2 Complete Replacement Filter Sets of 3 (6 Filters Total). Compatible with ALL Blueair 500 & 600 Classic Series Air Purifiers including: 501, 503, 505, 510, 550E, 555EB, 601, 603, 605 and 650E. Particle filters designed for maximum removal of airborne particles such as dust, pollen, pet dander, mold spores, and fabric fibers, resulting in cleaner air. Recommended changing filters out every 6-12 months, depending on air quality and use.