Definition As distance from the sea level increases, barometric pressure of the air falls; however, the O2 concentration remains unchanged as does water vapor pressure. At sea level barometric pressure is 760mmHg, with 47mmHg vapor pressure, leaving PO2 as 0.21 * (760-47) ≈ 150mmHg. At 19,000ft (Mount Kilimanjaro) the barometric pressure is 380mmHg and thus […]