Which option represents an open circuit breathing apparatus?

• A. Open circuit system
• B. Supplied air breathing system (SABA)
• C. Closed-circuit SCBA
• D. Half-face respirator

Answer: (A)

Open-circuit breathing means the air you breathe in is supplied from a source and your exhaled air is vented to the surroundings, with no air being recycled. This is the typical setup for an SCBA used by firefighters, where a cylinder provides the breathing gas and the exhaled breath simply exits to the environment.

The open-circuit system exactly describes this setup. In contrast, a closed-circuit (rebreather) recycles and scrubs exhaled air for reuse, so it isn’t open-circuit. A supplied-air system delivering air from an external source can vary, but it’s not defined as open-circuit by itself. A half-face respirator relies on filtered ambient air rather than a self-contained or external supply, so it isn’t an open-circuit breathing apparatus in the same sense.