Carbon Layer replacement. The activated carbon layer removes certain gases and odors from the air. This layer is what needs to be replaced on the HEPA drum filter. For best results, replace carbon layer filter every 3 months. Change indicator light is displayed on purifier when filter needs to be replaced. Compatible with TruSens Z2000 Air Purifier. Not washable. Life expectancy of 3 - 4 months. Package of 3.