COVID: Corsi-Rosenthal Box DIY air purifier works, studies say

This fall, teachers and parents shared on social media photos of homemade air purifiers for classrooms to help protect children from the spread of COVID-19.

But do these cheap purifiers really work?

According to the researchers, they have.

They are called Corsi-Rosenthal Boxes, a do-it-yourself air purifier first conceived in the summer of 2020 by two air purifier experts, Richard Corsi and Jim Rosenthal.

Since then, their effectiveness has been supported by scientists, researchers and an air filter manufacturing company.

Studies have shown that poor ventilation is associated with a much higher spread of COVID-19. But many buildings in Canada, including schools and workplaces, are still not equipped with proper ventilation systems or HEPA filters.

Unlike HEPA filters, which can cost $200 to $500, a Corsi-Rosenthal Box can be made for around $100 with materials that can be purchased at any hardware store. .

A York University engineering professor who carved Corsi-Rosenthal Boxes with him on campus last week said he heard about them on social media in mid-2021.

James Andrew Smith told in an email: “A number of ventilation engineers have been talking about them and they seem like a practical and easy way to get cleaner air into my school and into my home, ” James Andrew Smith told in an email.

“I don’t believe we’re doing enough to get clean air,” he added, adding that he feels better about the level of risk in his classroom when he uses a DIY air purifier. along with other measures.

The boxes also seem to have a lifespan.

Corsi said: “The guy behind me turned eight months ago a few days ago. in a Twitter video on Monday, referring to his own Corsi-Rosenthal Box humming over his shoulder. “Still doing its job, still lowering inhaled aerosols, including respiratory aerosols filled with viruses. It’s not rocket science, friends. “


There are many variations, but the basic Corsi-Rosenthal Box is made of four air filters, a box fan, and some cardboard, all glued together to form a cube. A guide from the University of California, Davis specifies the use of the MERV-13 filter.

The box fan must have an outward airflow and all edges must be sealed with duct tape. The last sixth side of the cube should be closed with cardboard or even a fifth air filter.

To make the filter more effective, you can also cut a circle out of a piece of cardboard and stick it on the outside of the box fan to reduce backflow.

The idea behind the Corsi-Rosenthal configuration is that the air entering the canister passes through multiple filters for cleaning, and the closed box block means that only filtered air is then blown into the room by the fan.

Studies show that while it cannot filter out as many particles as a HEPA filter, the Corsi-Rosenthal Box can circulate room air at a rate that sometimes exceeds that of a HEPA filter, removing virus particles. out of the air. at a fraction of the cost.

A study published in March in the journal Aerosol Science and Technology, which included Corsi as one of its authors, found that the boxes can rapidly filter an entire room at a much faster rate. compared to HEPA filters.

When considering the “cost per unit of air cleaning,” the study states that “DIY air filters are only one-tenth the initial cost of commercially available HEPA-based air filters.”

3M, a company that makes air filters, said in a press release in February that its scientists studied the Corsi-Rosenthal Box and that the DIY idea works.

“Indoor air is shared air,” Kelsey Hei, 3M Filtrete brand engineer, said in the statement. “Many viruses like COVID-19 are airborne and can become highly concentrated in poorly ventilated spaces. I’m glad to see there are so many people advocating for the importance of clean air, especially in schools. “

Another study published in September in the journal Science of the Total Environment looked at nine different configurations of DIY air purifiers and compared them to three HEPA filters.

Research shows that DIY options, including a single filter with a box fan, a Corsi-Rosenthal Box and a variation thereof using three filters and two box fans in a single block, are comparable or better than HEPA filters in terms of the rate at which they filter the air.


As it became clear that COVID-19 was spread by airborne transmission, scientists began to emphasize the importance of good ventilation as a tool to slow transmission.

An Italian study published last March that looked at more than 10,000 classrooms found that effective ventilation can reduce the transmission of COVID-19 in schools by more than 80%.

Infections in 316 classrooms equipped with mechanical ventilation systems were much lower than those without these systems. When the ventilation system cycled to completely replace classroom air with fresh air 2.4 times an hour, infections were reduced by 40%. The study found that if they replaced the air six times an hour, infections were reduced by 82%.

With some public health measures still in place to return to school, teachers are increasingly turning to tools like the Corsi-Rosenthal Box to make their classrooms safer.


As students return to classrooms this fall, teachers and parents have taken to social media to share photos of their Corsi-Rosenthal Box. On Twitter, the hashtag #CorsiRosenthalBox includes many creative designs that are kid-friendly.

A user in Manitoba posted a photo of a Corsi-Rosenthal Box for his son’s classroom that has been upgraded to look like a robot, with googly eyes, beard and arms.

Other designs use big eyes, bright paint, and a variety of animal ears to turn the boxes into friendly creatures.

In Quebec, parents and teachers have posted a petition calling on Jean-Francois Roberge, the Minister of Education, to allow them to provide Corsi-Rosenthal Boxes to their children’s schools, saying that the Citizens’ initiative was “put up against strong administrative opposition”.

The description reads: “Installing a homemade air purifier, called a Corsi-Rosenthal block, is an effective and inexpensive way to improve the air quality in indoor spaces.

The petition, which began last month, already has more than 1,600 signatures.

At York’s Lassonde School of Engineering, Smith says the carving around the Corsi Rosenthal Box spurred students to wonder and curiosity.

“How much do I care about my students? Enough to build them a #CorsiRosenthalBox with wheels to take to class,” he tweeted on Monday.

Smith and other faculty members have appealed to the Dean’s Office of York’s Lassonde School of Engineering to fund and distribute portable air purifiers to faculty, but these moves have so far been denied.

Smith told “Personally, I think they have no brains. “They’re easy to install, easy to operate, and not nearly the subject of controversy about vaccines or masks. They make the air cleaner to remove smoke, fumes, dust… they’re just overall a great idea. “

These boxes are being used in classrooms across the globe.

In November 2021, the Brown University School of Public Health in Rhode Island kicked off a project to build Corsi-Rosenthal Boxes for classrooms and study them, and has since installed these boxes in many classrooms and student lounges.

Oliver Patrick, a county councilor in the United Kingdom, who started a GoFundMe last month to deliver Corsi-Rosenthal Boxes to schools in his district, says he is concerned about the lack of health measures public in their area.

“I want to make sure we do the best we can to make schools as safe as possible for children,” Patrick wrote.

The California Department of Public Health listed the Corsi-Rosenthal Box as a recommended option for classrooms in its February newsletter to improve school air quality to combat COVID-19.


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