Nordic Pure 20x25x1 MERV 12 Pleated AC Furnace Air Filters: Actual Size: 19-1/2-Inch by 24-1/2-Inch by 3/4-Inch. Changing your air filter is the introductory step in insuring your HVAC scheme runs at it is optimal level this season. Generally, you will have to alter your 4-Inch air filters each 3 to 6 months. Particles in indoor air are measured in microns, which are 1/1000 of a millimeter. A humane hair is when it comes to 100 microns wide. A MERV 12 Nordic Pure air filter captures airborne molecules as little as 0.3 microns! Nordic Pure air filters do not kill germs, however, the filter media does not permit the growth of bacteria or mold. As a result, it is not necessary to add any potentially irritating chemicals to the air filter to kill germs. Nordic Pure air filters are made out of an electro statically-charged synthetic material that draws airborne particulates to it like a magnet. Electrostatic filters support trap airborne corpuscles to protect your family from allergy causing pollen, fine dust, smoke particles, airborne plant spores, bacteria, and some viruses.