The 501 effectively purifies air in rooms as large as 500 square feet(that's a 22 x 25 sq ft room!) It uses patented two-stage HEPASilent technology to capture 99.97% of the tiniest .1 micron particles, including allergy-provoking pollen grains, dust mites, mold spores and pet dander particles. This same filter technology earned the larger Blueair 501 Best of Class ranking in independent testing of Clean Air Delivery Rates. The 301 provides comparable performance for smaller rooms. Value-added features include quiet operation, low energy use, steel construction and easy maintenance: simply open the front and slide in replacement every 6 months.