Honeywell True HEPA Air Purifier offers high efficiency air cleaning with a permanent True HEPA filter that removes at least 99.97% of common household particles as small as 0.3 microns such as dust pollen tobacco smoke mold spores and cat dander. In addition independent lab tests have shown that it helps reduce airborne bacteria and viruses. The carbon pre-filter helps remove common household odors. The patented 360 degree air intake and discharge maximizes efficiency. The Intelli-Check electronic filter indicator reminds you when to clean the HEPA filter & when to change the pre-filter. Most effective for medium-large size rooms up to 235 square feet.