Tier1 Replacement for 21-1/2x23-3/8x1 Merv 11 Pleated Premium Air Filter/Furnace Filter 6 Pack More efficient than our MERV 8 filter, and captures even smaller particles (1-10 microns in size) Doesn't promote the growth of bacteria, mold, mildew, or fungi so they're a great choice for families with small children Similar to the 3M Filtrete Ultra Allergen filter Rated UL Class 2 by Underwriters Laboratories