The human body’s ability to withstand the extreme environment of deep water is dictated by the physics of pressure. Water pressure, known as hydrostatic pressure, increases significantly with depth due to the weight of the water column above the diver. This pressure increases by approximately one atmosphere (ATM) for every 10 meters (33 feet) of descent. While the body is largely incompressible because it is composed of about 60% water, the air-filled spaces within the body present the most significant vulnerability to rising pressure.
How Pressure Affects Air Spaces
The physical limit for a human descending underwater is governed by Boyle’s Law, which describes the relationship between pressure and gas volume. This principle states that as surrounding pressure increases, the volume of any gas decreases proportionally. The air spaces most affected are the lungs, sinuses, and middle ear, where internal pressure must be manually equalized with the external water pressure to prevent injury.
Failure to equalize these spaces results in barotrauma, commonly termed “the squeeze.” As the air volume shrinks, it causes damage to surrounding tissues. For example, the eardrum can be forced inward, or the soft tissue lining of the sinuses can rupture. For a free diver, the lungs are compressed substantially; a breath taken at the surface is reduced to half its volume at just 10 meters (33 feet). The limiting factor for deep free diving is the point where pressure causes the thoracic cavity to collapse, potentially leading to lung barotrauma.
The Danger of Gases Under Pressure
When a diver uses a breathing apparatus, the physical compression of air spaces is managed, but a new physiological barrier emerges: the danger of breathing gases at high pressure. This threat is explained by Henry’s Law, which states that the amount of gas dissolved in a liquid, such as blood and body tissues, is directly proportional to the gas’s partial pressure. As a diver goes deeper, the partial pressures of the breathing mix increase, forcing more gases to dissolve into the body’s tissues.
Nitrogen Narcosis
The first major consequence of increased gas solubility is nitrogen narcosis, often called the “rapture of the deep.” Nitrogen, which makes up 79% of air, acts as an anesthetic on the central nervous system at high partial pressures. Symptoms resemble alcohol intoxication, leading to impaired judgment and loss of motor skills. For a diver breathing standard air, narcotic effects become noticeable around 30 meters (100 feet). The functional limit for a recreational air dive is set near 40 meters (130 feet) due to the risk of irrational behavior.
Oxygen Toxicity
A potentially fatal threat at depth is oxygen toxicity, specifically Central Nervous System (CNS) toxicity. While oxygen is necessary for life, breathing it at a high partial pressure causes seizures and convulsions, which can lead to a diver losing their regulator and drowning. The maximum safe partial pressure for oxygen is 1.4 atmospheres for working dives. For a diver breathing regular air, this limit is reached at about 58 meters (190 feet), setting the absolute depth limit for standard compressed air diving.
The Absolute Physical Tolerance
Determining how much pressure the human body can structurally withstand requires separating fluid-filled components from gas-filled spaces. Since the body is mostly water, which is nearly incompressible, the theoretical limit of hydrostatic pressure tolerance is immense. This limit is only achievable if air spaces are completely eliminated or filled with fluid. The practical limit, however, is always imposed by the presence of gas, even in breath-hold diving.
Highly trained free divers have developed a physiological adaptation called “blood shift” that allows them to reach extraordinary depths. As the lungs compress, blood and other fluids shift from the limbs into the chest cavity. This fluid redistribution replaces the lost volume of air in the lungs, protecting the chest wall from the crushing force of the water. This adaptation allows divers to exceed the theoretical crush depth for an air-filled lung. The current No Limit free diving record stands at 214 meters (702 feet), a pressure of over 22 atmospheres. For these athletes, the limiting factor becomes oxygen deprivation or lung barotrauma, not skeletal failure.
Extending the Limits: Deep Diving Technology and Records
To push past the limits of gas toxicity in apparatus diving, specialized breathing gas mixtures are used. To combat nitrogen narcosis, nitrogen is partially or fully replaced with helium, an inert gas with a much lower narcotic effect. These mixtures, known as Heliox (helium and oxygen) or Trimix (helium, nitrogen, and oxygen), allow divers to descend to hundreds of meters while maintaining cognitive function. Oxygen is also precisely diluted in these mixes to keep its partial pressure below the toxic threshold.
For extreme, sustained depths, saturation diving is employed. Divers live in a pressurized habitat for days or weeks, breathing a Heliox mixture. The habitat pressure is equalized to the working depth, allowing the diver’s body tissues to fully “saturate” with the inert gas. This technique eliminates the need for daily decompression stops, as divers undergo a single, extended decompression at the end of their mission. Using these methods, the deepest depth a human has been exposed to was a simulated pressure equivalent to 701 meters (2,300 feet) of seawater, achieved during an experimental chamber dive in 1992.