When his daughter returned to school in the fall, Dave Pataky said he wanted to keep the risk of COVID-19 transmission to a minimum. 

Pataky, who has a background in physiology and neuroscience, had concerns about the HVAC system at Walton Elementary School in Coquitlam, B.C., so in September he volunteered to create an air-filter solution.

He built two Corsi-Rosenthal boxes — a do-it-yourself device consisting of furnace filters and a box fan — which he says were used at the school for about three months.

Over the Christmas break, he brought the boxes home for maintenance and discussed building around 25 more for the school. But in January, the local school district told him to put the brakes on his plans.

Pataky said the district told him the DIY filters were not approved by CSA Group, formerly known as the Canadian Standards Association.

"I had to abandon the idea," Pataky said. "So the kids are in their classroom, re-breathing each other's air, which is the perfect environment for something as highly contagious as Omicron."

While vaccines and masking continue to be important to curbing the spread of COVID-19, experts say proper ventilation and filtration are also key. 

Michael Brauer, a professor in the School of Population and Public Health at the University of British Columbia, says opening windows to bring in fresh air can reduce risk while filters, including homemade Corsi-Rosenthal boxes, are "something that complement ventilation."

Pataky said his child's school has MERV-9 filters — MERV stands for minimum efficiency reporting value — which fall short of MERV-13 filters, the standard set by the Education Ministry-led steering committee for HVAC systems. Even if it had better filtration, he says, a Corsi-Rosenthal box could only help. 

"All the engineers I've spoken to have said any extra filtration in a room is always better," Pataky said.

In a statement to CBC News, School District 43 said HVAC systems across Coquitlam schools have been inspected and are running at optimized levels.

"Student and staff safety is important to us and we expect that equipment brought into our schools meets Canadian safety standards, and homemade units do not meet that standard," the district said.

After she and her family came down with COVID-19 in September, Elizabete Costa offered to have her son bring his own high-efficiency particulate air (HEPA) filter into his classroom at Victoria's Monterey Middle School, which has MERV-10 filters.

She said the teacher and students were happy, but "the morning after I dropped off the HEPA filter, the principal removed it from the classroom and put it in a bag in his office."

When she asked why, she said the principal told her the school had sufficient filtration.

Costa later received a notification about a COVID-19 exposure in the classroom — the same week the filter was removed. 

"Even then they would not put the HEPA filter back," she said. 

B.C. Teachers' Federation (BCTF) president Teri Mooring says parents donating filters to schools is a symptom of a bigger problem. 

"It's a pretty sad commentary on our public school system that families and also teachers are feeling like they need to purchase their own HEPA filters to keep schools safe," she said.

Mooring said the BCTF's assessment of information collected on school ventilation found that "about half the districts in the province have quite a bit of work to do on ventilation systems."

The union has also advocated for greater information about school ventilation systems, which can now be found online.

B.C.'s Ministry of Education says it has invested $114.5 million to help school districts upgrade ventilation in thousands of classrooms around the province.

The ministry said a recent survey of 19 school districts found more than 1,500 standalone HEPA filters have been purchased through the latest round of funding.

All of the province's public schools have conducted regular inspection and maintenance of HVAC systems this year, the ministry said, and it's up to individual districts to decide where additional supports from families and communities are needed. 

The Public Health Agency of Canada says while HEPA filters can be used as an additional tool, their effectiveness against COVID transmission has not yet been demonstrated and should not replace adequate ventilation, physical distancing and hygiene measures. 

Brauer said the Omicron variant is so highly transmissible that stopping transmission is not likely, but measures such as masks and ventilation can help reduce risk. HEPA filters, he said, can be effective if appropriate for the size of the room, and he sees no downside to having them in classrooms.

"It's hard to imagine how it would hurt," he said. 

With files from Julia Wong and Natalia Goodwin

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