The Corsi–Rosenthal Box is a design for a do-it-yourselfair 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 and 850 cubic feet (17–24 m3) per minute (depending on fan speed), while costing roughly a tenth of commercial air filters. Engineers Richard Corsi and Jim Rosenthal created the five-filter 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.
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][11][14] Previous attempts to purify air with "DIY" box-like combinations of HEPA filters had been constructed in the past,[15][16] but there had never been an attempt to apply such a design to reduce the airborne load of viruses. In August 2020, Richard Corsi, an environmental engineer and the incoming Dean of Engineering at the University of California, Davis,[17] 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,[14] Corsi tweeted that Rosenthal really deserved the credit,[18] and that he preferred the name Corsi–Rosenthal Box.[19][20] HVAC experts Neil Comparetto and John Semmelhack modified the design in October 2020 to use 4 filters and a cardboard bottom, in a design called the Comparetto Cube.[21][22]
Design
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.[23][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.[20][13]
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.[23][24]
The units can be assembled in around 15 minutes, last for months, and cost between US$50 and $150 in materials.[20][13][11]
Efficacy
Airborne virus particulates range in size from 1 to 50 micrometres (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 and 239 cubic feet (4.7–6.8 m3) per minute (depending on fan speed) in an August 2021 case study by UC Davis researchers.[23] 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][25]
Researchers have expanded studies of these citizen science filtration units to evaluate their efficacy for reducing the levels of airborne volatile chemicals (VOCs); Dodson and colleagues showed the units reduced PFAS and phthalates.[26]
Safety
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.[27]
Corsi–Rosenthal Boxes have been used in schools to reduce risk of COVID-19 transmission,[28][29][30] sometimes also functioning as a STEM lesson with students learning about the science of air filtration and constructing the boxes themselves.[31] At the university level, engineering students are also running tests on the units.[32]
Homeless shelters and daycare facilities have also used Corsi–Rosenthal Boxes to mitigate COVID risk.[33]
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