Polluted air first passes through Alen's replaceable Pre-Filter to capture large particles, extending the life of the filter. In the second layer, air moves through a unique, electrostatically charged True HEPA material that attracts and captures dust, pollen, pet dander, mold spores and other airborne pollutants. We then add an antimicrobial Silver compound to the HEPA type material, protecting the filter from harmful bacteria* and microbe growth. In the final layer, clean air passes through a layer of Alen's Activated Carbon, reducing light odors and chemicals commonly found in the air from cleaning products, smoke, cooking, pets and various other household materials. These factors all combine to make the 45i Fresh filter the best choice for a room with a combination of multiple air quality issues.