This activated carbon pellet filter is specifically designed to remove pollutants from the air that can trigger allergies or cause flu-like symptoms. This carbon layer fits with Allergy & Flu Filter drum for all TruSens Medium Air Purifiers. This filter with activated carbon pellets offers a level of defense against unwanted air pollutants that can invade your home This layer is what needs to be replaced on the Allergy & Flu HEPA drum filter Filter replacement is essential for your air purifier to perform at its best. For best results, replace carbon filter every 3 - 4 months. Change indicator light is displayed on purifier when filter needs to be replaced Fits with the Allergy & Flu Filter drum for all TruSens Medium Air Purifiers. Pack includes 1 filter.