Despite skyrocketing school COVID -19 cases since the January school reopenings, decades of negligence and no fix on the horizon for the critical school ventilation situation, the Quebec government of Francois Legault and its public health director are still shamelessly denying the scientific evidence linking COVID-19 transmission to inadequate ventilation and stubbornly refuting the data from peer-reviewed studies confirming the effectiveness of air purifiers in reducing airborne transmission.

Why are school air purifiers so important?

For starters, 52,000 (58 per cent) of Quebec's 90,000 school classrooms and indoor spaces do not have any mechanical ventilation and most likely very few meet minimal international standards recommended in times of pandemic. Parents and teachers are left on their own to figure out how to reduce the risk of contagion in schools from inadequate ventilation systems. 

An interesting do-it-yourself mass-production solution has grabbed international attention by storm.

It is a simple but extremely effective air-purifier made from low-cost materials readily available from local hardware stores.

The device is the brainchild of Richard Corsi, Dean of the College of Engineering at the University of California, Davis and an indoor air quality expert and Jim Rosenthal, CEO of Tex-Air Filters who tested it.

It has been nicknamed the Corsi-Rosenthal box (or CR box), costs less than $150 and can be built in less than an hour by anyone who has solid experience at cutting duct tape with a pair of scissors.

Best of all, a recent study has shown that it demonstrated exceptional performance relative to most commercially available air purifiers.

Corsi said that some homemade units are delivering up to 600 cubic feet per minute (CFM) in clean air delivery rates (CADR).

"That’s phenomenal", he said. "That’s actually better than a lot of the more expensive HEPA-based portable air cleaners". 

Many underfunded school districts in several countries have turned this homemade technology into mass-production school and community events.

Students from a single classroom with a couple of parent volunteers can crank out a few dozen of those in a day, more than enough for the entire school.

It is a novel and rewarding way for students of all ages to integrate mathematics, science, art and writing skills into an exciting community project.

Theoretically, all Quebec classrooms could be equipped with CR boxes in 5-7 business days for about $20 million dollars, a fraction of the cost of equivalent commercial air-purifiers.

Government organizations and schools of public health agree that air purifiers using MERV-13 to 14 filters can mitigate the spread of the SARS-CoV-2 virus.

According to Ashish Jha, Dean of the Brown University School of Public Health, “The Corsi-Rosenthal Cubes are a great example of a hands-on public health intervention that can have an immediate impact on the school environment and our community’s overall health."

The Harvard School of Public Health and ASHRAE recommend targeting at least 5-6 air changes per hour (ACH) in classrooms for COVID-19 risk mitigation.

Note: this recommendation was made before recent, more transmissible variants, like Omicron.

It is fair to guess that extremely few Quebec classrooms come anywhere close to those levels, with the exception of those which have been fitted with air purifiers by the English school boards.

Using conservative estimates, two CR boxes per standard size classroom (1000 sq. ft. with an 8-foot ceiling) with 25 students could filter significantly more than six air changes per hour. In some areas like the nurse's station where symptomatic children may isolate, up to 10 air changes per hour are recommended.

Construction is amazingly simple; all you need is a 20-inch box fan, four 20x20 one-inch furnace filters rated no lower than MERV 13 (or the equivalent MPR 1900 rating), and some duct tape.

The CR box is also very eco-friendly, as the cardboard box from the fan can be used to make the bottom and the top of the device.

The resulting device is safe: the EPA funded a study by Underwriters Laboratories (UL) which tested these DYI fans, finding them not to present a fire hazard even in extreme, modified conditions.

Although MERV-13 filters are not as efficient as HEPA-rated filters in filtering out COVID-19 viral-size particles which are around 0.1 micrometre, the virus is trapped in respiratory droplets that are predominantly 1 micrometre in size and larger.

The good news is that the MERV-13 filter is at least 85 per cent efficient at capturing particles in that size range which can stay aloft for a long time.

Valuable information about the construction and other aspects of the CR-Box is available on this link.

It is unfortunate that Quebec did so little to improve inadequate school ventilation and that several English schools board had to fend for themselves to provide safe environments for their staff and children.

The English school boards could do so because they were protected by the Canadian constitution.

Sadly, French schools are not so lucky as they need to obtain government authorization to procure air purifiers, which is fiercely against their use.

People should not risk their lives to go to work.

CR boxes may not end the pandemic on their own, but along with other protective measures such as wearing N-95s or double masking, they do add an extra layer of protection terribly missing in Quebec schools.

The CR box is more than an air purifier: it is a powerful statement of community empowerment in response to this government's inept and harmful decisions.

COVID-Stop will hold an event on February 10th on safe schools which will speak about CR boxes. Details to come on their website and social media accounts.

  • Michael Levy, MPH (Master of Public Health), Environmental Health Specialist and Epidemiologist
  • Stéphane Bilodeau, ing., PhD, FIC, Indoor Air Quality Task Force Coordinator, World Health Network
  • Nancy Delagrave, Physicist, Scientific Coordinator of Covid-Stop