What is considered an oxygen deficient atmosphere?

An oxygen deficient atmosphere is defined as having an oxygen concentration of 19.5% or less. At this level, the oxygen is insufficient to support normal human respiration effectively and can lead to serious health risks for individuals exposed to such environments. The standard threshold for safe atmospheric oxygen levels is approximately 20.9%, which is the typical concentration found in the atmosphere at sea level.

Going below the 19.5% mark indicates a potential hazard for personnel, particularly in enclosed or poorly ventilated spaces. This can jeopardize the ability to sustain life and impair physical and cognitive functions. In addition to health concerns, oxygen levels below this threshold also impact combustion processes, as combustion requires a certain level of oxygen to maintain the chemical reactions necessary for fire to exist.

In summary, the definition for an oxygen deficient atmosphere is specifically linked to the clear demarcation of 19.5% oxygen concentration or lower, making this the correct choice for the question.