This is a tricky question. NFPA 654 (7.13.1.2) requires that protection be provided for air material separators that have an explosion hazard. Section 7.13.1.1 requires that air-material separators shall be located outdoors unless it is has explosion protection per 7.1.2.1 (venting through duct, suppression, innerting, containment, flameless venting) or if the volume is less than 8 cubic feet. Therefore it can be located indoors but still must be protected. The problem is that the volume is too small to vent or suppress. So the way I would interpret it is that it would have to be built to contain or innerted. I think that the best solution is to direct this question to NFPA 654 for a clarification.