alveolar air can be calculated because it will be directly proportional to its fractional composition in air. since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water. history the term hypoxemia was originally used to describe low blood oxygen occurring at high altitudes and was defined generally as defective oxygenation of the blood. in modern times there are a lot of tools to detects hypoxemia including smartwatches. in 2022 a research has shown smartwatches can detect short-time hypoxemia as well as standard medical devices.