Open-Circuit SCBA 1 : An SCBA in which exhalation is vented to the atmosphere and not rebreathed. There are two types of open-circuit SCBA: negative pressure or demand type and positive pressure or pressure demand type
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}