This DIY air purifier costs far less than a HEPA purifier, and studies say it works

The Corsi-Rosenthal Box is a DIY air purifier. Some are making theirs kid-friendly by adding unique design elements, while others are getting creative in how to transport them from office to class. (left: @roofeo24 on Twitter, right: James Andrew Smith)

This fall, teachers and parents have been sharing photos on social media of do-it-yourself air purifiers that they’ve made for classrooms to help protect kids from COVID-19 transmission.

But do these low-cost purifiers actually work?

According to researchers, they do.

They’re called Corsi-Rosenthal Boxes, a DIY air purifier that was first conceived of in the summer of 2020 by two air filtration experts, Richard Corsi and Jim Rosenthal.

Since then, their effectiveness has been backed up by scientists, studies and a company that makes air filters.

Studies have shown that poor ventilation is associated with 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 purifiers.

Unlike HEPA purifiers, which can cost from $200 to upwards of $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 has been carting his Corsi-Rosenthal Box with him across campus this past week says he heard about them on social media in mid-2021.

“Some ventilation engineers were talking about them and they seemed like a practical and easy way to get more clean air into schools and into my home,” James Andrew Smith told CTVNews.ca in an email.

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

The boxes also seem to have longevity.

“This guy behind me just turned eight months a couple days ago,” Corsi said in a Twitter video Monday, referring to his own Corsi-Rosenthal Box humming away past his shoulder. “Still doing its job, still lowering inhalation-dosed aerosol particles, including virus-laden respiratory aerosol particles. It’s not rocket-science, folks.”

 

WHAT ARE CORSI-ROSENTHAL BOXES?

There are variations, but the basic Corsi-Rosenthal Box is made of four air filters, one box fan and some cardboard, all taped together to make a cube. A University of California, Davis tutorial specifies to use MERV-13 filters.

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

To make the filter more efficient, you can also cut a circle out of a piece of cardboard and tape it overtop of the outward face of box fan to reduce backflow.

The idea behind the Corsi-Rosenthal configuration is that the air that enters the box will be passing through multiple filters for purification, and the sealed cube means that only filtered air will then be blown out into the room by the fan.

Studies indicate that while it won’t be able to filter out quite as many particles as a HEPA filter can, a Corsi-Rosenthal Box can cycle the air in a room at a rate that sometimes outstrips HEPA purifiers, removing viral particles from 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 were able to quickly filter an entire room at a much faster rate than the HEPA purifiers.

When looking at the “cost-per-unit-air-cleaned,” the study said that “the DIY air filter is approximately one-tenth the initial cost of a commercially available HEPA-based air cleaners.”

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

“Indoor air is shared air,” Kelsey Hei, a 3M Filtrete brand engineer, said in the release. “Many viruses like COVID-19 are airborne and can become highly concentrated in poorly ventilated spaces. I’m heartened to see 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 purifiers.

The study found that the DIY options, including a single filter with a box fan, Corsi-Rosenthal Boxes, and a variation on them that uses three filters and two box fans in a cube, were all comparable or better than HEPA purifiers in terms of the speed in which they filtered the air.

CLEAN AIR KEY TO LESS COVID TRANSMISSION

When it became clear that COVID-19 was spread through 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 efficient ventilation systems were able to reduce transmission of COVID-19 in schools by more than 80 per cent.

Infections were much lower in the 316 classrooms that were equipped with a mechanical ventilation system, compared to those without them. When the ventilation systems cycled to completely replace the classroom’s air with new air 2.4 times an hour, infections were reduced by 40 per cent. If they replaced the air six times an hour, the infections were reduced by 82 per cent, the study found.

• Read more: COVID-19 transmission in schools: Experts call for better ventilation, monitoring

With few public health measures still in place for back to school, teachers have been increasingly turning to tools like the Corsi-Rosenthal Box to make their classrooms safer.

THE CLASSROOM TAKEOVER

As students returned to classrooms this fall, teachers and parents took to social media to share photos of their Corsi-Rosenthal Boxes. On Twitter, the hashtag #CorsiRosenthalBox includes numerous creative designs aimed to be kid-friendly.

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

Other designs utilized big eyes, brightly coloured paints and a variety of animal ears to transform the boxes into friendly creatures.

In Quebec, parents and teachers have posted a petition calling for Jean-Francois Roberge, Minister of Education, to allow them to provide Corsi-Rosenthal Boxes to their children’s schools, saying citizen initiatives have “come up against strong administrative resistance.”

“Installing homemade air purifiers, known as Corsi-Rosenthal cubes, is an effective and inexpensive way to improve air quality in indoor spaces,” the description reads.

The petition, started last month, has more than 1,600 signatures.

At York`s Lassonde School of Engineering, Smith says that carting around a Corsi Rosenthal Box has spurred questions and curiosity from students.

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

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

“Personally, I think that they're a no-brainer,” Smith told CTVNews.ca. “They're easy to install, easy to operate, aren't nearly the controversial topics that vaccines or masks are. They make the air cleaner for smog, smoke, dust... they're just generally 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 launched a project to build Corsi-Rosenthal Boxes for classrooms and study them, and have since installed boxes in numerous classrooms and student lounges.

Oliver Patrick, a county councillor in the U.K., started a GoFundMe last month to provide Corsi-Rosenthal Boxes for schools in his district, stating that he was concerned about the lack of public health measures in place in his region.

“I want to make sure we do as much as we can to make sure school is as safe as possible for children,” Patrick wrote.

The California Department of Public Health listed Corsi-Rosenthal Boxes as a recommended option for classrooms in a February bulletin for improving air quality in schools to fight COVID-19.