Efficiency vs. Effectiveness - They are Not the Same
Over the past 3+ years there has been a sharp rise in interest related to indoor air cleaners. I have consistently seen confusion over the terms efficiency and effectiveness. This thread is about the difference. 1/
Richard Corsi, PhD, PE (Texas) on Twitter
A helpful & clear explanation! Some excerpts:
As an example, consider an air cleaner that removes close to 100% of particles that enters it, but that only has 1 cubic feet per minute (cfm) of air passing through it. 4/
In this example, the air cleaner has a Clean Air Delivery Rate (CADR) of only 1 cfm. In a typical K-12 classroom (say, 700 square feet with 9 ft ceiling height) this would yield an equivalent of only about 0.01 additional air changes per hour. 6/
So, 100% “efficient” may do very little to clean the air if the air flow rate through the device is small. This was the case for many quiet and non-filter-based air cleaners that were marketed in the 1990s and 2000s. 7/
On the other hand, it is possible to have a particle removal efficiency of, say, only 50%, e.g., only half of the particles (or particle mass) that enter the device are actually captured by the device, but to have that air cleaner be quite effective. What? 9/
Imagine a device that is only 50% effective at removing aerosol particles from indoor air (fraction removal efficiency = 0.5). Now, if the air flow rate though that device is 800 cfm it will have a CADR = 400 cfm (800 x 0.5). 10/
This is greater than most off-the-shelf HEPA air cleaners on the market and far greater than many other air cleaners. In this case, for the classroom described above the air cleaner would yield an equivalent of about 3.8 additional air changes per hour. 11/
This is why the Clean Air Delivery Rate (CADR) is a much better metric than filter removal efficiency when comparing air cleaners. 12/
