3 micron.Maytag M8-1056 whole house media filterNominal size 20" x 25" x 5" (Actual size 20 1/4" x 25 3/8" x 5 1/4")Removes atmospheric and household dust, coal and insecticide dust, mites, pollen, tobacco smoke, cooking smoke, grease, mold, fungi and more down to 0..