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


An Italian-based study published last week revealed the significant impact that effective ventilation systems in schools can have on limiting the transmission of COVID-19.

The experiment, conducted by the Hume foundation think-tank, observed the spread of COVID-19 in 10,441 classrooms across the Marche region of Italy between September 2021 and January of this year. Results showed that the number of COVID-19 infections reported in 316 classrooms with mechanical ventilation systems was significantly lower than the number reported among classrooms without these systems.

Devices that replaced the air in a classroom 2.4 times per hour reduced the number of infections by 40 per cent, according to the study. Additionally, four air replacements per hour brought down infections by 66.8 per cent, and six air replacements per hour reduced this number by 82.5 per cent.

“That's a large number, so as an epidemiologist, I'm always going to scratch my head and say, ‘What could account for that large number?” Colin Furness, an expert in infectious disease epidemiology from the University of Toronto, told in a phone interview on Wednesday. “It all depends on your starting point.”

Still, he said it doesn’t surprise him that effective ventilation can make a measurable difference in limiting the transmission of COVID-19 within closed spaces. However, this type of data among classrooms in Canada is limited, he said.

“We wouldn't know it in Canada because we have not done this kind of measurement at all,” Furness said. “Some individual teachers sneak in their own devices…but we're not doing this systematically, so we’ve got no clue.”

In the province of Ontario, where Furness is based, efforts were made to enhance ventilation systems before the start of the 2021 school year. However, not every classroom in the province is equipped with the tools it needs for proper ventilation or filtration, Furness said.

Less than 10 per cent of publicly funded schools in the province remain without mechanical ventilation, according to Grace Lee, spokesperson for Education Minister Stephen Lecce. Classrooms without mechanical ventilation are being prioritized for the use of high-efficiency particulate air (HEPA) filters, as are kindergarten classrooms, she said. The province recently stated it would be supplying school boards with up to 40,000 more HEPA filter units; these stand-alone filters help remove airborne particles floating in the air.

“We're still going to have winner and loser schools with respect to transmission,” said Furness. “And we've got no data to be able to say, ‘What kind of return did we get on the small amount of money that we did spend on ventilation?”

Joseph Fox is an HVAC engineer who works for a school board in Ontario. When he first came across the study, he said he felt a sense of relief as it adds to existing evidence that COVID-19 is airborne.

“It was a weight off my shoulders being able to see, finally, some proof that we can slow [transmission of] the virus down,” Fox told in a phone interview on Wednesday. “Everyone needs to understand how it spreads, and we have ways of…protecting people.”

The World Health Organization first brought up the possibility of airborne transmission of COVID-19 in specific indoor locations, such as crowded rooms with improper ventilation, in July 2020. If a disease is airborne, that means it commonly spreads through tiny respiratory droplets in the air, said Dr. Reza Afshari, a clinical professor at the University of British Columbia’s school of population and public health, who specializes in indoor air quality with respect to COVID-19. As a result, proper ventilation within closed spaces is especially important in limiting transmission of COVID-19, he said.

“Ventilation is helping to decrease the level of exposure [to COVID-19],” he told on Wednesday in a phone interview. “If the ventilation is good, we expect the relative risk of transmission to be lower.”

Ventilation involves exchanging the air between indoor and outdoor environments, said Fox. Air that people breathe indoors is exhausted outside, and replaced with outdoor air that is brought in. The exact extent to which ventilation can help reduce transmission of COVID-19 in classrooms, for example, can be difficult to calculate as it depends on a variety of factors, including how much of the virus is circulating in the room, how many students are present and how closely they’re sitting together, Afshari said.

Despite this, there is evidence that ventilation will reduce transmission to some degree, he said.

“If not ventilated, they are breathing in a fixed volume of air [and] the viruses are floating around so the risk [of transmission] is higher,” Afshari said. “If the ventilation is sufficient enough, the concentration of the virus in the air will be lower and other people will be less exposed.”


In the context of airborne diseases, they often spread in one of three ways, Fox said. The first is close-range, which involves standing within close proximity of an infected person and breathing in the same air as them. This presents the highest risk of transmission, said Fox. Another method of spread, although not as common, is long-range transmission, which is when someone enters a space where an infected person previously stood, and breathes in the air around them.

But in the context of a classroom, shared room transmission would be the largest concern, Fox said. This involves spreading a disease in congregate settings where groups of people spend long periods of time in the same room.

“If you're in the same room as someone for an extended period of time and the ventilation is not good enough, you will be sharing a lot of the air with other people, and that's when you could really get infected,” said Fox. “This makes it a higher-risk situation when it comes to the spread of airborne diseases.”

With younger children in particular, they may be less likely to maintain adequate personal space and are more likely to be in each other’s faces, Furness said. They’re also more likely to be actively physically engaged with each other and screaming, behaviours that can produce more aerosols and increase transmission, he said.

Other factors must be taken into consideration as well, Fox said, including the age of a building and whether or not it is ventilated according to modern standards, which usually involves mechanical ventilation systems. It’s less likely that older buildings would be equipped with mechanical ventilation, Fox said.

“You've got a long list of functional and behavioural factors that happen under that roof that we call a school that increase risk, and ventilation is a major piece of what to do about it,” said Furness.

Proper filtration is another way to reduce the risk of viruses such as COVID-19 spreading in schools, said Lexuan Zhong, an assistant professor in the University of Alberta’s faculty of engineering, who has conducted research on the spread of COVID-19 in buildings. Unlike ventilation, which exchanges the air in a room, filtration helps remove viral particles that exist within it.

But this does not involve modifying existing ventilation systems, and should not be considered a replacement for proper ventilation, Zhong said. Last month, Alberta’s government approved spending $6 million on new HEPA filters for Edmonton Public Schools.

“I think that’s a very smart strategy to use in schools because although ventilation is better, due to some of the older [building] designs…we’re not able to [limit transmission] as much as possible,” Zhong told on Wednesday in a phone interview. “So another way is to look at implementing these HEPA filters, which are very strong and can capture these very tiny aerosols in the air. We also can use this as a way to reduce the viral aerosol transmission in a space.”

Ahead of the return of students to class after the winter break, Edmonton Public Schools promised more frequent filter changes, as well as improved fresh air intake and the installation of high-quality filters for ventilation systems in 213 schools. According to a spokesperson from the school’s board, these measures “exceed the requirements outlined by the American Society of Heating, Refrigeration and Air-conditioning Engineers.”

As much as five per cent of British Columbia’s 24,670 classrooms are without mechanical ventilation, according to Scott McKenzie, senior public affairs officer for the government of British Columbia. Additional funding is also being used to purchase HEPA units for classrooms without mechanical ventilation, he said. Additionally, as of last month, about 58 per cent of Quebec’s 90,000 classrooms and learning spaces did not have mechanical ventilation.

The final important piece of the puzzle is masking, said Furness. Several provinces have lifted mask mandates for indoor public spaces, including schools, in recent weeks. Masking acts as an additional physical barrier to the spread of COVID-19 droplets, Furness said. It continues to be an important part of the solution to limiting the transmission of COVID-19 in closed, congregate settings and should be used in combination with ventilation and filtration, he said.

“It's ventilation, which is changing the air, filtration, which is scrubbing the air, and mask-wearing, which is source control [and] reducing what you're putting into the air – all three are needed,” Furness said.

But in the absence of these mandates, proper ventilation and filtration becomes even more crucial in closed spaces such as classrooms, Afshari said.

“We know when restrictions are lifted, the possibility of transmission goes up, that’s a fact,” he said.


It’s also key to ensure that each classroom has carbon dioxide monitors, Furness said. While not capable of measuring the risk of COVID-19 transmission specifically, these devices assess the amount of CO2 in a given space to determine how much exhaled air is in a room. Higher CO2 levels are a sign of lower ventilation, as more of the air a person breathes is shared by those around them. More often than not, it’s up to individual school boards to determine whether or not they will use CO2 monitors. This is the case in provinces such as Alberta, said Katherine Stavropoulos, spokesperson for Education Minister Adriana LaGrange. All new schools in the province, however, have been built with carbon dioxide monitoring, she said.

About 800 parts per million (ppm) is the level people should aim for when using carbon dioxide monitors, Fox said. At this level, about one per cent of the air is being shared by those in a given space, he said. If the ppms increase to 1,200, then two per cent of the air is shared, and so on. Carbon dioxide levels when standing outdoors are generally around 440 ppm, said Fox.

“The higher the CO2, the more exhaled air you are inhaling [and] the higher your risk of catching what may be dispensed in that room,” Furness said. “When those numbers get too high, we should be intervening on an urgent basis.”

Having this data would help reveal some of the patterns around the risk of transmission in schools, Furness said, whether factors pertain to the demographics of those in the building, or the configuration of the building itself.

“There's a bigger, longer term issue in terms of being smarter about how we design cognitive settings,” said Furness.

The goal is to build more concrete standards around indoor air quality, said Fox, particularly in the province of Ontario.

“[Air quality] needs to become part of our society and people need to start really being conscious about it,” said Fox. “This needs to be what everyone is always considering wherever they're going.”

In terms of short-term solutions, Furness advises HEPA filters in classrooms that don’t already have one, as well as reinstating mask mandates and providing students with N95 masks in particular. This is something school boards and governments should be working on together, he said.

Part of the solution also involves a shift in attitude towards the COVID-19 pandemic and spread of the virus in schools. Children continue to be susceptible to COVID-19, Furness said. Additionally, vaccination rates among children remain low in comparison to Canadian adults and even if children are vaccinated, similar to adults, this doesn’t completely protect them from infection, he said.

“We have a big problem with air quality in schools…especially with COVID,” said Furness. “It's fixable, we just have to do it.”

With files from Reuters, The Canadian Press.


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