The Blueair 500/600 Series Particle Filter uses patented HEPASilent® technology to remove 99.97% of harmful particles from the air such as pollen, dust, and pet dander from your home or office down to 0.1 microns in size. This filter is specifically designed as a replacement filter for the following Blueair air purifiers: 501, 503, 510, 550E, 555EB, 601, 603, and 650E. Its gradient structure is made up of thermally bonded fibers, free of chemicals and binders, naturally anti-bacterial, and fiber shedding resistant. The polypropylene and polyethylene fibers form a less dense filter media, requiring less air pressure, for greater airflow with a whisper silent delivery. This filter is made of 100% recyclable materials, so the only thing you'll add back into the environment is clean air. Blueair recommends changing the filter in your air purifier every six months.