An oxygen deficient atmosphere may result when the percentage is below ____.

An oxygen-deficient atmosphere is defined as one where the oxygen level drops below a certain threshold, posing health risks to individuals. The typical concentration of oxygen in the atmosphere is around 20.9%. When the oxygen concentration falls below 19.5%, it is considered insufficient to support normal physiological functions, leading to potential hypoxia, which can impair cognitive and physical abilities. This is especially critical for firefighters and others working in environments where breathing air with adequate oxygen is essential for safety and operational effectiveness. Maintaining oxygen levels at or above 19.5% is crucial, which is why this percentage is recognized as the standard limit for safety in various industries, including firefighting.