What is the normal level of oxygen in the environment?

The normal level of oxygen in the environment is typically around 21 percent, which is represented as 0.21 when expressed as a decimal. This percentage is crucial for the survival of aerobic life forms, including humans, as it is the concentration of oxygen in the atmosphere that is readily available for respiration. It supports combustion as well, which is a key aspect in firefighting and understanding fire dynamics.

Other percentages listed do not reflect the approximate oxygen concentration found in the atmosphere. Values like 0.15, 0.25, and 0.30 would indicate significantly altered atmospheric conditions, often detrimental to human health or indicative of different environments, such as higher altitudes or enclosed spaces where oxygen levels may be reduced or increased from the standard level. Thus, the correct representation of the normal environmental oxygen level is 0.21.