MONTREAL -- School officials across Montreal were caught off guard on Friday by a new pronouncement from the province: it doesn’t think air purifiers work as a way to limit COVID-19 spread in classrooms.

“We don’t recommend those filtration devices,” said Dr. Richard Massé of the provincial institute of public health during an announcement about the plan to reopen Quebec schools in person beginning next week.

The institute just did a study that found that air purifiers don’t limit virus transmission—which had some scientists and Montreal-area school boards scratching their heads, since there’s plenty of other research showing they do. 

“Air purifiers, they filter out 99.97 per cent of infectious particles emitted by humans,” Nancy Delagrave, a physicist who teaches at College de Maisonneuve, told CTV. 

“This is what the CDC says,” she said, referring to the American federal public health agency.

“The CDC highly recommends air purifiers, so I don’t understand where they’re coming from.”

Delagrave also pointed to a German study conducted last fall found that putting air purifiers in classrooms made students six times less likely to contract COVID-19.

She was one of over 300 scientists who signed an open letter calling on the provincial health minister to take aerosol transmission in classrooms more seriously.

But Massé said that according to the Quebec researchers, air purifiers are not only useless in classrooms -- relatively large spaces -- but potentially they can help spread the virus around if improperly installed. 

Given that, they can also give people a false sense of security, he said.

“In a class where you don’t know who can be infected, just putting one device in one spot is kind of impossible to be sure it’s going to be really efficient in any way,” said Massé.

“It could create a false security perception by people, that if you have [air purifiers], maybe you could relax the other measures,” he said. “But clearly the other measures are the most effective.”

The government also says that according to its study, Quebec schools are mostly in top shape anyway when it comes to ventilation and air quality.

Of 330 schools whose air quality was tested for the study, only about 10 of them—three per cent—needed immediate attention, according to the province.

The rest were all found to have optimal air quality.


Both of Montreal’s English school boards said Friday that they did their own research and will stick with their own plans.

English Montreal School Board chair Joe Ortona said he was also baffled by the lack of new precautions for schools in general.

“It seems that they’re not taking any measures in schools whatsoever, which is inconsistent with what thye’re doing with everything else,” he told CTV News.

Earlier this winter, the EMSB spent $1.75 million on air purifiers and improved air filtration and it stands by that decision, it said in a statement.

The board “undertook an extensive assessment of all 55 buildings” that serve as schools, it said.

“We did our due diligence before making this decision,” said Ortona. The 806 new air purifiers will actually be installed a month ahead of schedule—Ortona said they should all be in place by end of January. 

The Lester B. Pearson School Board spent half a million dollars on air purifiers, enough for its 17 buildings that don’t have mechanical ventilation, and it’s confident they will help, it said.

"The decision to purchase and install the upgraded filters and HEPA air purifiers was made after consulting best practices in other jurisdictions around the world," the board wrote in a release Friday afternoon.

Board officials "acted based on scientific studies conducted by, amongst others, Harvard University, Yale University, the [U.S. CDC public health agency] and the World Health Organization."

Those sources had their own recommendations "regarding the types of benefits the purifiers and filters offer in terms of promoting better air quality and their ability to significantly limit the transmission of aerosol particles," the board wrote.

The EMSB had said it planned to push the province to reimburse it for the cost of the purifiers. On Friday, Education Minister Jean-François Roberge wouldn’t say if the province was considering that. 

Watch the videos above for a longer interview about aerosols with Delagrave.