Air consists of approximately what percent Oxygen?

• A. 21 Percent Oxygen
• B. 15 Percent Oxygen
• C. 50 Percent Oxygen
• D. 10 Percent Oxygen

Answer: (A)

Air is made up mostly of nitrogen and oxygen, and the oxygen portion is about 21 percent by volume at sea level. That means roughly one-fifth of the air you breathe is oxygen, with nitrogen making up most of the rest. This 21% figure is important for both life support and combustion in fire safety contexts. Other numbers—like 15%, 50%, or 10%—don’t match normal ambient air: 15% would be noticeably oxygen-deficient and unsafe to breathe, 50% would be an unusually enriched environment, and 10% is far below breathable levels.