In order to determine how air filters help to remove particles in a space, it is important to understand that filter effectiveness is not the same thing as filter efficiency. As it is currently defined, filter efficiency refers to the ability of a filter to capture particles in a single pass test. Filter effectiveness refers to the ability of a filtration system to remove particles in a space. As such it requires both filter efficiency and air flow. Higher air flow can raise filter effectiveness. (As we have seen in higher removal of all particle sizes by a MERV 13 Corsi-Rosenthal Box air cleaner than a HEPA). Lower air flow can result in filter effectiveness far below tested filter efficiency values.
Many air filters are made with what is called “electret media.” This electret media gives higher efficiency on small particles and lower resistance than strictly “mechanical” media. As a result, it provides higher filter effectiveness.
The lower resistance of electret filters means that they provide higher airflow. Many of the pleated filters sold in North America today use electret media – including virtually all of the higher efficiency MERV 11 and MERV 13 pleated filters. Currently, MERV 11 and MERV 13 filters that do not use electret media cannot be used in many residential and many commercial HVAC systems because they do not allow for sufficient airflow for the system to operate properly.
One disadvantage of electret media is that it decreases in filter efficiency in use. Particles captured by the filters “mask” some of the electret effect. A test has been developed to “pre-condition” filters to determine the level of this efficiency drop. This “pre-conditioning” step is referred to as Appendix J and is a part of the ASHRAE 52.2-2017 filter test standard. However, it has always been controversial and it is not mandatory. There have been questions raised about its repeatability, cost and accuracy.
The proposed ASHRAE 241p Standard on the Control of Infectious Aerosols includes a provision that filters used should be tested with Appendix J and have a MERV A designation. This is for all filters MERV 11 and above. The provision would take effect in 2025. The intent is to ensure the filters used will remove airborne pathogens (filter effectiveness) throughout their useful life. Unfortunately, it will not have this result without sufficient airflow.
In fact, it may have exactly the opposite effect. The increased resistance of non-electret filters may decrease the air flow through the filters and may decrease the amount of cleaned air delivered to the space.
It seems counterintuitive to say that a more efficient filter could be a less effective filter. Our minds immediately go to the fact that there are particles going through the less efficient filter that would have been captured by the more efficient filter. But numerous tests are showing that the less efficient filter with the higher airflow is actually the more effective filter. In other words, particles need to make it to the filter to be removed by the filter.
Several years ago we did a test of a Corsi-Rosenthal Box made with MERV 11 filters versus a $1,000 high-end HEPA air cleaner. We found that the MERV 11 CR Box outperformed the HEPA on all particle sizes – including 0.5 um. This is remarkable in that the tested particle removal efficiency of a MERV 11 filter in the 0.3 um to 1 um range is only 20% while a HEPA is 99.97%.
This CR Box vs. HEPA air cleaner was done in December of 2020. Since then many more similar tests have been conducted. All have turned out the same. The higher airflow of the Corsi-Rosenthal box using high efficiency pleated filters (MERV 13) outperforms the HEPA air cleaner on the full range of particle sizes. It is important to note that all MERV 11 and MERV 13 filters used in these tests use electret media.
Here is another example from Eben Cross of QuantAQ using their Modulair-PM particle counter. All particles in this graph are under 1 micron.
The Canadian National Collaborating Centre for Environmental Health has authored an extensive review of the research done on Corsi-Rosenthal boxes.
Eykelbosh, A. Do-it-yourself (DIY) air cleaners: Evidence on effectiveness and considerations for safe operation. Vancouver, BC: National Collaborating Centre for Environmental Health. 2023 Jan. It is available here: Do-it-yourself (DIY) air cleaners: Evidence on effectiveness and considerations for safe operation | National Collaborating Centre for Environmental Health | NCCEH – CCSNE
All of the research has confirmed the initial tests done several years ago. The Corsi-Rosenthal Boxes, with their higher air flow, outperform HEPA air cleaners. A more efficient filter does not result in more effective air cleaning.
Here is a partial listing of the peer-reviewed articles that have been written on the effectiveness of Corsi-Rosenthal Boxes.
1. Cadnum JL, Bolomey A, Jencson AL, Wilson BM, Donskey CJ. Effectiveness of commercial portable air cleaners and a do-it-yourself minimum efficiency reporting value (MERV)-13 filter box fan air cleaner in reducing aerosolized bacteriophage MS2. Infection Control & Hospital Epidemiology. 2022:1-3. Available from: https://doi.org/10.1017/ice.2022.5.
2. Dal Porto R, Kunz MN, Pistochini T, Corsi RL, Cappa CD. Characterizing the performance of a do-it-yourself (DIY) box fan air filter. Aerosol Science & Technology. 2022;56(6):564-72. Available from: https://doi.org/10.1080/02786826.2022.2054674.
3. Holder AL, Halliday HS, Virtaranta L. Impact of do-it-yourself air cleaner design on the reduction of simulated wildfire smoke in a controlled chamber environment. Indoor Air. 2022;32(11):e13163. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1111/ina.13163.
4. May NW, Dixon C, Jaffe DA. Impact of wildfire smoke events on indoor air quality and evaluation of a low-cost filtration method. Aerosol and Air Quality Research. 2021. Available from: http://dx.doi.org/10.4209/aaqr.210046.
5. Van Valkinburgh K, Mohammadi Nafchi A, Mousavi E, Blouin V, Kaye N, Metcalf AR. Assessing
mitigation strategies to reduce potential exposures to indoor particle release events. Aerosol and Air
Quality Research. 2022;22(9):220054. Available from: http://dx.doi.org/10.4209/aaqr.220054.
6. Derk RC, Coyle JP, Lindsley WG, Blachere FM, Lemons AR, Service SK, et al. Efficacy of do-it-yourself
air filtration units in reducing exposure to simulated respiratory aerosols. SSRN. 2022 Nov. Available from:
7. Gasparrini W, Akter S, Russell B, Assanah F, Brugge D, Cole M, et al. Testing the efficacy of the ‘CorsiRosenthal’ box fan filter in an active classroom environment. ChemRxiv. 2022.
8. Srikrishna D. Can 10× cheaper, lower-efficiency particulate air filters and box fans complement
High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic? Science of The Total
Environment. 2022 Sep 10;838:155884. Available from: https://doi.org/10.1016/j.scitotenv.2022.155884
All of these studies use electret MERV 13 pleated filters. We tested Corsi-Rosenthal Boxes using MERV 13 filters. After 6 months of continuous use, we found that they still performed up to MERV 13 efficiency levels. Testing the Efficiency of MERV 13 Corsi-Rosenthal Boxes In Use – Tex-Air Filters (texairfilters.com)
Theoretically, a more efficient filter with the same air flow would be the more effective filter. But this requires a more powerful fan and creates some additional problems with noise and energy usage. Also, the reality is that the vast majority of HVAC equipment will not be upgraded to meet this need. It becomes a question of what one can do with the resources at hand. It appears from the data we have seen that means using the filters with the best combination of filter efficiency and flow would be the most effective. It also means that the Standard covering the “control of infectious aerosols” needs to be concerned with filter effectiveness – not just filter efficiency.