What is the percentage of oxygen in normal air?

The percentage of oxygen in normal air is approximately 21%. This value is significant in various fields, including medicine and environmental science, as it reflects the typical composition of Earth's atmosphere. Oxygen is essential for the respiration of aerobic organisms, and understanding its concentration helps EMTs assess patients in situations where oxygen levels can fluctuate, such as in certain medical emergencies or high-altitude scenarios.

The other options reflect misunderstandings about air composition. For instance, while nitrogen constitutes about 78% of air, the percentage given in the correct answer accurately highlights the importance of oxygen. The choice of 30% is misleading, as this level is much higher than what is found in natural atmospheric conditions, which would not occur outside of controlled environments or oxygen therapy. Lastly, stating that air consists of 100% oxygen is incorrect, as breathing pure oxygen over extended periods can be harmful due to the risk of oxygen toxicity. Hence, 21% is the recognized and crucial percentage for oxygen in the air we breathe.