What is the percentage level that indicates oxygen deficiency in the air?

The percentage level that indicates oxygen deficiency in the air is 19.5%. Normal atmospheric air contains about 21% oxygen. A level below 19.5% indicates that the air has become oxygen-deficient, which can pose serious health risks. At this concentration, individuals may begin to feel symptoms of hypoxia, which can lead to impaired cognitive function, fatigue, and, with prolonged exposure, can result in more severe health issues. Understanding this threshold is crucial for safety in environments where oxygen levels may be reduced, such as confined spaces or certain industrial settings.