It is important to only use genuine Blueair replacement filters to maintain optimal performance and warranty. Filters that are not Blueair genuine will void your units warranty. GENUINE Blueair Replacement Filter is compatible with Blueair Classic 501, Blueair Classic 503, Blueair Classic 510, Blueair Classic 550E, Blueair Classic 555EB, Blueair Classic 601, Blueair Classic 603, Blueair Classic 650E, Blueair Classic 505, Blueair Classic 605 PARTICLE and ACTIVATED CARBON combine to create the Blueair SmokeStop filter that, with the Classic air purifier, reduces pollen, dust, pet dander, mold spores, bacteria, odor, VOCs, Tobacco smoke and other gaseous pollutants. RECOMMENDED to change filter once every 6 months, or dependent on usage. This filter is eco-friendly and can be recycled after it's changed. The 500/600 Series air purifiers use 3 separate filters, this replacement pack contains a set of 3 filters. 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 also 100% recyclable, so the only thing you?re adding back into the environment is clean air.