Most of the world's population lives at or near sea level, where their ancestors lived for thousands of generations. Because the human body is adapted to life at sea level, it must make physiological adjustments to the decrease in atmospheric pressure at higher altitudes. These adjustments are true physiological adaptations that appear to have evolved out of the survival advantage that tolerance to O2 deprivation affords the body. The amount of O2 in the atmosphere declines as altitude increases, exposing the body to hypoxia, which produces the same effects as certain disorders of cardiopulmonary function.
At sea level the air column above Earth exerts a force approximately equivalent to the weight of a column of mercury (Hg) 760 millimeters (29.9 inches) high. This height of mercury, placed in a barometer, exactly counterbalances the normal sea level pressure of Earth, 1 ATA, or 1 bar (1000 millibars). On ascent to altitude barometric pressure falls because the atmosphere is less dense owing to the lower weight of the air column above it. Atmospheric pressure falls more rapidly at attitude than might be predicted from the weight of the air column because as one ascends there is less compression of the air from the gas above it. Thus,