The human body is constantly subjected to one atmosphere absolute (ATA) of pressure at sea level. Underwater, pressure increases rapidly, adding approximately one ATA for every ten meters of descent. For example, a diver at forty meters experiences five times the pressure felt on the surface. The ultimate limit to human survival is a complex intersection of physical crushing forces and the chemical effects of breathing gases under high pressure.
The Mechanical Limits of Underwater Pressure
The body is mostly liquid and largely incompressible, but air-filled spaces are immediately subject to pressure changes. Barotrauma, or pressure injury, occurs when a pressure difference exists between an internal gas space and the surrounding ambient pressure. The primary air spaces affected are the lungs, sinuses, and middle ear, which must be constantly equalized during descent. Failure to equalize creates a negative pressure gradient, which can pull fluid and tissue into the space, potentially resulting in a ruptured eardrum or sinus damage.
The lungs face the greatest mechanical challenge, especially in free diving without a compressed gas supply. As a diver descends, the volume of air in the lungs decreases inversely to the pressure increase, following Boyle’s Law. This compression continues until the lungs reach their residual volume, the minimum amount of air remaining after a maximal exhale, typically reached around thirty to forty meters.
Diving beyond the residual volume results in lung squeeze, a form of pulmonary barotrauma. To exceed this depth, the body relies on a physiological defense mechanism known as blood shift, which pools blood into the chest cavity to help equalize the internal pressure. If the blood shift is insufficient, the negative pressure can cause capillaries in the lungs and airways to rupture, leading to symptoms like coughing up blood-tinged sputum.
The Physiological Impact of High-Pressure Gases
For divers breathing compressed gas, the absolute limit is the toxic and narcotic effects of gases at high partial pressures, rather than mechanical forces. The partial pressure of a gas is the true measure of its effect on the body. Since the percentage of a gas remains constant in the breathing mix, its partial pressure increases linearly with depth and total ambient pressure.
Oxygen toxicity is the most immediate threat, divided into Central Nervous System (CNS) and Pulmonary types. CNS oxygen toxicity is an acute concern that occurs quickly, typically when the oxygen partial pressure exceeds 1.4 ATA. When breathing standard air, this limit is reached around fifty-seven meters. The resulting chemical reaction in the brain can trigger seizures, confusion, and convulsions without warning. Since a convulsion underwater almost certainly leads to drowning, the CNS limit is the immediate maximum depth constraint for divers using air.
Pulmonary oxygen toxicity affects the lungs over a longer duration. It develops when a diver is exposed to an elevated oxygen partial pressure, such as above 0.5 ATA, for many hours or days. The symptoms are gradual, resembling a severe respiratory illness with a persistent cough, chest burning, and reduced lung capacity. This type of toxicity is primarily a limiting factor in saturation diving, where divers may spend weeks at depth.
Nitrogen narcosis, often called the “rapture of the deep,” begins to impair cognitive function around thirty meters. Nitrogen, normally inert, acts as an anesthetic on the central nervous system under high pressure. This effect is similar to alcohol intoxication, causing impaired judgment, poor motor control, and difficulty concentrating. At depths greater than ninety meters, the effects can become debilitating, leading to hallucinations and loss of consciousness. This neurological impairment makes standard air unsafe for deep dives, compelling professional divers to replace nitrogen with less narcotic gases like helium.
Defining the Absolute Limits of Human Survival
The maximum pressure a human can withstand depends entirely on the technology and gas mixtures used. For an unassisted free diver, the practical depth limit is dictated by lung squeeze. Highly trained athletes have pushed this barrier, with the current “No Limits” record exceeding 250 meters. These extreme feats are only possible due to the body’s maximized blood shift response, which compensates for the extreme lung compression.
For assisted diving, specialized gas mixtures like heliox (helium and oxygen) or trimix (helium, oxygen, and nitrogen) bypass the limitations of nitrogen narcosis and oxygen toxicity. Helium is substituted for nitrogen to eliminate the narcotic effect, and the oxygen percentage is reduced to keep its partial pressure below the toxic threshold. While this allows divers to reach greater depths, it introduces High-Pressure Nervous Syndrome (HPNS), which involves tremors and neurological issues at depths beyond 160 meters.
To counteract HPNS, a small amount of nitrogen is added back into the helium-oxygen mix, creating trimix and extending the survivable depth. The deepest simulated dive in a hyperbaric chamber reached 701 meters, while the open-sea record for an ambient pressure dive is 534 meters. The theoretical absolute limit for a diver breathing an optimal gas mixture is estimated to be between 1,000 and 1,200 meters, where the sheer pressure on the nervous system causes irreversible psychotic and neurological dysfunction.