A gas filtration system including an orificed barrier having a plurality of orifices through which gas can be forced, each orifice being sealed by a filter bag via which said air can pass and be filtered, each filter bag having a mouth opening biased to an open condition to force a mouth opening seal of the filter bag against a seat of the orifice by an underlying resiliently deflectable band that is deformable inwardly to facilitate the seating with yet allow removal of the filter bag from the barrier, each filter bag defining a passage but with an enclosed end region such that at least some solid matter can be filtered from the passing air and is contained inside the bag.