Which is the most common type of SCBA for use in an IDLH atmosphere?

• A. Negative pressure
• B. Positive pressure
• C. Open-circuit
• D. Supplied-air

Answer: (B)

In IDLH atmospheres, the priority is to prevent contaminants from leaking into the breathing zone. A positive-pressure SCBA does this by maintaining a slight positive pressure inside the facepiece, so air flows outward if there’s a small leak. That means even with a marginal seal breach, contaminated air is kept from entering the facepiece, giving the wearer a safer margin in dangerous environments. This is why positive-pressure SCBA are the standard choice for IDLH work.

Negative-pressure designs rely on a tight, perfect seal to keep contaminants out; if the seal isn’t flawless, contaminated air can be drawn in as the wearer inhales. While open-circuit versus closed-circuit describes how the air is used and exhausted, the crucial safety factor in IDLH is the positive pressure itself. Supplied-air systems deliver air from an external source and aren’t self-contained like SCBA, so they aren’t the typical SCBA type used in IDLH.