By Jim Rosenthal, CAFS
In the Fall of 2022, the EPA did chamber testing on DIY air cleaners to determine their effectiveness on reducing wildfire smoke. This resulted in an article in Indoor Air entitled “Impact of do-it-yourself air cleaner design on the reduction of simulated wildfire smoke in a controlled chamber environment” by Amara L. Holder, Hannah S. Halliday and Larry Virtaranta.
Here are the results of their tests:
The results are consistent with other studies and show that the DIY air cleaners using MERV 13 filters with the highest level of air filter media produce the best results. The Corsi-Rosenthal Box (CR Box) had a CADR of 400 CFM followed by the “wedge design” using two filters with a CADR of 270 CFM and the unit using a 4″ MERV 13 filter with a CADR of 250 CFM. It is interesting to note that the addition of the “shroud” to the 1-filter design increased the CADR by approximately 40%. It is also interesting that the Commercial HEPA (Turbo) had about the same CADR as the 1″ MERV13 filter on a box fan.
The researchers also studied the effects of “filter loading.” This was done by pre-loading the test filters with either ASHRAE test dust or simulated wildfire smoke. Dust loaded filters were generated by a commercial lab using the ASHRAE 52.2 dust loading procedure. Smoke loaded filters were generated in the EPA Open Burn Test Facility using the following procedure:
“The facility was filled with smoke from small batches of pine straw and three air cleaners were operated simultaneously to reduce the smoke concentrations and load the filters. The process was repeated until the air cleaners ceased to reduce smoke concentrations in the chamber.”
It is very important to understand that this was not a test of the longevity or charge durability of the MERV 13 filters. As we can see in the above chart, both the “smoke loaded” and the “dust loaded” air cleaners using these filters had a significant drop in CADR.
The filters that were “loaded” with smoke from the fire in the chamber “were dark and had a distinct smoke smell.” With this level of exposure, they needed to be replaced. This would be true for all types of filters – electrostatic or mechanical. Using filters with this level of smoke build up would result in unacceptable smoke odors in the building.
For some reason, the researchers tested the filters that had “ceased to reduce smoke concentrations in the chamber.” Not surprisingly, they found that: “The DIY air cleaner was almost completely ineffective when used with dirty filters.”
Unfortunately, some people have interpreted these “findings” to mean that DIY air cleaners have limited utility in combatting wildfire smoke. As seen from the above chart, they are very effective and provide for the biggest benefit at the lowest cost.
How long will the filters last in a wildfire smoke situation? Of course, it depends on the amount of smoke exposure. But inside a closed building, we are probably looking at months rather than days. The best way to confirm effectiveness is with a particle monitor. There are inexpensive monitors that can give an acceptable “ballpark” reading. Change the filters when needed – not before, not after.