Electrostatic filter. 20 in. x 20 in. Captures large, airborne allergens like mold spores and large dust particles. Excellent for high-velocity equipment. Lasts 3 months. 3M 600 Microparticle Performance Rating (MERV 8). No unnecessary antimicrobial chemicals added. "Filtrete" filtering material will not support the growth of bacteria, mold, mildew or fungi in normal use.