What is the percentage at which an atmosphere is considered oxygen deficient?

An atmosphere is considered oxygen deficient when the percentage of oxygen is below 19.5 percent. This threshold is significant because normal atmospheric air contains about 21 percent oxygen. Levels below 19.5 percent can lead to various health issues, such as impaired cognitive function, dizziness, or loss of consciousness, depending on the degree of deficiency and the duration of exposure. Therefore, recognizing 19.5 percent as the cutoff helps ensure safety in environments where inhalation of oxygen is critical, such as in confined spaces or during certain industrial operations.