A number of companies, including Filtrete, Honeywell, TrueBlue and Rheem, manufacturer furnace air filters. These filters comes in different sizes, so check that a new filter is compatible with the furnace before making a purchase.
Furnace filters are designed to protect the furnace by preventing dirt, pollen and other larger particulates from entering the equipment. As a side effect, air filters may also help keep the air cleaner inside the home. Most homeowners have seen the simple paper-and-fiber filter secured with a cardboard border in their home furnaces. Manufacturers also produce thicker filters designed to help remove even smaller particles from the air as it enters the home. Some filters may be as thick as 5 inches, but most are about 1 inch thick.
Many companies produce name brand and generic air filters. The best quality filters are manufactured by Filtrete, as stated by Consumer Reports. Filtrete is a 3M brand that produces a wide range of filters, from inexpensive 1 inch filters to larger filters for allergy sufferers. Another popular brand is Honeywell, which produces filters for furnaces and home air purifiers.
These products also come in rectangular or square shapes to fit in specific home furnace models. Always look up the furnace model number and check to see which filters are compatible with the furnace.