Multistage filtration requires the use of two or more stages of high efficiency filters.
Air exchange ratio defines the number of times per hour that a room's total air is drawn through the purifier. Typically a higher number of hourly exchanges reduces the dust contamination level at a faster rate, and will maintain the target dust level for a longer period. As the air filters accumulate dust, the air flowing through the air filters decreases due to the restrictions in airflow. This decrease in airflow can negatively affect the hourly air exchanges to a point where the target dust levels cannot be maintained. PAT purifiers each contain a sophisticated electronic control and monitoring system to recommend when any of the three stages of filter need replacement as well as indicating the system differential pressure.