Safe Home Duo4 4 Inch Furnace Filter, Twin Layers for Dirt and Chemicals 20 x 25 x 4 Inch furnace filter our best air filter for HVAC systems with 4 or 5 inch air filter slots . Twin layered disposable filter media absorbs dirt and chemical fumes up to 6 times more than the Safe Home 1 Inch Duo furnace filter. Duo4 traps hazardous chemicals, odors and allergens . Note Check with Your HVAC tech before replacing any furnace filter. This is a MERV 8. Older systems may lack capacity for this now standard HVAC filter. Polyester media filter with chemical filter uses 300 grams/sq. ft. of 50/50 activated carbon and potassium permanganate based FormaldaSorb TM to trap gases, odors and chemicals including formaldehyde, benzene, toluene, ammonia and ozone. No other filters are as effective at removing dust mites, mold spores, pollen, odors, cleaning chemicals, ozone, paint fumes, and common gasses like formaldehyde which triggers asthma and allergies. Made in USA. Captures up to 95% of common household allergens. Eliminates odors from pets, paint, cooking and more. High density carbon plus advanced compound, FormaldaSorb TM , a patented potassium permanganate product, in second layer. Note This furnace filter is not a control device for radon, carbon monoxide or carbon dioxide. Limited Warranty Free of manufacturing and material defects . If product is found defective within 30 days of purchase, it will be replaced at no additional charge. Product is not packaged for sale outside of the United States