Silver nanoparticles in the Eco Silver pre-filter prevent the growth of microorganisms on the filter, including bacteria and viruses. Coconut carbon in the pre-filter neutralizes odors, smoke, pollutants and allergens. True HEPA filtration removes up to 99. 9% of airborne particles as small as 0. 3 microns in size including smoke, dust, pollen, pet hair and dander, mold spores and dust mites. Recommended to replace HEPA filter every 6 months and pre-filter every 3 months for maximum efficiency, or more often depending on air quality & environmental factors. Fits Hunter air purifier models HP400, HP400BLS, HP400BLB, HP400WHS, HP400WHGR. Package contains one (1) replacement HEPA filter and two (2) pre-filters.