TRUTH: CADR is primarily focused on delivering clean filtered air. In a scientific study, CADR = Airflow rate of the device (Qac) x the filtration efficiency (ε).  

(R. J. Shaughnessy and R. G. Sextro: What Is an Effective Portable Air Cleaning Device? A Review. Journal of Occupational and Environmental Hygiene .169-181, 2006, April).

The EPA defines CADR as “a measure of a portable air cleaner’s delivery of contaminant-free air, expressed in cubic feet per minute."  "For example," the EPA report continues, "an air cleaner that has a CADR of 250 for dust particles can reduce dust particle levels to the same concentration as would be achieved by adding 250 cfm of clean air." 

 It shows a clear picture that CADR is not a measure of particle removal but of lowering particle concentration by adding filtered air, that is, a dilution. 

If the ultimate goal were simply to lower particle CONCENTRATION, there are two ways to achieve it: 

  • the addition of clean filtered air or  
  • the removal of particles.

Think of the air as lemonade that can be made less sweet by either adding more water or adding less sugar. Adding air is tantamount to adding more water to the lemonade. There is no less sugar, but the dilution generates a similar "reading" (taste) on your tongue. 

When running the CADR tests, the fixed positioning of the particle sensor and the dilutive effect of recirculating clean air into the test chamber with a fan producing 300~400cfm throughout all tests all weaken the utility of CADR as a measure of effectiveness. Instead, they contribute to the misconception that particles are removed in equal proportion to the CADR levels. 

Last but not least, the CADR tests are done at the highest fan speed setting in a lab. But 99 percent of the time you’ll run your machine on low-to-medium settings due to its annoying noise. Be noted that running on low or medium settings will supply much less clean air than the published CADR. 

CADR is largely a measure of the clean filtered air, but the measurable impact of introducing this air is very short-term if removal is not accomplished as well.