With a good mask-to-face seal and an oxygen flow rate of 15 L/min, the nonrebreathing mask is capable of delivering up to ______% inspired oxygen.

• A. 80
• B. 90
• C. 70
• D. 100

Answer: (B)

Nonrebreathing masks deliver very high FiO2 when you have a good seal and a high oxygen flow, because the reservoir bag fills with oxygen and most of the inhaled gas comes from that reservoir rather than ambient air. With a flow of 15 L/min, the bag stays filled and the one-way valves minimize dilution with room air, so the inspired oxygen concentration can reach around 90%. It isn’t guaranteed to be 100% because, in practice, a tiny amount of ambient air can mix in and some oxygen is consumed by the patient, plus dead space in the mask means not every breath is pure reservoir gas. So, 90% is the best-supported figure for this setup.