Hypobaria refers to an environment in which the total ambient gas pressure is less than 1 atmosphere (1013 mbar). This definition is independent of the composition of the gas.

For purposes of respiration a meaningful hypobaric restriction is the Armstrong Limit at 63 mbar, defined by the water vapor pressure at normal body temperature (37°C). Below this pressure any bodily fluids exposed to the atmosphere (such as in the eyes, mouth, or lungs) will begin to boil, posing an immediate threat to survival. It should be noted that blood does not boil under these conditions, unless a cut exposes it to the atmosphere. Maintaining pressure above the Armstrong Limit does not guarantee that respiration is possible, as this is further restricted by the need for a sufficiently high O2 pressure (see Hypoxia). If this requirement is not met, a pressure suit is necessary for survival, just as in a vacuum environment. That said, environmental pressures above the Armstrong Limit significantly reduce the danger associated with a pressure suit leak. Thus, it is a useful benchmark in emergency situations on spacecraft or high altitude aircraft, as well as the terraforming of planets with thin atmospheres like Mars.