Leitz Trusens Replacement Carbon Layer for TruSens Z-3000/Z-3500 Air Purifier, 3 Pack. Filter replacement is essential for your air purifier to perform at its best. The activated carbon layer removes certain gases and odours from the air. This layer is what needs to be replaced on the HEPA drum filter. For best results, replace carbon filter layer every 3 months. Change indicator light is displayed on purifier when filter needs to be replaced. Compatible with Leitz TruSens Z-3000/Z-3500 Air Purifier.