How does the Fellowes Air Purifier know when filters need to be changed?

The product takes into consideration the quality of the air it cleans as well as the period of time it is being used in order to determine the amount of dirt and pollutants that have been captured by/on the filters.  Once this amount has raised a level where the efficiency of the machine is about to start decreasing, the machine tells the user it's time to replace the filters.