What does a closed-circuit SBCA (Self-Contained Breathing Apparatus) do?

A closed-circuit self-contained breathing apparatus (SBCA) is designed to recycle the air that the user exhales. Unlike open-circuit systems that vent exhaled air outside, the closed-circuit system captures the exhaled carbon dioxide and removes it, allowing the user to then breathe the same air after it has been purified and enriched with oxygen. This efficient design allows for longer usage durations in environments where oxygen is limited and increases the overall safety and effectiveness of operations such as firefighting, where breathable air may be scarce.

In this context, the other options do not accurately represent the function of a closed-circuit SBCA. Providing oxygen from an external source implies an open circuit system that takes in air from the environment. Increasing airflow could refer to various breathing apparatus designs but isn’t specific to closed-circuit operations. Filtering environmental air might apply to devices intended to cleanse ambient air, but this is not the operation of a closed-circuit system, which focuses on recycling already exhaled air.