3m Filtrete™ Ultra Allergen Reduction Furnace Filter Filtrete™ Ultra Allergen Reduction Furnace Filter Ultra filter outperforms all other 1" filters. 90% effective at capturing large allergens like mold and pet dander, as well as microscopic allergens like pollen, dust and smoke. Also effective against bacteria and particles that can carry viruses. Delivers three months of filtering performance. Six filters per carton. 20 x 25