Pressure is defined as the force exerted over a specific area, measured in pounds per square inch (PSI). The baseline for human tolerance is the standard atmospheric pressure at sea level, approximately 14.7 PSI. The human body is adapted to this pressure, and deviations above or below this level introduce physiological challenges. Tolerance depends heavily on whether the pressure change is sustained (like in deep water) or transient and rapid (such as from an explosion).
Limits of Tolerance to Low External Pressure
The greatest danger in a low-pressure environment is the lack of oxygen and the physical effects of gas expansion, not the vacuum itself. The initial concern when pressure drops is hypoxia, or inadequate oxygen supply. The “death zone” for unacclimated individuals begins below approximately 5.4 PSI, where the partial pressure of oxygen is too low to sustain normal brain function, quickly leading to confusion and unconsciousness.
The absolute mechanical limit is the Armstrong Limit, occurring at approximately 0.906 PSI. This is the point where ambient pressure equals the vapor pressure of water at normal body temperature. Exposure below this limit causes ebullism, the boiling of low-vapor-pressure body fluids like moisture in the mouth, eyes, and lung air sacs. Although the circulatory system prevents blood from boiling, the rapid expansion of gases in the lungs and gastrointestinal tract causes immediate and catastrophic internal damage.
Limits of Tolerance to High Hydrostatic Pressure
Tolerance to sustained high pressure, such as in deep-sea diving, is not limited by the compression of the body’s tissues, which are mostly liquid. The limiting factors arise from the effects of compressed breathing gases on the nervous system. As a diver descends, the total pressure increases by about 14.7 PSI (one atmosphere) for every 33 feet of depth.
One of the first significant limitations is nitrogen narcosis, where the high partial pressure of nitrogen causes cognitive impairment similar to alcohol intoxication. This typically begins to affect divers noticeably around 100 feet of depth, where the total pressure is about 58.8 PSI. To overcome this, deep divers replace nitrogen with helium, creating a breathing mixture called Heliox.
Substituting helium introduces High-Pressure Nervous Syndrome (HPNS), which manifests as tremors, dizziness, and decreased mental function. HPNS effects begin to appear at pressures around 191 PSI (approximately 400 feet of depth) and worsen as pressure increases. Commercial saturation divers manage these effects using specialized habitats and gas mixtures. The deepest simulated dive record reached 1,043.7 PSI, while the deepest open-water scuba dive record corresponds to a total pressure of about 485.1 PSI.
The Impact of Rapid Overpressure
The body’s tolerance to rapid pressure increases (overpressure) is dramatically different from its response to sustained hydrostatic pressure. This trauma is often caused by a blast wave from an explosion and is measured as the pressure above the ambient atmospheric pressure. Air-filled organs are the most vulnerable to this sudden shock wave due to the massive pressure differential created across tissue interfaces.
The threshold for mechanical damage starts low, with the tympanic membrane, or eardrum, being the most sensitive organ to blast injury. A transient overpressure of only 5 PSI is enough to cause the eardrum to rupture. Severe injuries involve pulmonary barotrauma, where the pressure wave damages the lungs. Lung contusions and hemorrhage begin at overpressures of approximately 15 PSI, and overpressures in the range of 60 to 80 PSI are considered potentially lethal, resulting in widespread fatalities.
Physiological and Technological Factors Modifying Pressure Tolerance
While the inherent physiological limits are relatively fixed, technology and training allow for the temporary expansion of these boundaries. For low-pressure environments, pressurized suits maintain a cabin-like atmosphere, keeping external pressure above the 0.906 PSI Armstrong Limit. These suits ensure the body’s internal water does not vaporize and maintain a sufficient partial pressure of oxygen for breathing.
For high-pressure environments, the primary technological solution is manipulating the breathing gas mixture. Saturation diving involves living in a pressurized habitat for weeks, allowing the body to equalize to the depth pressure while using specialized mixtures like Heliox or Trimix to mitigate narcotic and toxic effects. Physiologically, the mammalian diving reflex—which reduces heart rate and centralizes blood flow—offers a temporary, innate increase in pressure tolerance for breath-hold divers. These modifications allow humans to push beyond the limits of unassisted survival.