What percent of oxygen is typically present in normal air?

Normal air consists of approximately 20.9% oxygen. This value is recognized as the standard atmospheric concentration of oxygen at sea level. The presence of this specific percentage of oxygen is critical for human respiration and supports combustion processes, as it is sufficient for sustaining life and enabling fires to burn.

While the other options present values that are close to the accurate percentage, they do not accurately represent the composition of normal air. For instance, 19.5% is considered the minimum oxygen level for safe breathing, but it is lower than typical air concentration. Similarly, values like 21.5% and 22.0% exceed what is present in the atmosphere, as these amounts would indicate a higher concentration than natural air conditions. Thus, the recognized concentration of oxygen in the atmosphere is indeed about 20.9%.