What does the term "negative pressure" refer to in the context of SCBA?

The term "negative pressure" in the context of SCBA refers to the vacuum created inside the facepiece. When a firefighter inhales, the movement of their diaphragm creates a lower pressure in the facepiece compared to the outside atmosphere, which helps draw air through the filter or directly from the SCBA cylinder. This negative pressure is crucial for effective breathing, as it ensures that air is pulled into the facepiece rather than allowing contaminated air to enter. This mechanism allows for a proper fit and seal, which is essential for the safety of the wearer in hazardous environments. The creation of this vacuum effect helps maintain the integrity of the breathing apparatus and ensures the wearer receives clean, breathable air while in unsafe atmospheres.