Your pre-filter is the first line of defense when it comes to your purifier’s four-stage filtration system for cleaning the air. The pre-filter is the black foam layer that covers the exterior of your filter. It’s designed to capture larger particles such as dust and cat or dog hair. Keeping your pre-filter clean will extend the life of your filter and keep your purifier working at peak efficiency. Pre-filters can be removed and cleaned by vacuuming, but some customers prefer to discard their heavily soiled pre-filter and exchange it for a fresh one. This pre-filter is intended only for the BreatheSmart Classic.