The CADR (Clean Air Delivery Rate) Level is another industry standard used to measure the rate of delivered clean air. So how do the air purifier manufacturers measure the clean air delivery rate and arrive at the numbers? It is measured by measuring the processing air volume of the 3 most common contaminants in our atmosphere. They are smoke, dust and pollen. For each contaminant, the higher the CADR level, the more efficient it is at processing the contaminant. So you should pick an air purifier with high CADR numbers for all three components. More on Best Air Purifier.