Replacement Filter B is a combination filter that incorporates multiple layers of filtration including: a pre-filter, charcoal filter and true HEPA filter, to help improve indoor air quality. Filter should be replaced approximately every 6-8 months depending on use. HEPA Replacement filter for GermGuardian model series: AC4825, AC4300, AC4350, AC4900 and Black+Decker model BXAP048. True HEPA captures up to 99. 97% of airborne particles including dust mites, pet dander, mold spores, plant pollens and debris. Pre-Filter: Captures initial dust and allergen particles. Charcoal Filter (aka Activated Carbon): Charcoal/Activated Carbon contributes to the control and prevention of the growth of bacteria, germs and mold. Manufacturer recommends replacing filter every 6 months