Check the listing for details. 4 Pack - HEPA 500 F397 Activated Carbon Pre-Filter. Condition: New. Listed at 149.99 USD. Filter upgrade boosts HEPA 500 functionalityHEPA filtration is increasingly seen as the single most important safety device available for creating a safe environment for jobsite workers. Now, with the Activated Carbon Filter, the DefendAir can play an even more important role in keeping an enclosed environment clean and safe – both for workers and for occupants.The activated carbon filter now equips the 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.