Nordic Pure 24x24x4 Pleated Air Filters MERV 12 plus carbon are made of a dual material where the MERV rated material has been fused together with a carbon layer. This results in less flow restriction and 33% more surface area for cleaner smelling air. Odorous gases and vapors (VOC) are attracted to and held by the activated carbon material while maintaining excellent air flow properties. All Nordic Pure pleated air filters are manufactured using a heavy duty, moisture resistant beverage board frame. There is a metal mesh support grid bonded to the pleats for extra support. MERV 12 carbon furnace air filters are comparable to HVAC filters with the rating of MPR 1500-1900. This high-performance HVAC replacement filter will help keep your air conditioning and heating systems running efficiently, saving money on utilities and costly maintenance.The effective life of your AC air filter depends on both the inside and outside conditions in which you live. It is important to regularly change your furnace air filters especially during any home improvements for fresh, clean air in your home or office. We recommend that you change your air filter every 3-6 months to maintain maximum efficiency and economy.Important Note: Almost ALL furnace filters have a different \"actual size\" than the nominal size printed on the air filter. It's best if you measure your air filter for the EXACT size and compare it to our \"Actual Size\".