Return grill media air filter, 20 X 20 inch, provides high efficiency, long life alternative to 1-inch filter. Patented design, unique to industry, captures significant amount of airborne particles in air that circulates through filter. Mounts in most return filter grills. Pleated for greater media capacity. Low pressure drop provides comfortable air flow. Maintains equipment efficiency longer than standard filters. Minimum Efficiency Reporting Value (MERV)-10. Efficiency ranges are defined for small particles, E1 = 0.3 to 1.0 microns; medium particles, E2 = 1.0 to 3.0 microns; and large particles, over 3.0 microns. Fractional efficiency: E1=31%, E2=61%, E3=86%.