The human body’s ability to withstand underwater pressure is not limitless. No single depth or pressure can be endured, as physiological factors and the rate of pressure increase with depth play significant roles. The body’s response depends heavily on managing air spaces and the speed of pressure changes.
The Mechanics of Underwater Pressure
Atmospheric pressure at sea level is approximately one atmosphere absolute (ATA). As one descends into water, pressure rapidly increases due to the weight of the water column above. For every 10 meters (or approximately 33 feet) of saltwater depth, pressure increases by an additional 1 ATA. This means at 10 meters deep, the total pressure is 2 ATA, and at 20 meters it is 3 ATA.
The relationship between pressure and gas volume is fundamental. Gases compress under pressure; as pressure increases, gas volume decreases proportionally. Conversely, as pressure decreases during ascent, gas volumes expand. This principle is central to many physiological challenges underwater.
How Pressure Affects the Human Body
Increasing underwater pressure impacts the human body, particularly its air-filled cavities. Barotrauma, or “squeeze,” occurs when there is a pressure difference between an air-filled space in the body and the surrounding water. Common examples include ear and sinus squeezes, causing pain, eardrum damage, or bloody noses. Lung squeeze, or pulmonary barotrauma, is a condition where air in the lungs compresses to a volume less than the residual volume, leading to tissue damage, fluid accumulation, and bleeding. This can result in chest pain, difficulty breathing, and coughing, sometimes with blood-tinged sputum.
Beyond mechanical compression, dissolved gases also pose risks. Nitrogen narcosis, often called “the rapture of the deep” or “the martini effect,” occurs when increased nitrogen partial pressure at depth affects the central nervous system. Symptoms begin around 30 meters (100 feet) and can include impaired judgment, euphoria, disorientation, and reduced motor skills, similar to alcohol intoxication. As depth increases, these symptoms can worsen, potentially leading to confusion, hallucinations, and impaired decision-making.
Oxygen can become toxic at elevated partial pressures. Central nervous system (CNS) oxygen toxicity can lead to neurological issues, including convulsions and unconsciousness, especially at partial pressures above 1.4 ATA. Pulmonary oxygen toxicity, affecting the lungs, develops over longer exposures to elevated oxygen levels and can cause inflammation and tissue damage.
Decompression sickness (DCS), commonly known as “the bends,” is another concern. It arises when inert gases, primarily nitrogen, dissolve into the body’s tissues under high pressure and then form bubbles as pressure decreases too rapidly during ascent. These bubbles can form in or migrate to various parts of the body, causing a wide range of symptoms. Joint pain, often described as “deep” and “boring,” is the most frequent symptom. More severe forms can affect the neurological system, leading to numbness, paralysis, or even death, or impact the respiratory system, causing chest pain and breathing difficulties.
Mitigating Pressure’s Dangers
Divers employ various strategies and technologies to manage underwater pressure risks. Specialized breathing gas mixtures are used for deeper and longer dives. Enriched air nitrox (EAN) contains a higher percentage of oxygen and less nitrogen than regular air, allowing for longer bottom times at shallower depths by reducing nitrogen absorption. Gases containing helium, such as heliox or trimix, are used for very deep dives to mitigate nitrogen narcosis and reduce oxygen toxicity risk. Helium is less narcotic than nitrogen and is absorbed and released by the body more quickly.
Careful decompression procedures prevent decompression sickness. Divers follow ascent rates and perform decompression stops, which are planned pauses at specific depths during ascent. These stops allow dissolved inert gases to safely off-gas from the body’s tissues, preventing bubble formation. Safety stops, a 3-5 minute pause at 5-6 meters (15-20 feet), are recommended for most dives to provide an additional margin of safety.
Hyperbaric chambers treat decompression sickness and are used in saturation diving. For DCS treatment, a hyperbaric chamber re-pressurizes the diver to reduce nitrogen bubble size, allowing them to redissolve and be eliminated. In saturation diving, divers live in a pressurized environment for extended periods, becoming fully saturated with inert gas. This allows them to work at great depths for long durations, with decompression occurring only once at the end of the mission, often taking days or weeks. Proper training and adherence to dive plans minimize risks.
Pushing the Limits of Depth
Human capability to withstand underwater pressure varies depending on the diving method. Free diving, which involves breath-holding without breathing apparatus, is limited by air compression in the lungs and the body’s physiological response to pressure changes. While the body is mostly incompressible water, air spaces like the lungs are highly affected. Highly trained free divers have achieved depths like Herbert Nitsch’s 214 meters (702 feet) in “No Limits” free diving (2007) and Alexey Molchanov’s 131 meters in constant weight freediving. These feats risk lung squeeze and other pressure-related injuries.
SCUBA diving and saturation diving involve breathing compressed gases, allowing humans to go deeper and stay longer. With specialized gas mixtures and controlled decompression, professional divers can work at depths of several hundred meters. Saturation divers can operate at depths of over 300 meters, living in pressurized habitats for weeks. The primary limiting factors for these deep dives are the narcotic effects of gases like nitrogen, oxygen toxicity, and the complex logistical challenges of managing decompression over extended periods. Even with advanced technology, the human body’s tolerance to high pressure and gas physiology presents challenges.