Fraction of inspired oxygen

Fraction of inspired oxygen (F_IO₂), correctly denoted with a capital I, is the molar or volumetric fraction of oxygen in the inhaled gas. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric F_IO₂. Natural air includes 21% oxygen, which is equivalent to F_IO₂ of 0.21. Oxygen-enriched air has a higher F_IO₂ than 0.21; up to 1.00 which means 100% oxygen. F_IO₂ is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity, but there are applications when up to 100% is routinely used.

Often used in medicine, the F_IO₂ is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the F_IO₂ may remain constant while the partial pressure of oxygen changes with the change in barometric pressure.

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