Commercial grade Pre-filters capture large particulates like dust and hair and keep carbon filters working effectively. Compatible with the Fellowes AeraMax PRO AM3, AM3S, AM4 and AM4S Air Purifiers. Replace the Pre-filters when replacing the Carbon Filter, approximately every 6-12 months, depending on carbon filter size and usage conditions, in order to ensure that the Air Purifier remains effective. Pre-filters can be changed more frequently in high particulate environments. Pack of 4 Pre-filters.