may be for example be a cloth fabric textile or other material suitable for ventilation in preferred embodiments the material is suitable for a filtration application wherein the filtration is used for removing insects or other flying and or airborne contaminants from air that is to be delivered into a building in particular a building used for food processing the mesh may be between 150 and 800 microns alternative grades of mesh or other kinds of materials are also envisaged to be useful for use as part of the present invention each filter bag 1 includes a mouth opening 9 via which air can pass into the filter bag body to then pass through the filter bag by virtue of a pressure differential between opposite sides of the barrier preferably at the mouth 9 the filter bag is constructed in a manner to allow for the mouth to be moved between a relaxed condition as shown in fig5 and a deformed condition as shown in fig1 in the deformed condition a resilient band or cuff region 10 of the filter bag will bias the mouth back to the relaxed condition of fig5 each orifice 7 and filter bag at its cuff region is dimensioned so that in the.