What is considered a normal oxygen level?

A normal oxygen level in the atmosphere is typically around 20.9%. This value represents the concentration of oxygen in the air we breathe at sea level under standard atmospheric conditions. Maintaining an adequate level of oxygen is crucial for both human health and effective functioning in environments where respiratory safety is a concern, such as during hazmat operations.

While options like 20.0% and 21.5% may seem close, they do not accurately reflect the standard atmospheric composition. The concentration of 19.5% is also significant as it marks the minimum level of oxygen required for safe breathing; levels below this can lead to hypoxia. Therefore, option C is the only choice that represents the typical, healthy, and safe oxygen concentration present in the environment.