The Corsi–Rosenthal Box is a design for a do-it-yourself air purifier that can be built comparatively inexpensively. It consists of four[1] or five[2][3] HVAC particulate air filters that form a cube and a box fan to draw air through the filters. The seams of the cube are sealed with duct tape. A 2022 study found the clean air delivery rate on the five-filter design was between 600–850 cubic feet (17–24 m3) per minute (depending on fan speed), costing roughly a tenth of commercial air filters. Engineers Richard Corsi and Jim Rosenthal created the design during the COVID-19 pandemic, with the goal of reducing the risk of infection by reducing the levels of airborne viral particles in indoor settings.

An example of a homemade unit

Background and history


COVID-19 is primarily transmitted through the air[4][5][6] and superspreading events are generally associated with indoor gatherings where the virus is allowed to accumulate in the air.[7][8] In response, and on recommendations of infectious disease researchers,[9][10] engineers began to consider how improved ventilation may reduce risk of infection by reducing the amount of virus in indoor air.[11]

Richard Corsi in 2022

Air purification units (room or multi-room) with HEPA filtration range widely in price from under $50 to considerably more than US$500.[12][13][14][11][15] In August 2020, Richard Corsi, an environmental engineer and the incoming Dean of Engineering at the University of California, Davis,[16] spoke with Wired reporter Adam Rogers about an idea he had for combining multiple store-bought filters with a box fan to improve the efficiency of home-made air filter designs.[11] Rogers contacted Jim Rosenthal, the CEO of filter manufacturer Tex-Air Filters, who had collaborated with Corsi at the University of Texas and in the Texas chapter of the Asthma and Allergy Foundation of America, to run some tests on a single air filter attached to a box fan.[2] Inspired by Corsi's idea to use multiple filters, Rosenthal later came up with a five-filter design. Rosenthal named it after Corsi,[2] although after a New York Times article mentioned the boxes by that name,[15] Corsi tweeted that Rosenthal really deserved the credit,[17] and that he preferred the name Corsi–Rosenthal Box.[18][19]


Instructions for making the box using materials available at North American hardware stores

The Corsi–Rosenthal Box design consists of four[1] or five HVAC filters of effectiveness MERV13 or higher, which form the side walls of a box.[2][3] Four-filter design variants use 20-inch wide filters, secured with duct tape and a piece of cardboard forming the bottom of the box.[20][1] In Rosenthal's design from 2020, two 16×20×2-inch filters and three 20×20×2-inch filters form five sides of the cube.[2] For both, a 20-inch (500 mm) box fan makes the last side of the cube and is duct taped to the filters, sealing the system so that air is drawn through the filters and out of the box.[19][14]

Rosenthal later improved the design by adding a shroud on the fan: this cardboard cutout covers the corners of the box fan to improve the system’s efficiency by reducing backflow.[20][21]

The units can be assembled in around 15 minutes, last for months, and cost between US$50 and $150 in materials.[19][14][11]



Airborne virus particulates range in size from 1 to 50 microns (μm). Rosenthal used his HVAC company's testing equipment to run an informal test of the design, in which he found that around 60% of 1 μm particles were removed by the system, and almost 90% of 10 μm particles were removed.[11] The clean air delivery rate (CADR) of a four-filter, US$75 design were estimated at between 165–239 cubic feet (4.7–6.8 m3) per minute (depending on fan speed) in an August 2021 case study by UC Davis researchers.[20] In April 2022, a team based at UC Davis published a study of a Corsi–Rosenthal Box that used five two-inch MERV-13 filters. They found that this design's "effective clean air delivery rate [CADR] increase[d] with fan speed, from about 600 to 850 ft3 min−1 (1019 to 1444 m3 h−1)".[3] Based on the cost of their design, this output amounted to $0.08 per CADR, or roughly ten times cheaper than commercial air purifiers, with quieter operation.[3]

A study of a home-built air purifier to remove wildfire smoke, using a box fan and filter mounted in a window, showed that particulate matter between 1 and 10 μm in size was reduced by about 75%. Wired wrote that this study suggests such filters may effectively filter similarly sized virus particles.[11][22]

Researchers have expanded studies of these citizen science filtration units to evaluate their efficacy for reducing the levels of airborne volatile chemicals (SVOCs); Dodson and colleagues showed the units reduced PFAS and phthalates.[23]



A 2021 study by Underwriters Laboratories found that attaching filters to a box fan in a do-it-yourself configuration did not present a fire hazard from increased heating of the fan motor windings.[24]


Students with decorated Corsi-Rosenthal Boxes at a competition at University of California, Davis in 2022

Corsi–Rosenthal Boxes have been used in schools to reduce risk of COVID-19 transmission,[25][26][27] sometimes also functioning as a STEM lesson with students learning about the science of air filtration and constructing the boxes themselves.[28] At the university level, engineering students are also running tests on the units.[29]

Homeless shelters and daycare facilities have also used Corsi–Rosenthal Boxes to mitigate COVID risk.[30]

Corsi–Rosenthal Boxes have also seen use in the 2023 wildfire season in North America, especially as smoke pollution caused hazardous air quality conditions across heavily populated areas in the Eastern United States.[31][32][33]


  1. ^ a b c Sun, Lena H. (14 January 2023). "After three years of covering covid, I built my own air filter". Washington Post. ISSN 0190-8286. Retrieved 8 June 2023.
  2. ^ a b c d e Rosenthal, Jim (22 August 2020). "A Variation on the "Box Fan with MERV 13 Filter" Air Cleaner". Tex-Air Filters. Archived from the original on 13 November 2021. Retrieved 13 November 2021.
  3. ^ a b c d Dal Porto, Rachael; Kunz, Monet N.; Pistochini, Theresa; Corsi, Richard L.; Cappa, Christopher D. (2022). "Characterizing the performance of a do-it-yourself (DIY) box fan air filter". Aerosol Science and Technology. 56 (6): 564–572. Bibcode:2022AerST..56..564D. doi:10.1080/02786826.2022.2054674. S2CID 248406422.
  4. ^ Morawska, Lidia; Cao, Junji (June 2020). "Airborne transmission of SARS-CoV-2: The world should face the reality". Environment International. 139: 105730. doi:10.1016/j.envint.2020.105730. PMC 7151430. PMID 32294574.
  5. ^ Morawska, Lidia; Milton, Donald K (6 July 2020). "It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19)". Clinical Infectious Diseases. 71 (9): 2311–2313. doi:10.1093/cid/ciaa939. ISSN 1058-4838. PMC 7454469. PMID 32628269. Archived from the original on 9 March 2022. Retrieved 9 October 2021.
  6. ^ Zhang, Renyi; Li, Yixin; Zhang, Annie L.; Wang, Yuan; Molina, Mario J. (30 June 2020). "Identifying airborne transmission as the dominant route for the spread of COVID-19". Proceedings of the National Academy of Sciences. 117 (26): 14857–14863. Bibcode:2020PNAS..11714857Z. doi:10.1073/pnas.2009637117. ISSN 0027-8424. PMC 7334447. PMID 32527856.
  7. ^ Wainer, Gabriel (27 October 2020). "How to prevent COVID-19 'superspreader' events indoors this winter". The Conversation. Archived from the original on 5 October 2021. Retrieved 9 October 2021.
  8. ^ Lewis, Dyani (23 February 2021). "Superspreading drives the COVID pandemic — and could help to tame it". Nature. 590 (7847): 544–546. Bibcode:2021Natur.590..544L. doi:10.1038/d41586-021-00460-x. PMID 33623168. S2CID 232037743.
  9. ^ Noorimotlagh, Zahra; Jaafarzadeh, Neemat; Martínez, Susana Silva; Mirzaee, Seyyed Abbas (February 2021). "A systematic review of possible airborne transmission of the COVID-19 virus (SARS-CoV-2) in the indoor air environment". Environmental Research. 193: 110612. Bibcode:2021ER....193k0612N. doi:10.1016/j.envres.2020.110612. PMC 7726526. PMID 33309820.
  10. ^ Greenhalgh, Trisha; Jimenez, Jose L; Prather, Kimberly A; Tufekci, Zeynep; Fisman, David; Schooley, Robert (May 2021). "Ten scientific reasons in support of airborne transmission of SARS-CoV-2". The Lancet. 397 (10285): 1603–1605. doi:10.1016/s0140-6736(21)00869-2. ISSN 0140-6736. PMC 8049599. PMID 33865497.
  11. ^ a b c d e f Rogers, Adam (6 August 2020). "Could a Janky, Jury-Rigged Air Purifier Help Fight Covid-19?". Wired. ISSN 1059-1028. Archived from the original on 25 September 2021. Retrieved 9 October 2021.
  12. ^ Air Purifier Ratings and Reviews. Consumer Reports. Many purifiers. Wide price range. Each purifier page has a shop button for prices. No need to sign in to see prices.
  13. ^ HEPA air cleaners search at Amazon. Wide price range.
  14. ^ a b c Emanuel, Gabrielle (17 August 2021). "DIY: How To Build A Cheap, Effective Classroom Air Filter". WGBH. Archived from the original on 17 August 2021. Retrieved 9 October 2021.
  15. ^ a b Mandavilli, Apoorva (7 October 2020). "The plexiglass barriers at tonight's debate will be pretty useless, virus experts say". The New York Times. Archived from the original on 13 November 2021. Retrieved 13 November 2021.
  16. ^ Bartl, Aditi Risbud (2 July 2021). "Richard Corsi Appointed College of Engineering Dean". College of Engineering. Archived from the original on 9 October 2021. Retrieved 9 October 2021.
  17. ^ @CorsIAQ (7 October 2020). "Thanks for the shout out, @Don_Milton. Great article. One correction. I mentioned the concept of a portable air cleaner with walls made of filters in an interview. But it was actually @JimRosenthal4 who built a unit shortly thereafter & deserves credit (gr8 craftsmanship!)" (Tweet) – via Twitter.
  18. ^ @CorsIAQ (8 October 2020). "I am good with that! Thanks for this, Don" (Tweet) – via Twitter.
  19. ^ a b c Lapook, Jon (7 October 2021). "New air purifiers filter at least 90% of COVID-carrying particles, researchers say". CBS News. Archived from the original on 7 October 2021. Retrieved 9 October 2021.
  20. ^ a b c Pistochini, Theresa; McMurry, Robert (August 2021). Testing Different Configurations of Do-it-yourself Portable Air Cleaners (PDF) (Report). Archived (PDF) from the original on 4 October 2021. Retrieved 9 October 2021.
  21. ^ Rosenthal, Jim (4 November 2020). "How to Improve the Efficiency of the "Box Fan and MERV 13 Filter" Air Cleaner". Tex-Air Filters. Archived from the original on 13 November 2021. Retrieved 13 November 2021.
  22. ^ Tham, K.W.; Parshetti, G.K.; Balasubramanian, R.; Sekhar, C.; Cheong, D.K.W. (2018). "Mitigating particulate matter exposure in naturally ventilated buildings during haze episodes". Building and Environment. 128: 96–106. doi:10.1016/j.buildenv.2017.11.036. Archived from the original on 2 December 2021. Retrieved 9 October 2021.
  23. ^ Dodson, Robin E.; Manz, Katherine E.; Burks, Shaunessey R.; Gairola, Richa; Lee, Nina F.; Liu, Yun; Pennell, Kurt D.; Walker, Erica D.; Braun, Joseph M. (10 January 2023). "Does Using Corsi–Rosenthal Boxes to Mitigate COVID-19 Transmission Also Reduce Indoor Air Concentrations of PFAS and Phthalates?". Environmental Science & Technology. 57 (1): 415–427. Bibcode:2023EnST...57..415D. doi:10.1021/acs.est.2c05169. ISSN 0013-936X. PMC 9876422. PMID 36562547.
  24. ^ "Wildfire Safety Research: An Evaluation of DIY Air Filtration" (PDF). Underwrites Laboratories Chemical Insights. July 2021. Archived (PDF) from the original on 11 May 2023.
  25. ^ Coker, Jonny (24 February 2023). "In response to COVID-19, Las Cruces middle schoolers are taking action to improve the air quality of their school". KRWG Public Media. Retrieved 11 June 2023.
  26. ^ Graham, Kristen A. (22 September 2022). "These parents are making DIY air purifiers for Philly schools. They want one in every city classroom". Philadelphia Inquirer. Retrieved 11 June 2023.
  27. ^ Soular, Diana Alba (27 February 2023). "In southern Arizona, an entire K-8 school tackles dust, COVID-19 with DIY air purifiers". Las Cruces Sun-News. Retrieved 11 June 2023.
  28. ^ Wolf, Marin (23 August 2022). "How a $100 box is changing the way people protect themselves against coronavirus". Dallas News. Retrieved 11 June 2023.
  29. ^ Yorio, Kara (4 February 2022). "Corsi-Rosenthal Boxes Help Clear the Air at Schools Across the Country". School Library Journal. Retrieved 12 June 2023.
  30. ^ Hannah, Douglas (7 March 2022). "The Homemade Air Purifier That's Been Saving Lives During the Covid-19 Pandemic". Smithsonian Magazine. Retrieved 12 June 2023.
  31. ^ Ritzen, Stacey (9 June 2023). "TikTok Is Teaching People to Make DIY Air Purifiers". Men's Journal. Retrieved 11 June 2023.
  32. ^ McCarthy, Ciara (9 June 2023). "How a DIY air cleaner with Fort Worth roots can help during wildfire season". Fort Worth Star-Telegram. Retrieved 11 June 2023.
  33. ^ Dupuy, Beatrice (9 June 2023). "The Surprising Effectiveness of DIY Air Purifiers". Time. Archived from the original on 11 June 2023. Retrieved 11 June 2023.