DuPont Replacement Carbon for TruSens is specially designed carbon filter offers a level of defense against pet odors. This carbon layer is what needs to be replaced on the Pet Filter HEPA Drum. Compatible with all TruSens Air Purifiers. 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.