The question of how much pressure a human can withstand does not have a single numerical answer, as tolerance depends entirely on the type of pressure and the speed of its application. Pressure is most commonly measured in pounds per square inch, or PSI, which quantifies the force exerted over a one-square-inch area. The human body is a fluid-filled system designed to maintain equilibrium with its surrounding environment. Therefore, survival at extreme pressure levels hinges on maintaining a balance between the external force and the internal pressures within the body’s air-filled spaces.
Understanding Pressure Measurement and Human Tolerance
The standard pressure at sea level, the baseline environment for human life, is approximately 14.7 PSI, referred to as one atmosphere absolute (1 ATA). Pressure is measured in two primary ways: absolute pressure (PSIA) and gauge pressure (PSIG). PSIA uses a perfect vacuum as its zero point, thus including the 14.7 PSI of the atmosphere. PSIG measures pressure relative to the surrounding atmospheric air, starting at zero at sea level.
The body’s internal pressure, such as the gases in the lungs and sinuses, is naturally equalized to the external atmospheric pressure. When an individual changes elevation, the body must adjust its internal pressure to match the new external pressure. A significant and rapid imbalance between internal and external pressure is the primary source of danger, dictated by the physical laws governing gases.
Physiological Limits of High (Hyperbaric) Pressure
Hyperbaric pressure, found deep underwater, increases by approximately 14.7 PSI (1 ATA) for every 33 feet of descent. Since the body is mostly fluid and incompressible, the pressure itself does not crush tissues. Instead, survival limits are set by the toxicity of breathing gases under high pressure and their effects on the central nervous system.
Sustained human tolerance in saturation diving has reached levels around 485 to 530 PSI (33 to 36 ATA). The deepest recorded simulated dive transiently exposed humans to pressure equivalents nearing 1,029 PSI (70 ATA). At these extreme depths, the primary physiological barrier is High Pressure Nervous Syndrome (HPNS), caused by the direct effect of pressure on nerve cell membranes. HPNS manifests as tremors, muscle spasms, dizziness, and cognitive impairment, requiring specialized breathing gas mixtures for mitigation.
Gas Toxicity Limits
Another major limit is oxygen toxicity, where the partial pressure of oxygen causes oxidative damage. CNS oxygen toxicity can lead to seizures and loss of consciousness, while pulmonary oxygen toxicity causes lung damage. Nitrogen gas, which makes up most of the air we breathe, also becomes narcotic above 60 to 75 feet of seawater, a condition known as nitrogen narcosis. To safely reach extreme depths, divers must replace nitrogen with less narcotic gases like helium.
Physiological Limits of Low (Hypobaric) Pressure
Low external pressure, or hypobaric conditions, are experienced at high altitudes or in a vacuum. At high altitude, the primary danger is severe hypoxia, a lack of oxygen reaching the tissues. Although the percentage of oxygen in the air remains constant at 21%, the total atmospheric pressure drops significantly, lowering the partial pressure of available oxygen. Unacclimated humans experience cognitive and physical impairment without supplemental oxygen above 10,000 feet.
The absolute lower limit of pressure tolerance is marked by the onset of ebullism, which occurs at approximately 63,000 feet, known as Armstrong’s Limit. At this height, the ambient pressure drops to about 0.9 PSIA. Below this limit, the lack of external pressure causes water in the body’s exposed fluids, such as saliva, tears, and moisture in the lungs, to spontaneously boil and turn into gas.
The near-vacuum state causes rapid swelling due to the formation of water vapor bubbles beneath the skin and within soft tissues, alongside the immediate cessation of effective lung function. While immediate death is likely from severe hypoxia, the body’s internal pressure, maintained by the circulatory system, may offer a brief protective effect against immediate catastrophic organ failure if recompression occurs quickly.
Acute Dangers of Rapid Pressure Changes
The most dangerous pressure-related injuries result from the rapid rate of change, not the absolute pressure limit. Rapid compression or decompression creates a significant pressure differential between gas trapped inside the body and the surrounding environment. This imbalance is governed by Boyle’s Law, which states that the volume of a gas is inversely proportional to the pressure exerted on it.
Barotrauma
The primary mechanical injury from a rapid pressure change is barotrauma, physical damage to tissues surrounding trapped gas. During a rapid ascent, expanding air in the lungs can cause pulmonary barotrauma, potentially rupturing lung tissue and leading to a fatal arterial gas embolism. Rapid descent can cause barotrauma in the middle ear or sinuses when internal air spaces cannot equalize fast enough to match the increasing external pressure.
Decompression Sickness (DCS)
Decompression Sickness (DCS), often called “the bends,” results from a rapid reduction in pressure following high-pressure exposure. Under high pressure, inert gases like nitrogen dissolve into the body’s tissues and blood according to Henry’s Law. If pressure is reduced too quickly, the dissolved gas forms bubbles within the tissues and bloodstream, similar to opening a carbonated drink. These gas bubbles cause severe joint pain, neurological symptoms, and spinal cord damage, making controlled, slow decompression necessary for survival after deep exposures.