Artograph 1520 Filter Pak (4 ea. mid-filter & poly-carb pads) - 4 each mid-filter & poly-carb filter pads. The mid-filter is a non-woven air-laid polyester substrate with a green PVC binder on the exit side. Poly-carb filter is a carbon-impregnated polyester offering high odor and further particle-trapping efficiency. Note: Both filters should be replaced at the same time, approximately every three months with regular use or when replacing the Pre-Filter Roll.