Filtrete 1900 Ultimate Allergen Reduction Filter, 14x20x1 (4-Pack) Nominal Size 14x20x1 Actual Size 13.7x19.7x1 93% effective at capturing large airborne allergens, such as pollen, mold spores and dust mite debris. 1900 Microparticle Performance Rating Captures microscopic particles like smoke, smog, dander, bacteria, and particles that carry viruses and odors Lasts up to 3 months.