In which atmospheric condition is the oxygen level less than 21%?

An oxygen deficiency occurs when the concentration of oxygen in the atmosphere falls below the normal level of 21%. In typical atmospheric conditions, oxygen makes up approximately one-fifth of the air we breathe. Various scenarios can lead to lowered oxygen levels, such as high altitudes, enclosed spaces, or environments that consume oxygen, like certain industrial settings.

Understanding this concept is essential for safety, particularly in scenarios involving fire and emergency response, where maintaining appropriate oxygen levels is critical for firefighter health and operational success. In contrast, the other choices refer to situations where oxygen is either present in sufficient amounts or in heightened levels, thereby not representing a deficiency. This distinction emphasizes the importance of recognizing atmospheric conditions that can negatively impact safety.