from Pressure Changes
Divers employ techniques and procedures to protect their bodies from the physical and chemical effects of pressure changes. To counteract barotrauma, divers must perform equalization maneuvers frequently during descent, such as the Valsalva technique for the ears and sinuses. Maintaining a continuous, steady breath throughout the dive, especially during ascent, prevents pulmonary barotrauma.
Mitigating the risk of decompression sickness requires strict control over the ascent phase. Divers must limit their ascent rate to a slow speed, typically no faster than 30 to 60 feet (9 to 18 meters) per minute, allowing nitrogen to off-gas safely. Mandatory safety stops, where the diver pauses at a shallow depth for several minutes, further aid in the controlled release of dissolved nitrogen. Dive computers track depth and time, providing real-time guidance on nitrogen absorption and safe ascent profiles.
The risk of nitrogen narcosis and oxygen toxicity is managed by controlling the depth and the composition of the breathing gas. Recreational divers are advised to stay within limits where narcosis is minimal, typically above 100 feet. Technical divers use specialized gas mixtures, such as trimix, which replaces some nitrogen with less narcotic gases like helium, to safely explore greater depths.