An oxygen-deficient atmosphere is defined as one where the percentage of oxygen in air is less than what percentage?

An oxygen-deficient atmosphere is specifically characterized as having an oxygen percentage that is less than 19.5 percent. This threshold is significant because, at this level, the oxygen concentration falls below what is considered safe for human breathing under normal conditions. A concentration of 19.5 percent is the minimum oxygen level required to sustain most physical activities without leading to adverse health effects.

When the oxygen level dips below this point, individuals may start to experience symptoms such as increased breathing rates and impaired cognitive functions, and further decreases can lead to serious health risks including unconsciousness or death. Therefore, the correct answer emphasizes the importance of maintaining a safe and breathable environment, particularly for first responders and individuals working in hazardous conditions.

The other percentages mentioned are either at or above normal atmospheric levels (which is approximately 21 percent for healthy air), making them unsuitable for defining an oxygen-deficient atmosphere.