Which statement is true about simple facemasks concerning room air?

• A. They are open systems because they can entrain room air
• B. They are closed systems because they entrain room air
• C. They cannot entrain room air
• D. They are neither open nor closed

Answer: (A)

The key idea here is how a simple facemask interacts with ambient air. A simple facemask does not form a tight, sealed circuit around the face, so when you inhale, air from the room can mix with the oxygen being delivered. That entrainment of room air means the system is open rather than closed—the atmosphere outside the mask becomes part of the inhaled gas. Because of this, the inspired gas is a variable blend of the oxygen flow and room air, and the FiO2 depends on how much room air is drawn in. In a truly closed system, there would be no exchange with room air and the gas composition would stay fixed regardless of breathing, which isn’t the case with a simple facemask. So, the statement that a simple facemask is an open system because it entrains room air best captures how this device works in practice.