Once you have chosen the type of filter required in an air purifier, the second most important aspect to consider before buying an air purifier is its CADR, or simply called Clean Air Delivery Rate.
CADR is the speed at which the air comes out of the filter. The CADR is measured in cubic meters per hour. Since the filter has to suck all the air in the room and push it out it, the CADR helps determine how effective the air purifier is at cleaning the air in the room. It is advised that the purifier should completely clean the air in the room at least once every 20 minutes and ideally at least once every 12 minutes.