What is the normal air-to-oxygen ratio when inspired oxygen is about 40%?

• A. 1:3
• B. 1:1
• C. 3:1
• D. 2:1

Answer: (C)

When you mix room air with supplemental oxygen, the resulting FiO2 depends on how much oxygen you add relative to air. Air is about 21% O2, so a mixture of three parts air to one part O2 yields FiO2 around 0.41 (calculation: O2 from air is 0.21×3 = 0.63, plus 1.0 from the pure O2 gives 1.63 O2 in 4 parts, 1.63/4 ≈ 0.41). This is essentially about 40% oxygen.

So the best way to achieve roughly 40% inspired oxygen is to use a 3:1 air-to-oxygen ratio. Ratios like 2:1, 1:1, or 1:3 would push FiO2 higher or lower than 40% (for example, 2:1 yields about 47% FiO2, 1:1 about 60%, and 1:3 well above 80%), not matching the target.