Tier1 Replacement for 21x21x1 Merv 13 Ultimate Air Filter/Furnace Filter 6 Pack More efficient than our MERV 11 filter, and captures even the tiniest particles (less than 1 micron 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 Ultimate Allergen filter Rated UL Class 2 by Underwriters Laboratories