How does a closed circuit breathing apparatus function?

A closed circuit breathing apparatus functions by filtering and recycling exhaled air while adding oxygen to ensure that the individual using the apparatus has a fresh supply of breathable air. This type of system is designed to minimize the loss of air and is efficient in conserving resources, making it especially valuable in environments where breathable air is limited or requires management, such as in confined spaces or during extended firefighting operations.

In this system, exhaled air is passed through a chemical scrubber or filter that removes carbon dioxide and other contaminants. The purified air is then blended with a supply of oxygen before being delivered back to the user. This continuous recycling process allows the user to sustain longer durations without needing to replace air tanks frequently, enhancing safety and operational effectiveness in hazardous conditions.

The other options do not accurately describe the operation of a closed circuit breathing apparatus—fresh air is not continuously supplied from an external source, exhaled air is not simply vented outside, and there is typically no direct connection to an external air supply that would substitute the self-contained system inherent in close circuit designs.