The CADR rating of an air purifier takes into account inefficiencies with the air purifier due to cleaning air that had recently already been cleaned.
Think about it this way: an air purifier designed in a loop that directs all clean air coming out of the air purifier back into the air purifier would not do a great job at cleaning a room, even though the HEPA filter is 99.99% effective.
Learn more about: The difference between CADR and airflow »
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