Plastic Filter Units A 4mm filter gives 60% retention of dust particles (replacement filters 80 x 80mm 494-7975 & 120 x 120mm 494-7981 The finger guard is attached to the fan with screws (not supplied). The mesh screen and filter are then retained by the outer cover which clips over the finger guard Note Filter units ( 696-3534 , 696-3531 , 221-336 & 221-342 ) include filter pads. Filter pad replacements ( 494-7975 and 494-7981 ) are also available separately for filter units 221-336 & 221-342 .