Abstract: High efficiency air filters, including cores of pleated media sealed within a metal frame, are supported by parallel tracks suspended from overhead structure of an enclosed room. The filters are arranged both side-by-side and end-to-end in closely adjacent relation to present an essentially continuous, downwardly facing surface forming all or part of the overhead boundary of a space wherein downward, laminar air flow is provided. The filter units are sealed to one another, or to the adjacent walls, continuously around the periphery at the lower ends by caulking material which is injected from below into the spaces between the units to provide a totally sealed system, whereby the only path for air to enter the room is from above through the high efficiency filter media.