4 Inch HVAC Furnace Air Pollution Filter for 4 or 5 Inch Return Air Filter Slots Duo 4 by Safe Home Filters, disposable furnace filter, is 16 x 25 x 4 inches. Air filter fits tightly in furnace return air filter slot. SafeHome Duo 4 MERV 8 polyester plus VOC filter has twin filters. Traps hazardous chemicals, odors and allergens allergy dust filter and a potassium permanganate and activated carbon filter for odors and volatile organic compounds such as fumes from formaldehyde. Note All furnace filters with external frames are made slightly smaller than the noted dimensions in order to fit into the furnace filter slots. This is an industry standard. Note Check with your HVAC technician to verify your system can use a MERV 8 filter before purchasing this or any other MERV 8 filter. Install this MERV 8 with Air Flow First Going through the White Media. Made in USA. CarbonWeb carbon-filled filter media in the second layer has low air resistance and a high capacity. Its gas phase, 60% activity granular activated carbon has early five million square feet of surface area per pound Self sealing edges for maximum effectiveness at capturing allergens, odors, and chemicals from the air in furnace filter slots, temporary needs, or custom sizes in air return grills. Duo4 replaces any 4 thick home air filter in any location and can last up to 9 months. Note This furnace filter is not a control device for radon, carbon monoxide or carbon dioxide. Safe Home Duo4 Filter 16 x 25 x 4 inch filter captures 95% of common household allergens. Eliminates odors from pets, paint, cooking, and more. Uses high density carbon and advanced compounds. FormaldaSorb TM , a patented potassium permanganate product plus the activated carbon in the second layer of the filter trap gasses that can cause health issues.Twin filter pad MERV 8 polyester white ASHRAE rated allergen filter plus an industrial chemical and odor filter, i.e. the activated carbon filter pad. Made in USA. Limited Warrant