In an ideal world, all the dirty air in a room would pass through the air purifier just once, giving a 100% clean room when the room’s volume of air has passed through the filter. However, some air will often ‘track back’ and pass through the air purifier again. This leads to an inefficiency since clean air is being re-cleaned.
Air purifiers may also have leakage around the HEPA filter, as well as through the filter (if the HEPA filter is not 99.99%). These factors are reflected in the fact the CADR is lower than airflow.
Learn more: The difference between CADR and airflow »