Most air filters are from spun fiberglass, which is the same material that makes up your attic insulation, and a cardboard frame to help it keep its shape. Other pleated air filters are made from pleated paper. These HVAC air filters are inserted into your return air duct and help prevent things like mold spores, lint, pet dander, dust, bacteria, and other contaminants from entering into your HVAC system and circulating back into your home.