This activated carbon pellet filter is specifically designed to help remove pet odours from your surroundings that can aggravate allergy symptoms | This carbon layer fits with the Pet Filter HEPA drum on all TruSens Large Air Purifiers | Specially designed carbon filter offers a level of defence against pet odours | This carbon layer is what needs to be replaced on the Pet Filter HEPA Drum | Filter replacement is essential for your air purifier to perform at its best | For best results, replace carbon filter every 3 - 4 months (depending on usage) | Change indicator glows red on your air purifier when the filter needs to be replaced | Fits with the Pet Filter HEPA drum for all TruSens Large Air Purifiers | Pack includes 1 filter