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. This layer is what needs to be replaced on the particulate air 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-2000 and Z-2500 Medium Air Purifier.