 RABS doors are only opened at the start of an operation to perform equipment setup, and must be locked thereafter until the conclusion of operations.In contrast, other RABS designs allow for rare door openings in specified circumstances.Because this "open RABS" allows for a door to be opened to the surrounding cleanroom (albeit into a fully HEPA-filtered perimeter around the RABS structure) during aseptic operations, the design allows for higher contamination hazard than a RABS that is kept closed.