Which of the following describes a closed circuit breathing apparatus?

A closed circuit breathing apparatus functions by recycling the user's exhaled air. In this system, the exhaled air passes through a carbon dioxide scrubber that removes the CO2, allowing the clean, oxygen-rich air to be breathed back in by the user. This process minimizes the need for a constant supply of fresh air from external sources, making the apparatus highly efficient in environments where air is limited, such as during prolonged operations or in confined spaces.

This effectiveness is particularly important in firefighting and rescue operations, as it allows personnel to work in hazardous conditions without the need for frequent air supply replenishment. The recycled nature of the air not only conserves oxygen but also reduces the weight and bulkiness associated with carrying large tanks of compressed air.

The other options do not accurately describe the closed circuit breathing apparatus. Providing fresh air from an external source refers more to open circuit systems. Creating oxygen from exhaled air is not a function of these devices, as they rely on the oxygen present in exhaled air rather than generating new oxygen. Lastly, while some closed circuit systems may use compressed air cylinders, their defining feature is the method of recycling air, rather than depending solely on compressed tanks, which is characteristic of open circuit systems.