Corsi–Rosenthal Box

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An example of a homemade unit

The Corsi–Rosenthal Box, also called Corsi–Rosenthal Cube and Comparetto Cube, is a design for a do-it-yourself air purifier that can be built comparatively inexpensively. It was designed during the COVID-19 pandemic, with the goal of reducing the levels of airborne viral particles in indoor settings.

Background and history[edit]

Since COVID-19 was declared a pandemic by the World Health Organization on 11 March 2020,[1] evidence, including increasing amounts of peer-reviewed research, has been accumulating that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus causing COVID-19, is airborne.[2][3][4] Superspreading events are generally associated with indoor gatherings.[5][6] In response to the emerging evidence and recommendations of infectious disease researchers,[7][8] engineers began to consider how improved ventilation may reduce indoor viral loads.[9]

Air purification units with HEPA filtration can be expensive, often costing considerably more than US$500.[10][9][11] In August 2020, Richard Corsi, an environmental engineer and the incoming Dean of Engineering at the University of California, Davis, (UC Davis)[12] 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.[9] 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.[13] Inspired by Corsi's idea to use multiple filters, Rosenthal later came up with a 5-filter design. Rosenthal named it after Corsi,[13] although after a New York Times article mentioned the boxes by that name,[11] Corsi tweeted that Rosenthal really deserved the credit,[14] and that he preferred the name Corsi–Rosenthal Box.[15][16]


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

The original Corsi–Rosenthal Box design consisted of five furnace filters, preferably of effectiveness MERV13 or higher, which formed the sides and bottom of a cube.[13] A 20-inch (500 mm) box fan is placed on top and duct taped to the filters, sealing the system so that air is drawn through the filters, up and out of the box.[16][10] An updated design, also known as a Comparetto Cube,[17] uses four filters and a cardboard base that can sit directly on the floor.[18] Rosenthal later improved the design further by adding shrouds made of cardboard or similar materials to cover the corners of the box fan to improve efficiency and reduce backflow.[19][20]

The filtration units can be assembled in around fifteen minutes, last for months, and cost between US$50 and $150 in materials.[16][10][9]


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.[9] Clean air delivery rates (CADR) of a US$75 design were estimated at between 165 and 239 (depending on fan speed) in an August 2021 case study by UC Davis researchers.[19] In October 2021, Corsi told GBH News that "People are now reporting 600 cubic feet per minute (280 L/s) in clean air delivery rates. That's phenomenal. That's actually better than a lot of the more expensive HEPA-based portable air cleaners".[10]

In April 2022, a team based at UC Davis published a study of a Corsi–Rosenthal box that used five 2-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)".[21] 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.[21]

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 may be suggestive of the efficacy of similar filters to filter virus particles similar in size to the particles studied.[9][22]

Researchers have expanded studies of these citizen science filtration units to evaluate their efficacy for reducing the levels of airborne volatile chemicals, including semivolatile organic compounds (SVOCs).[23] Dodson and colleagues recently demonstrated that levels of SVOCs are also reduced by a functioning Corsi-Rosenthal Box.


  1. ^ Cucinotta, Domenico; Vanelli, Maurizio (19 March 2020). "WHO Declares COVID-19 a Pandemic". Acta Bio Medica Atenei Parmensis. 91 (1): 157–160. doi:10.23750/abm.v91i1.9397. PMC 7569573. PMID 32191675. Archived from the original on 9 March 2022. Retrieved 9 October 2021.
  2. ^ 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.
  3. ^ 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.
  4. ^ 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.
  5. ^ Wainer, Gabriel. "How to prevent COVID-19 'superspreader' events indoors this winter". The Conversation. Archived from the original on 5 October 2021. Retrieved 9 October 2021.
  6. ^ 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. Archived from the original on 22 September 2021. Retrieved 9 October 2021.
  7. ^ 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.
  8. ^ 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.
  9. ^ 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.
  10. ^ a b c d 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.
  11. ^ 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.
  12. ^ 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.
  13. ^ a b c 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.
  14. ^ @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.
  15. ^ @CorsIAQ (8 October 2020). "I am good with that! Thanks for this, Don" (Tweet) – via Twitter.
  16. ^ 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.
  17. ^ Trethewey, Ross (17 January 2021). "How to Make a DIY Air Filter". This Old House. Archived from the original on 13 November 2021. Retrieved 13 November 2021.
  18. ^ Rosenthal, Jim (4 July 2021). "IAQ Research-Practice in Action: The Corsi/Rosenthal Box Air Cleaner". Tex-Air Filters. Archived from the original on 13 November 2021. Retrieved 13 November 2021.
  19. ^ a b 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.
  20. ^ 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.
  21. ^ a b 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. doi:10.1080/02786826.2022.2054674. S2CID 248406422.
  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. doi:10.1021/acs.est.2c05169. ISSN 0013-936X. PMC 9876422. PMID 36562547.