Human exposure to hyperbaric pressure occurs while diving and during hyperbaric oxygen therapy. Hypobaric exposures occur on commercial plane flights where the cabin pressure equals about 2438.4 meters (8000 feet); however, professional aviators, particularly military personnel, mountain climbers, research subjects, and astronauts are exposed to much greater extremes in low-pressure environments. The two main concerns about hypobaric exposure relate to absolute pressure upon the human body and the total oxygen available to the body.
An understanding of the gas laws is foundational to understanding how pressure changes affect humans. The gas laws describe the relationship between temperature, volume, and pressure for a given amount of gas. For example, Charle's law states that for a given pressure, the volume is proportional to the temperature. Therefore, a gas expands when heated as long as the pressure is allowed to remain the same. However, if gas cannot expand such as when trapped inside the middle ear or a nasal sinus, the pressure will increase. For a given volume the pressure will vary with the temperature of the gas. The thermoregulatory system in the human body does not typically allow for more than a few degrees Celcius variance in body temperature. Boyle's law states that for a given temperature the volume is inversely proportional to the pressure. This law explains why sinuses or middle ear (which are normally fixed volume gas-filled spaces) may hurt when during altitude or pressure changes. Finally, Dalton's law notes the total pressure of a mixture of gases equals the partial pressure exerted by each gas. This concept is important given that the air humans breathe is a mixed gas of nitrogen (approximately 78%), oxygen (approximately 21%), and trace other gases.
If the human body is exposed to a low enough absolute pressure, then surface fluids (tear film, saliva, and the air-exposed surface of alveoli) will begin to boil at normal body temperature. This occurs at around an altitude of 60,000 feet (approximately 11.4 miles or 18.3 kilometers) depending on exact atmospheric conditions. This altitude has been named "Armstrong's limit" or "Armstrong's line" and is named after an early American aerospace medicine physician, Harry G. Armstrong. When blood boils, this is called "ebullism" and is the trapping of gases released from blood under the skin. Ebullism is painful but recoverable to full function as human experience has shown.
Reduced oxygen levels in the body can occur for various reasons. Hypoxia is the general term for low oxygen content in the blood or at the tissue level. Hypoxic hypoxia is hypoxia secondary to low alveolar oxygen exchange in the lungs and can be caused by either a low oxygen availability or due to low surface area for the gas exchange. In this article, "hypoxia" means hypoxic hypoxia due to low oxygen availability in the environment. While the percentage of oxygen remains a constant 21% as one increases in elevation, the total pressure of oxygen decreases because the total pressure of all gases combined decreases. The barometric pressure at sea level is around 760 mm Hg with some variation depending on the weather. Therefore at sea level, oxygen is only 21% of that total or 160mm Hg. Inside the lungs and alveoli, the temperature remains approximately 37 degrees Celsius (98.6 degrees Fahrenheit). As one ascends in altitude, the total atmospheric pressure goes down which necessarily means the oxygen available for breathing goes down as well.
While there is some variation from person to person, the effects of hypoxia are accepted to begin at 3048 meters (10,000 feet). These effects include reduced light reception and decreased ability to distinguish colors. As a person ascends in altitude, their body compensates with increased depth of respiration, increased rate of respiration, and increased heart rate in an attempt to maintain oxygen delivery to the tissues. Further ascent leads to extreme fatigue and reduced mental capacity. Exposure to atmospheric conditions in approximately 7620 to 10,363 meters (25,000 to about 34,000 feet) results in death if supplemental oxygen is not used. Under 34,000 ft 100% oxygen in a tight-fitting mask will deliver near ground level oxygen to the tissues. Pressure suits or pressurized cockpits must be used beyond this level to maintain near sea level oxygenation to the tissues.