For use with the TruSens Z-3000 Air Purifier.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.The carbon prefilter not only captures odours and gases, but it also offers the first level of defence against dust and pet hairThis layer is what needs to be replaced on the HEPA drum filterFor best results, replace the carbon layer filter every 3 monthsChange indicator light is displayed on purifier when filter needs to be replaced