When the particle size and filter efficiency of any two air cleaners are the same (as they often are), the Clean Air Delivery Rate (CADR) is key to choosing the more effective system. CADR test results are expressed in cubic feet per minute (CFM), with a number rating for three "yardstick" pollutants: tobacco smoke, dust and pollen. The higher the CADR test numbers, the better the overall ability of the unit to clean your indoor air. CADR results reflect:
- the size of the particle removed
- what percentages of particles are removed
- the volume of air actually moving through the system
Air volume is often described as air exchange (the number of times the total volume of air in the room is processed by the unit within a given period of time). Some manufacturers substitute air exchange rates for CADR results, but they are not equivalent. CADR numbers give a much more precise report of an air purifier's performance.
CADR works the same way, rating not just how much air is cleaned nor just what percentages of particles are removed, but the overall performance of the filtration system when both factors are examined.
Let's say that both Air Purifier 1 and Air Purifier 2 have a filter efficiency of 99.97% for particles that are 0.1 micron in size. The difference between the two units is the amount of air filtered through the unit (airflow air exchange rate). Air Purifier 1 provides an air exchange rate of 3 per hour, which means that the entire volume of air in the space is filtered through the unit three times per hour. Air Purifier 2 provides an air exchange rate of 5 per hour, filtering the air in the space five times per hour. Air Purifier 2 is better than Air Purifier 1; it has the same filter efficiency but filters the air in the room more frequently. Like CADR, this is based on both filter efficiency and the amount of air filtered.
An air purifier can have lower filter efficiency and higher air exchange rate than another air purifier, yet still deliver a better Clean Air Delivery Rate. That is why using CADR to compare air purifiers is the best way to find the best air purifier for you.