535. Which of the following devices allows the user to remain in an oxygen deficient space for the longest period of time without interruption?

A direct compressed air supply allows for the longest duration in an oxygen-deficient environment because it utilizes a remote air source, unlike self-contained units limited by onboard cylinder capacity. Only air-supplying respirators are suitable for oxygen-deficient spaces, as they provide breathable air rather than filtering contaminants. Canister masks, pure oxygen apparatus, and demand-type oxygen apparatus all have limited durations due to their reliance on finite onboard supplies.