What is the pressure within the facemask relative to atmospheric pressure?

What is the pressure within the facemask relative to atmospheric pressure?

• A. 0 psi above atmospheric
• B. 0.01 psi above atmospheric
• C. 0.036 to 0.054 psi above atmospheric
• D. 0.5 psi above atmospheric

Answer: (C)

The facepiece of an SCBA is kept under a small positive pressure relative to the surrounding air. This slight overpressure, about 0.036 to 0.054 psi above atmospheric, corresponds to roughly 1 to 1.5 inches of water gauge. That tiny push outward through any gaps or leaks is what prevents contaminated air from leaking into the mask, even if the sealing isn’t perfect. It also helps ensure a clean supply of breathing air is always available as you inhale, while exhaled air is vented through the exhalation path. So the best choice reflects a small, steady positive pressure above ambient—not zero, not a negligible 0.01 psi, and not as high as 0.5 psi—because this level balances protection with comfortable breathing.

The facepiece of an SCBA is kept under a small positive pressure relative to the surrounding air. This slight overpressure, about 0.036 to 0.054 psi above atmospheric, corresponds to roughly 1 to 1.5 inches of water gauge. That tiny push outward through any gaps or leaks is what prevents contaminated air from leaking into the mask, even if the sealing isn’t perfect. It also helps ensure a clean supply of breathing air is always available as you inhale, while exhaled air is vented through the exhalation path.

So the best choice reflects a small, steady positive pressure above ambient—not zero, not a negligible 0.01 psi, and not as high as 0.5 psi—because this level balances protection with comfortable breathing.