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 air 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 air 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.