CADR Rate, Is It Really A Good Means Of Comparison?

Almost every website out there that has an air purifier on it also has a brief mention of the Clean Air Delivery Rate (CADR). They like to say that the "CADR ratings are impartial", and that this is the best way to compare different air purifiers. But is it really?

The CADR is a measure of the airflow times the filtration efficiency. Because there are two variables that go into the calculation of the CADR, an increase in one can offset a decrease in the other and still result in an improved CADR rate.

For example, a unit that has a flow rate of 200cfm and a 50% efficient filter has a CADR rate of 100. A unit that has a flow rate of 100cfm and a filter efficiency of 100% will also have a CADR rate of 100. While both machines have the same CADR rate, the second one is the better air cleaner."

The CADR is then misleading, and does not appear to be a good means of comparing dissimilar air purifiers. The Association of Home Appliance Manufacturers (AHAM), which provides CADR certification, does not explain why they believe the CADR is a good measure of comparison other than stating that it is.

The CADR is biased towards air purifiers that can move a lot of semi-clean air, rather than air purifiers that can clean the air very well, but move air at a slower rate.

This bias is a result of the test procedures used to calculate the CADR. A sample air purifier is placed in an enclosed room (1008 cubic ft) that is then filled with contaminants up to a specific level. At this point, the air purifier is turned on. The air is sampled every 2 minutes for a period of only 20 minutes. After 20 minutes of filtration, the final air quality is tested and the CADR is calculated.

There is a major problem with this test: the extremely short time period over which it is conducted. In order to score well on this test, many manufactures have designed their products to circulate a lot of air very quickly and only filter out the larger contaminants (cfm=cubic feet per minute). After 20 minutes, the high cfm/low filtration air purifiers have removed more of the particles from the room than the low cfm/high filtration air purifiers. Unfortunately, after 4 hours of operation, the high cfm/low filtration air purifier will still be blowing around the same small particles that escaped its filter earlier. However, the low cfm/high filtration that did not score as well on CADR will have cleaned the room's air to a much higher degree.

Another problem with the CADR is that it does not indicate a products ability to filter out bad odors, bacteria, or volatile organic compounds. As defined on the CADR website: "CADR is a measure of the appliance's ability to reduce smoke, dust, and pollen particles".

While pollen can irritate someone's allergies, and dust and smoke are not pleasant to breathe, most of the pollutants that you really should not be breathing are ignored when calculating the CADR. In conclusion, the CADR is a good measure of how quickly the air purifier makes the air look clean, and not how clean the air really is.