Getting the Recommended ACH With More Affordable Fan-and-Filter Models (COVID-19 Series, Part 7 of 7)
Table of Contents
Enhancing Indoor Air Quality to a Meaningful Level for COVID-19 Prevention Can Be Done Practically with Fan-and-Filter Models
In this final installment of our series, we will introduce an accessible and cost-effective solution to the COVID-19 problem: MERV 13 fan-and-filter air cleaners. As we have explored in our previous post, achieving the optimal 6-12 air changes per hour (ACH) is crucial for reducing the airborne transmission of COVID-19. ACH represents the number of times the air in a space is refreshed in an hour.
While traditional HEPA purifiers are effective, they can be costly and impractical for widespread use in homes, classrooms, and offices. However, MERV 13 fan-and-filter devices can help achieve the necessary 6-12 ACH to ensure cleaner and safer indoor air, and can do so more efficiently than HEPA purifiers. Join us as we introduce the science behind these models, their effectiveness, and how you can create and implement them in your spaces to protect against airborne viruses.
Fan-and-Filter Models Are More Efficient Than HEPA Purifiers
In the past, Do-It-Yourself (DIY) air filtering “boxes” have been used to achieve high air changes per hour (ACH) to clear out contaminants such as wildfire smoke and sawdust in workshops. They are made with a combination of MERV 13 filters (filters recommended for removal of very small particles including smoke, viruses, bacteria, allergens, molds, funguses, and other contaminants) and different types of fans, and are easy to assemble. These air purifiers have been shown to be effective against COVID-19, and their use has sky-rocketed, from the popular DIY Corsi-Rosenthal box (also referred to as “CR box”, image 1) [1] to newer renditions using quieter PC fans in arrangements with different sizes and shapes [2].
Although MERV 13 filters are rated to remove 85-90% of the types of particles the SARS-CoV-2 virus travels on, and HEPA purifiers filter 99.7% of particles, fan-and-filter air cleaners made with MERV 13 filters can clean a room more efficiently than most commercial HEPA purifiers due to their ability to push more air through them at higher rates.
Katherine Ratliff, lead Environmental Protection Agency (EPA) scientist, conducted research on CR box efficiency, and stated:
“These CR boxes really work. The Corsi-Rosenthal box works against infectious aerosols in the air. The results are really powerful. Three different sets of biochamber testing data show that these air filters reduce the amount of infectious virus in the air and capture both smaller and larger sized particles. CR boxes are more effective at reducing concentrations of infectious aerosols in indoor air than some of the more expensive technologies that we tested.” [3]
How Do Fan-and-Filter Models Work?
Imagine your house is on fire and your only two options are using an expensive golden spoon or an ugly hose to put out the fire. Which one do you choose? You choose the one that carries or delivers the most water. In the same way, fan-and-filter models can achieve a higher clean air delivery rate (CADR) than HEPA purifiers, and deliver more clean air faster. CADR is a great metric to compare air cleaners because it is the volume of clean air that a purifying unit delivers, typically measured in cubic feet per minute (cfm), or sometimes in cubic meters per hour (cmh) which can be converted to cfm. Studies show that these DIY purifying boxes are more cost-effective than traditional HEPA purifiers as well, giving more CADR per dollar. CADR is listed by the manufacturer for most HEPA purifiers, with smoke CADR being the measurement needed for the purposes of infection control for diseases spread through the air like COVID-19.
For example, a 2022 study that compared three HEPA purifiers with fan-and-filter air cleaners [4] found that DIY air purifiers cost 5 to 10 times less and perform better than the best-in-class HEPA purifiers in the market (see image 2).
Image 2: Estimated CADR for Different Types/Brands HEPA Purifiers and Fan-and-Filter Boxes and Cost per Unit
Fan-and-filter boxes have proven to give high CADRs (often between 200-700 cfm depending on the types and number of filters used in the design), and they can be made at just a fraction of the cost. They use MERV 13 or higher air filters combined with a fan, such as a box fan or computer fans, arranged into different dimensions of box-shapes that are able to pass large amounts of air through them quickly, filtering with each cycle, and resulting in efficient removal of airborne contaminants like wildfire smoke, allergens, VOCs, and viruses that spread through the air like COVID-19. There is an instruction page on how to make a CR box below in image 3.
How Many Air Cleaners Does Your Space Need?
HEPA purifier manufacturers often list a square footage size recommended for their units. However, this number does not reflect the amount needed for that size space for the purposes of infectious aerosol reduction. In order to determine the purifier needs for your space, we look to the Clean Air Delivery Rate, or CADR, which is a measure of how much air the purifier can clean in a fixed amount of time and space.
Before picking out purifiers, it is important to know the total CADR needed for your particular space in order to achieve your targeted air changes per hour (ACH). You can use a tool, such as this tool to calculate the total combined CADR needed [5] to input your particular space’s size in order to reach a target ACH (such as Air Support Project’s recommended 6-12 ACH). To use it, you will need to know:
- the length, width, and height of the space (in feet), and
- the number of total air changes per hour you would like (put 0 for the current ACH and then choose a number between 6 and 12 for your target ACH)
Inputting the information into the tool will give you the total combined CADR as cfm needed for your space.
Next, you will need to add up the individual CADR for the air cleaners you are using or would like to use.
- For HEPA purifiers, it is listed by manufacturer as smoke CADR, typically already in cfm. But if it is listed as cmh (cubic meters per hour, more common outside the US) you can convert it to cfm using this cmh to cfm converter. Other tips when using a commercial purifier:
- CADR is typically measured when the purifier is run on maximum fan speed, and can be significantly lower when the fan is run at lower speeds. If listed, use the CADR for the speed you will run it at (you can use the decibels– or noise level– to help you determine the level you think you will use, with 45 dB or less considered low, 46-55 dB as medium, and higher than 56 dB as high).
- Avoid HEPA purifiers that use ionizers which, according to the EPA and ASHRAE, produce ozone pollution and can be bad to breathe in; or if there is an option, turn the ionizer off.
- If listed as HEPA or true HEPA, the filter has to pass certain standards. However, if it is listed as HEPA-type or anything else, then you will have to check how the size and percent of particles filtered was verified and determine the quality on your own.
2. Fan-and-filter models often have CADR listed on their instructions, or, if sold as a kit, on the kit website. A typical CR box made with four 20”x20” MERV 13 filters and a 20” Lasko box fan (assembly instructions are available below in image 3) will give an estimated smoke/ infectious aerosol CADR of 234 cfm per unit according to an EPA study [6].
After adding all air cleaning units’ individual CADRs together, the sum should reach or surpass the total combined CADR needed for your space (from the tool above).
Remember, devices with higher CADR will reach higher ACHs with fewer total units needed. When using more than one unit in a room, keep them on opposite ends of the room, with a few feet on each side before any walls or desks, so as not to obstruct air flow. This helps get better air mixing and thus more efficient filtering of the space.
We recommend using fan-and-filter boxes made with MERV 13 or higher air filters to achieve the targeted ACH, since you will be able to reach the CADR needed at a more practical cost than HEPA purifiers. It can be expensive to reach the ACH needed using HEPA purifiers alone.
The Need for a Commercial Option
You may wonder, if these fan-and-filter air cleaners are so great, why have they not become ubiquitous in classrooms, offices, and homes around the world? Although DIY fan-and-filter boxes are recommended for use by CDC and NIOSH for reducing COVID-19 [7], some agencies, like the EPA [8], suggest only using them temporarily and finding more permanent solutions in commercial options for long-term risk-reduction. They cite that commercial options offer better consistency.
Customers also often vie for commercial options for air cleaners, citing that DIY Fan-and-Filter Boxes require time and ability to put together, which can be intimidating for people like teachers in schools.
Others are looking for a more aesthetically pleasing option, and/or quieter units than the DIY models.
In going with our earlier metaphor of your house being on fire, and the options being a golden spoon or an ugly hose, many people are still choosing the more aesthetically pleasing golden spoons, or in this case, HEPA purifiers– and often with not enough CADR for the space to reach the air changes per hour needed to get infection-reducing results. We need a better option than the golden spoon or the ugly hose. We need an innovative commercialized option that uses the best of fan-and-filter technologies while also responding to the needs in cost, sound, and aesthetics. Which brings us back to our statement made at the beginning of this series: Modern day problems require modern day solutions.
Air Support Project’s Solution: Education and Technological Innovation
The need for IAQ education and innovative technologies to remedy the current COVID-19 situation are the inspiration for the work here at Air Support Project. Not only do we create education like this COVID-19 Series, but we also have created and Open Source Fan-and-Filter Box Prototype, a model that uses the technology of the DIY fan-and-filter boxes, and strives to find the perfect size, cost, sound, style, and performance to produce commercially for use in homes, classrooms, and offices around the world. We aim to bring this product to the market in the upcoming year. You can learn more about our current prototype at our Blog: Breathing Innovation, and follow our journey on Twitter, LinkedIn, and Mastodon as we progress in developing a polished finished product.
Check out image 5 below to see how MERV 13 fan-and-filter models (including our open-source proof of concept prototype) compare to HEPA purifiers. PDF with clickable links viewable upon clicking on the image or a download is available at the bottom of this post.
Air Support Project’s prototype is advancing as a low-cost, high-CADR purifier, distinguishing itself from competitors in both the fan-and-filter kits and commercial HEPA purifier sectors. Our pre-production, proof-of-concept prototype CADR has yielded promising results in initial in-house testing, achieving 600-800 cfm. We are currently enhancing the design with production-grade components and collaborating with engineers to reduce noise levels. Our goal is to bring to market a final, tested, and certified product priced between $100-$200 within the next year. Although these are ambitious goals, and are subject to change, we are aiming to provide a product that is safe, economical, functional, and refined for use in homes, classrooms, offices, and community buildings across the US, and beyond. Stay tuned for updates!
In the meantime, we are working hard to empower you with the education, tools, and know-how to stay safe today! Let’s review!
Concluding our COVID-19 Series
Throughout this series, we have highlighted the importance of achieving high indoor air quality to reduce the transmission of COVID-19. DIY fan-and-filter models offer a practical, cost-effective solution to achieve the recommended 6-12 ACH, significantly enhancing room air cleaning power compared to traditional HEPA purifiers. These MERV 13 fan-and-filter models provide a higher clean air delivery rate (CADR) at a fraction of the cost, making them accessible for widespread use in various settings.
Although DIY solutions are highly safe and effective, there is a need for more permanent and aesthetically pleasing commercial options to meet the diverse needs of users. At Air Support Project, we are committed to advancing these technologies and providing accessible solutions to improve air quality and safeguard public health.
Remember that air filtration is best when combined with other practices that reduce the density of viral aerosols. Ventilation reduces carbon dioxide (CO2) build-up, and thus reduces the amount of time viruses remain infectious in the air, while also helping to clear them out of the air. Actions like opening the windows and/or turning on mechanical ventilation can help supplement air filtration methods to achieve the 6-12 ACH recommended to mitigate COVID-19 risks.
Wearing masks made to filter out aerosols– often referred to as respirators or N95 masks– helps protect you in the space and time it takes for ventilation and filtration to clear the virus from the air. Ventilation and filtration are especially important for one-way masking situations, significantly increasing the mask’s ability to lower risks for longer amounts of time.
We learned that we can take actions today that will prevent one infection from turning into a super-spreading event. Improving indoor air quality not only reduces illnesses, but also reduces the associated absences, harmful long-term health risks, and negative economic impacts.
We also learned that modern spaces that have been built to contain air for energy efficiency purposes have the inadvertent effect of also trapping in indoor pollutants and contaminants – from COVID-19, to infiltrating wildfire smoke, to allergens like dust, pollen, mold, and animal dander. Modern trends, like spending much of our day in indoor environments, as well as gathering together in higher densities, have further increased our exposure. Society has already taken measures to clean water, monitor outdoor air quality, and regulate food safety. The time to add similar measures to clean indoor air is now! We can start today using the new information, strategies, and tools in this series to clean the air we breathe.
As we conclude our COVID-19 series, (Thinking Outside the Box: An Introduction to our 7-Part COVID-19 Series) we encourage you to stay informed, implement these practical solutions, and join us in the movement to create safer indoor environments for everyone. Remember, the power to create healthier spaces is within our reach. Stay protected and proactive as we work together towards a safer future. And remember: “Know Better. Breathe Better.”
[1] Porto, R. D., Kunz, M. N., Pistochini, T., Corsi, R. L., & Cappa, C. D. (2022). Characterizing the performance of a do-it-yourself (DIY) box fan air filter. Aerosol Science and Technology, 56(6), 564–572. https://doi.org/10.1080/02786826.2022.2054674
[2] Rosenthal, J. (2023, June 5). Comparing the Performance of Corsi-Rosenthal Boxes Made with Box Fans and PC Fans. Tex-Air Filters. https://www.texairfilters.com/comparing-the-performance-of-corsi-rosenthal-boxes-made-with-box-fans-and-pc-fans/
[3] Owens, L. (2023, November 2). EPA testing shows the power of DIY air filters to trap viruses. UConn Today. https://today.uconn.edu/2023/10/epa-testing-shows-the-power-of-d-i-y-air-filters-to-trap-viruses/#
[4] Srikrishna, D. (2022). 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, 838, 155884. https://doi.org/10.1016/j.scitotenv.2022.155884
[5] Estimate CADR of the portable air cleaner required to raise current ACH to need. (n.d.).https://www.urmc.rochester.edu/dentistry/research/air-ventilation/CADR-calculation.html
[6] EPA Office of Research and Development, & Ratliff, K. (n.d.). EPA ORD’s Corsi-Rosenthal Box bioaerosol testing results. In EPA ORD’s Corsi-Rosenthal Box [Report]. https://today.uconn.edu/wp-content/uploads/2023/10/EPA-ORD-CR-Box-Bioaerosol-Results-embargoed-10-30-23-11am-est.pdf
[7] The effectiveness of DIY air filtration Units | Blogs | CDC. (2023, August 11). https://blogs.cdc.gov/niosh-science-blog/2023/02/03/diy-filtration/
[8] Air cleaners, HVAC filters, and coronavirus (COVID-19) | US EPA. (2024, May 6). US EPA. https://www.epa.gov/coronavirus/air-cleaners-hvac-filters-and-coronavirus-covid-19
Sarah Masih, MD, MBA
Sarah Masih is the Executive Director of Air Support Project and has extensive experience in project management, fundraising, and communications. Her deep commitment to public health and her collaborative approach with community organizations and health experts make her a vital force in the fight for cleaner air and healthier communities.
Published under Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/).
SHEA O'NEIL
Volunteer
Creator of COVID-conscious advocacy works: writings, art, and social networking. BA degree in psychology, with over 11 years of continued education studies in allergy and autoimmune research, and continued research the past 3 years into airborne irritants and COVID-19. Parent, disabled rights advocate.
S. Riley
S. Riley is a mutlidisciplinary content creator and technology enthusiast with a background in crowdfunding and project management. He holds a BA in English, and is grateful to be able to utilize his unique skill set in service of Air Support Project's humanitarian mission.
Melissa Smallwood
Melissa Smallwood is Science and Technology Policy researcher with a background in neuroscience, psychology, and social science, and the author of The Future of Long COVID: A Threatcasting Approach. They are interested in how disability and healthcare justice intersect with emerging technologies, and are an advocate for COVID mitigation strategies to help end the pandemic.
Published under Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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DIY AIR CLEANER COMPONENTS
Main components of a DIY fan-and-filter air cleaner available in hardware store.
DIY AIR CLEANER INSTRUCTIONS
Detailed instructions on how to assemble a DIY fan-and-filter air cleaner.
MERV 13 VERSUS HEPA AIR CLEANERS
A chart that compares the CADR, sound, and cost for purifiers that use MERV 13 versus HEPA filters.
COVID Blog Series
Discover how to protect yourself and others from COVID-19 and other respiratory-spread pathogens. Our COVID blog series is applicable to all airborne contaminants. Know Better, Breathe Better.
what's in the air we breathe?
On average, we breathe 6 liters every minute, mainly indoors. Indoor air quality is often worse than outdoor air which underscores our need for cleaner air and safer spaces. Our air is made up of microparticles such as dust, pollen, mold, viruses, pet dander, and VOCs. These particles can enter our bloodstream through our lungs after we inhale them. The air we breathe impacts our daily and long-term well-being and health. Follow us at Air Support Project. We want you to know better so you can breathe better.