What percentage of oxygen is contained in exhaled air?

Analyze the question: You're asked for the approximate percentage of oxygen in air that a person breathes out (exhaled air).

Concept: Inhaled and exhaled air differ in composition because gas exchange happens in the alveoli of the lungs. Oxygen diffuses into the blood, and carbon dioxide diffuses out of the blood.

Typical composition:

| Gas | Inhaled air | Exhaled air | |---|---|---| | Oxygen | about 21% | about 16% | | Carbon dioxide | about 0.04% | about 4% | | Nitrogen | about 78% | about 78% | | Water vapour | variable | high (saturated) |

Step by step:

1. Inhaled air has about 21% oxygen (option A, which is the inhaled value, not the exhaled one).
2. Only about 5% of the oxygen is taken up by the body in each breath.
3. So exhaled air contains 21% − 5% ≈ 16% oxygen.
4. Checking the other options: 4% is the carbon dioxide in exhaled air (C), and 0.04% is the carbon dioxide in inhaled air (D). Less than 0.01% (E) is not a value for any major gas here.

Answer: The exhaled air contains about 16% oxygen, which is option B.

Tip: Remember the pairs "21 → 16" for oxygen and "0.04 → 4" for carbon dioxide. Oxygen drops by about 5 points, while carbon dioxide rises about 100 times. NECO, WAEC and JAMB often test this comparison, so learn the full inhaled and exhaled table.