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Fraction of inspired oxygen (FiO2) is the fraction or percentage of oxygen in the volume being measured. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric FiO2. Natural air includes 21% oxygen, which is equivalent to FiO2 of 0.21. Oxygen-enriched air has a higher FiO2 than 0.21; up to 1.00 which means 100% oxygen. FiO2 is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity. If a patient is wearing a nasal cannula or a simple face mask, each additional liter/min of oxygen adds about 4 percentage points for each of the first 3 liters and only 3 percentage points for every liter thereafter to their FiO2 (for example, a patient with a nasal cannula with 4L/min of oxygen flow would have an FiO2 of 21% + (3 × 4%)+(1 × 3%) =36%).
Often used in medicine, the FiO2 is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the FiO2 may remain constant while the partial pressure of oxygen changes with the change in barometric pressure.

Source: Wikipedia