A submicron particle filter unit of the Hepa type adapted to be interposed between a source of pressurized gas and a work-processing region to provide an environment therefor free of particles and other contaminants. The membrane of the unit is fabricated from a continuous, elongated, porous metal sheet accordion-folded to define a uniform series of pleats forming a media pack having an entry and an exit face, the pack being clamped with gaskets between the side plates of a frame or welded or otherwise joined thereto. Pressurized gas from the source applied to the entry face of the pack is filtered therethrough to yield at its exit face a laminar flow of gas which creates a clean environment for the work-producing region.