What type of respirator should be used in an oxygen-deficient environment?

In an oxygen-deficient environment, a supplied-air respirator is the appropriate choice because it provides the user with a continuous supply of breathable air from a separate source instead of relying on the ambient air. This type of respirator is designed to protect the wearer in situations where the oxygen concentration is below safe levels, such as confined spaces, where there might be a risk of asphyxiation or other serious hazards due to low oxygen availability.

An air-purifying respirator is not suitable for oxygen-deficient environments, as it operates by filtering contaminants from the air; it does not supply oxygen. Similarly, disposable face masks do not provide protection in such hazardous conditions, as they are primarily used for filtering out larger particles but do not protect against low oxygen levels. Filtered respirators also focus on purifying existing air and would fail to provide the necessary oxygen in an environment where it is lacking. Therefore, the supplied-air respirator is the only option that ensures safety by supplying adequate oxygen for breathing in environments that do not have sufficient levels.