The Dri-Eaz Activated Carbon Filter (F397) equips the DefendAir HEPA 500 to capture a wide variety of volatile organic compounds (VOCs), including soot and smoke, food odors, fumes from paint and paint thinners, sewer odors, and many other compounds. No modification of the DefendAir is required – users simply place the new carbon filter inside the cabinet in front of HEPA filter, and then re-secure the cover. Capacity of the carbon filter will vary widely depending on conditions, but it should be replaced when a decline of effectiveness is noted. Sold in packs of four individually-wrapped filters.