Compatible Particle Replacement Filter for BLUEAIR Classic 500/600 (3packs)Replacement filter for Blueair 501, 503, 505, 510, 550E, 555EB, 601, 603, and 650E Highly efficient HEPA filter easily essential for households with Children, Pets, Captures 99% of harmful particles such as dust, pollen, and smoke Recommended to change filter once every 6 months, or dependent on usage Energy saving Blueair air filters save energy due to a low density design and requiring less pressure to push air through the filters, the filters are