The pre filter pad works as the first barrier to stop big particles, and it helps to lengthen the lifetime of filter; Pre filter pads are recommended to be changed every 9 months. Both the pre-filter pad and the filter are washable and budget friendly. Re-install the filter before it’s completely dry. The filter can catch particles down to 0.3 microns in size, allowing cleaner air to pass through. The filters are recommended to be changed every 18 months.