Negative Pressure SCBA : An SCBA in which the pressure inside the facepiece, in relation to the pressure surrounding the outside of the facepiece, is negative during any part of the inhalation or exhalation cycle.
NFPA 1981 Standard on Open-Circuit Self-Contained Breathing Apparatus for Fire and Emergency Services, 2002 ed. https://www.nfpa.org/assets/files/AboutTheCodes/18 52/1852-F2007-ROP.pdf, (Std: Open-Circuit SCBA for Fr and Emr Srvc - Amer) {432 NFC}