Final answer:
Air-purifying respirators require at least 19.5% oxygen to function properly. Gases like oxygen can be stored in small, compressed containers and released for use as needed while maintaining adequate partial pressure for respiration.
Step-by-step explanation:
For air-purifying respirators to work effectively, there must be at least 19.5% oxygen present in the atmosphere. When an environment has low oxygen levels, supplemental oxygen can be provided from a tank. Because gases are highly compressible, oxygen can be stored in a small container in a liquid state and released as needed. The partial pressure of inspired oxygen in the lungs, known as the alveolar PO₂, is typically around 150 mm Hg, which is sufficient for the oxygen to flow from the lungs into the bloodstream where it has a partial pressure of around 100 mm Hg. Air, which is about 21% oxygen, exerts a partial pressure, and the minimum permissive oxygen concentration is essential for both respiration and the operation of air-purifying respirators.