An atmosphere is considered oxygen-deficient if it contains less than what percentage of oxygen?

An atmosphere is deemed oxygen-deficient when the concentration of oxygen falls below 19.5 percent by volume. This threshold is significant because it indicates a level of oxygen that may not support normal physiological functions, leading to symptoms such as confusion, shortness of breath, and increased heart rate. In typical atmospheric conditions, the normal oxygen level is approximately 21 percent, which is necessary for proper respiratory function in humans. Therefore, the value of 19.5 percent is adopted as a standard for recognizing a potentially hazardous environment. This is why the correct answer pertains directly to the established safety guidelines for fire fighters and other personnel working in potentially dangerous situations.