Thread by @CorsIAQ

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/

When thinking about air cleaners, make sure not to conflate efficiency and effectiveness.
An air cleaner that is intended to remove aerosol particles from indoor air can be close to 100% efficient at removing particles as they flow through the device. 2/

That sounds great, right? Maybe. Maybe not. In fact, a device that is 100% efficient (captures all particles that move though it) may be highly ineffective. What? How can this be? 3/

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 case, the number or mass of particles entering the device is tiny and the difference in particle concentration (in #/cm3 or micrograms per cubic meter) in indoor air would change immeasurably from before to after activating the air cleaner. 5/

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/

Many of those devices were "efficient" at removing particles but were not very effective due to low air flow rates. And some emitted ozone sufficient to drive measurable formation of ultrafine indoor secondary organic aerosols & a wide spectrum of oxygenated reactive gases. 8/

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/

The CADR is equal to the fractional particle removal for a single pass through the filter (100% removal = 1.0 on a fractional basis, 50% removal = 0.5) multiplied by the volumetric flow rate through the air cleaner (e.g., in cubic feet per minute). 13/

The most common definition for effectiveness (E) relates to the concentration of particles (or particle mass; make sure you understand which) in room air before and after using an air cleaner and is defined as:

E = 1 - Cac/Cnoac

14/

Here, E is effectiveness (a number between 0 and 1 --- the larger the better), Cac is the concentration of particles or particle mass in room air when the air cleaner is activated and steady-state conditions are achieved (the concentrations no longer changes with time). 15/

Cnoac is the concentration of particles or particle mass in room air at steady-state without an air cleaner being used. So, if Ca = 3 ug/m3 and Cnoac = 10 ug/m3, then E = 0.7 (70% removal of particles (or particle mass) from indoor air. 16/

Note that when a company that sells air cleaners states that it can be used for rooms of certain square footage, that statement is (or should) be based on an assumed ceiling height (to determine volume), air exchange rate, and prescribed value of E (effectiveness). 17/

With a little bit of mass balance magic, E can be written in terms of multiple particle removal mechanisms before and after air cleaner use, including CADR. But that’s for another tweet.

18/

Upshot: Don't be sucked into only considering efficiency of an air cleaner for removing particles. It's about the multiplicative product of single pass removal fraction & volumetric flowrate. The greater the CADR, the greater the effectiveness for a given indoor space. 19/

The #CorsiRosenthalBox is highly effective relative to many other air cleaners because it has a high CADR (single pass removal fraction x air flow rate through the device). 20/

Because of changes to Twitter I'll keep this account but will tweet less & spend more time writing about #IAQ issues & uploading info at CORSIAQ.com. Hope to see you there! Until then, stay well & keep breathing cleaner indoor air. Just do it, folks! end/