Micro Allergen Defense Air Filters 18 Inch by 20 Inch by 1 Inch help reduce allergens in your air by turning your central heating and cooling system into a whole house air cleaner. Developed to pull in and trap unwanted particles such as pollen and pet dander from the air, these filters help capture allergens as well as large particles like lint and household dust. Designed with exclusive Filtrete (tm) Brand 3-in-1 technology from 3M to pull in and trap unwanted air particles while letting cleaner air flow through. Attracts and captures microscopic particles like smoke and smog and large allergens like pollen from the air passing through the filter. Outperforms fiberglass, washable and non-electrostatic 3 month pleated 1 inch filters. For maximum performance, change your filter at least every 90 days.