Which air-to-oxygen ratio corresponds to an FiO2 of 0.50?

Which air-to-oxygen ratio corresponds to an FiO2 of 0.50?

• A. 3:1
• B. 1:1
• C. 5:1
• D. 1.7:1

Answer: (D)

When mixing oxygen with room air, the resulting FiO2 sits between 0.21 (air) and 1.00 (pure oxygen). The ratio of air to oxygen determines how high FiO2 climbs. If you denote the air flow as A and the oxygen flow as O, FiO2 is (0.21A + 1.00O) divided by (A + O). To reach FiO2 of 0.50, set (0.21A + O)/(A + O) = 0.50 and solve for O/A: 0.21A + O = 0.50A + 0.50O 0.50O − O = 0.50A − 0.21A −0.50O = 0.29A O/A = 0.58 So the air-to-oxygen ratio A/O is about 1/0.58 ≈ 1.72, roughly 1.7:1. This means for every part of oxygen, there are about 1.7 parts air, which gives an FiO2 of about 0.50. That’s why the 1.7:1 ratio best matches the target FiO2. A 1:1 mix would give a higher FiO2 (around 0.60), while much more air (like 3:1 or 5:1) would lower FiO2 well below 0.50.

When mixing oxygen with room air, the resulting FiO2 sits between 0.21 (air) and 1.00 (pure oxygen). The ratio of air to oxygen determines how high FiO2 climbs. If you denote the air flow as A and the oxygen flow as O, FiO2 is (0.21A + 1.00O) divided by (A + O).

To reach FiO2 of 0.50, set (0.21A + O)/(A + O) = 0.50 and solve for O/A:

0.21A + O = 0.50A + 0.50O

0.50O − O = 0.50A − 0.21A

−0.50O = 0.29A

O/A = 0.58

So the air-to-oxygen ratio A/O is about 1/0.58 ≈ 1.72, roughly 1.7:1. This means for every part of oxygen, there are about 1.7 parts air, which gives an FiO2 of about 0.50.

That’s why the 1.7:1 ratio best matches the target FiO2. A 1:1 mix would give a higher FiO2 (around 0.60), while much more air (like 3:1 or 5:1) would lower FiO2 well below 0.50.