Scuba Diving International (SDI) Computer Nitrox Practice Exam

What amount of oxygen does air typically contain?

1. 15%

2. 21%

3. 25%

4. 30%

The correct answer is: 21%

Air typically contains approximately 21% oxygen. This percentage is a fundamental aspect of the Earth's atmosphere and is crucial for supporting life. In the context of scuba diving and the use of nitrox, understanding the composition of air is essential when considering the differences between regular air and enriched air nitrox, which has higher percentages of oxygen. When divers use nitrox mixtures, they are conscious of the increased oxygen levels, which allows for longer bottom times and shallower ascent rates, but they must also be aware of the risks associated with oxygen toxicity at higher partial pressures. Knowing that standard air contains 21% oxygen provides a baseline for divers to recognize how different gas mixtures will impact their dives and what precautions must be taken with various oxygen levels.