Filter replacement is essential for your air purifier to perform at its best. The replacement carbon filter removes certain gases and odours from the air. The carbon pre-filter is the layer that needs to be replaced on the HEPA drum filter. The change indicator light is displayed on the purifier when the filter needs to be replaced. For best results replace every 3 months. The Leitz Replacement Carbon Filter is compatible with Leitz TruSens Z-3000 and Z-3500 Large Air Purifier.