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A empty teacher's desk is pictured in an empty classroom at Mcgee Secondary school in Vancouver on Sept. 5, 2014. Guidelines recently issued by the Public Health Agency suggest that schools should move activities outdoors 'where possible,' open windows, 'ensure that the ventilation system operates properly' and adjust ventilation systems to increase air exchanges.JONATHAN HAYWARD/The Canadian Press

Students go back to school in less than a month. While many boards have been focusing on reduced class sizes, personal protective equipment and hand-washing, one source of COVID-19 threat has been overlooked: the air.

There is growing concern that the virus can be transmitted through air over longer distances. This could have huge implications for how buildings – including schools – function. And to address the risk demands checking and upgrading ventilation systems. That’s a complex and expensive exercise in building science, made worse by the run-down state of many Canadian public schools.

It’s time to get started.

COVID-19 spreads most easily through what scientists call droplets. When an infected person breathes out, that air contains moisture that can carry viral particles to other people. But once those particles dry out, they can be aerosolized, hang in the air and travel long distances.

This is a common means of transmission for some diseases, such as measles. COVID-19 may travel this way as well. In May, 36 scientists published a multidisciplinary paper on the subject, calling for “engineering controls” to address the problem. And in July, 239 scientists asked the World Health Organization to look into the risk of airborne transmission. That risk appears small; it is not zero.

Most COVID-19 spread has been indoors. Some may have been via aerosols. After all, most buildings in North America have streams of air that recirculate, around and around, being diluted by some intake of fresh air. If the air is not diluted enough, or filtered appropriately, COVID-19 could travel this way. The likelihood increases if you have more people crowded into a space – such as a classroom.

Guidelines recently issued by the Public Health Agency suggest that schools should move activities outdoors “where possible,” open windows, “ensure that the ventilation system operates properly” and adjust ventilation systems to increase air exchanges.

In short: More fresh air is better. A higher level of air filtration helps, ideally with a HEPA filter. That means changing air filters, using more effective HEPA filters if possible, maintaining and repairing ventilation equipment, and opening windows where possible.

I spoke to four mechanical engineers with relevant experience, and their consensus was that this is a knotty problem for schools. Details will vary from building to building. Some older schools have “exhaust-only” systems, where all the fresh air comes through windows. (If they still open, that is.) Some, from the 1970s and 1980s, have systems that take in only small amounts of fresh air.

Thus far, Canadian schools have made no decisive effort to test and repair systems. Why? “There is an almost total lack of direction,” said engineer Ian Jarvis of Enerlife Consulting, who has worked extensively with schools on energy-efficiency projects. “Nobody knows exactly what needs to be done.”

Justin Downey, a mechanical engineer with the consulting firm RWDI, said the basic goal should be to maximize fresh air. “The question is, how do I make sure that I’ve got as much air flow as possible, and it’s providing dilution to as much of the room as possible?” When renovation projects are not possible, using in-room air-cleaners can help, he said – together with open windows.

Spot testing can also provide some answers about the presence of the virus. Mr. Downey’s firm is advising its clients on a technology called polymerase chain reaction (PCR). Now used by Canadian health authorities for COVID-19 screening in patients, it can also be used to spot the virus within the air in a room and it is used in facilities such as food-processing plants. Appropriate air-testing devices can be purchased at a cost of tens of thousands of dollars, he said, and then offer results within 90 minutes. “It can provide an early warning sign,” he said. “This should be a huge part of the response in the short term.”

Mr. Jarvis suggests that every school building should undergo a thorough inspection of its ventilation systems to see whether they are functioning as designed, and the rate at which they exchange air.

A large-scale upgrade of these systems would also save school boards money in the long run, he said, since their ventilation equipment often is poorly maintained or improperly set. “It’s a huge opportunity for utility cost savings,” he said, “and to address climate change” by reducing the emissions from schools. He suggested that federal funding for climate-change mitigation could be applied to such projects.

But any audit could turn up ugly results. Many schools are simply falling apart. This is particularly true in Ontario, where 20 years of provincial governments have been “deferring maintenance” – in other words, not fixing things – but elsewhere, too.

That problem won’t go away on its own. And COVID-19 isn’t disappearing any time soon. A nationwide program of improving ventilation systems could save many lives. And if not, this work would have other health and environmental benefits – and, in many cases, has to get done anyway. Why not start now?

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Editor’s note: (Aug. 13, 2020) An earlier version of this article included an incorrect description of PCR.