What percentage of oxygen is in the air we breathe?

What percentage of oxygen is in the air we breathe?

• A. 30%
• B. 10%
• C. 20%
• D. 50%

Answer: (C)

Oxygen makes up about one-fifth of the air we breathe. In standard atmospheric composition, oxygen is roughly 21 percent, with the rest largely nitrogen and other trace gases. Because 20 percent is a close, simple approximation of that 21 percent, it’s the best match among the options. The other numbers are not close to the actual mix: 30 percent would overstate oxygen, 10 percent would understate it, and 50 percent would be far too high. So about 20 percent is the correct approximation.

Oxygen makes up about one-fifth of the air we breathe. In standard atmospheric composition, oxygen is roughly 21 percent, with the rest largely nitrogen and other trace gases. Because 20 percent is a close, simple approximation of that 21 percent, it’s the best match among the options. The other numbers are not close to the actual mix: 30 percent would overstate oxygen, 10 percent would understate it, and 50 percent would be far too high. So about 20 percent is the correct approximation.