Abstract: A diffuser of the type used in supplying filtered air to a clean work area includes a filter housing within which is supported a filter panel. Gaskets are carried on the upper and lower peripheral surfaces of the panel and an adjustable, resilient clamp operably engages the filter and biases the filter and the upper gasket to form a knife edge seal with a portion of the filter housing. A protective grill assembly is sealingly latched to the housing and also forms a knife seal with the lower gasket on the filter panel. Further, a curtain support member is disclosed which is carried by the grill assembly and seals an isolation curtain airtight to the ceiling.