Linsey Marr, of Virginia Tech, says "there's no question" HEPA, or high-efficiency particulate, filters can improve air quality.

And she doesn't see how they could be harmful  — "unless people take apart the HEPA air cleaner and grab the filter and start, you know, rubbing it on their face."

Sixty schools in New Brunswick have no integrated mechanical ventilation systems, as of September. Unlike the 234 schools in the province that do have integrated ventilation systems, these schools must rely instead on opening doors and windows to circulate air.

While other provinces, such as Ontario, spent millions on HEPA filters for schools before classes started last fall, New Brunswick plans to hire an outside expert to review the "competing science" surrounding the issue and provide recommendations, Cardy has said.

HEPA-filtered air purifiers like this one, in a Toronto public school, can reduce the concentration of some viruses in the air by capturing small particles, such as the water droplets that can carry the coronavirus, says Marr. (Evan Mitsui/CBC)

Earlier this week, before students returned to virtual learning following the Christmas holidays instead of in-person classes because of an ongoing surge of COVID cases and hospitalizations, CBC News asked Cardy why HEPA filters weren't used in schools at the start of the pandemic.

"When I went and asked experts in my department and folks across government about HEPA filters, they said, actually it's not quite as clear cut as that. There's actually a lot of information that it might actually make things worse," he said.

"So no problem at all, admitting when we're off-base on something. But the last thing I want to do is add to the list of things that we're off-base on by starting off, by saying that this is definitely going to be a sure fix to a problem when we don't yet have that evidence."

Marr, who has been a leading voice on the airborne transmission of COVID-19 since the beginning of the pandemic, contends Cardy spoke to the "wrong experts, or they haven't caught up on the more recent information."

Earlier in the pandemic, there was much debate among virologists about how COVID-19 is spread. Many believed the primary mode of transmission was through large droplets.

But Marr says the highly transmissible Omicron variant has made it clear that COVID-19 is an airborne virus and we must act accordingly to stop it from spreading.

Sixty of New Brunswick's 294 schools do not have adequate ventilation systems, as of September. (Ivanoh Demers/Radio-Canada)

"We know from basic physics … if you have a filtration system in a smoky room, that's going to bring down the amount of smoke particles in the room," she said.

There have also been some in-hospital studies that are not yet peer-reviewed, which found less bacteria and viruses, including COVID-19, in the air, when HEPA filters were turned on.

"So, you know, between basic physics and these empirical, observational studies, it's very clear that HEPA filters can reduce the amount of virus in the air," said Marr.

The Public Health Agency of Canada says the effectiveness of HEPA filters in reducing the transmission of the SARS-CoV-2 virus "hasn't yet been demonstrated."

But HEPA filters can be used "as an additional protection in situations where enhancing natural or mechanical ventilation is not possible," it advises.

"When properly used, portable air filtration devices with high-efficiency particulate air (HEPA) filters have been shown to reduce the concentration of some viruses from the air."

On Dec. 24, the federal agency released guidance on choosing a portable air purifier.

It comes on the heels of a video offering COVID-19 guidance on ways people can improve ventilation and air filtration in their home.

"Ventilation is a key way to help prevent the spread of COVID-19," according to the description.

It replaces stale indoor air with fresh outdoor air.

"This helps to reduce the levels of infectious particles in the air," the video states.

People can open windows and doors to create a cross-breeze of fresh air.

They can also use air filters with their heating, ventilation, and air conditioning (HVAC) system or portable air purifiers with a HEPA filter, it advises.