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Samira Mokenan stands on the balcony of her apartment at Rebecca Towers in Hamilton, Ontario. The building has been struggling with a serious COVID-19 outbreak, with 110 cases.

Peter Power/The Globe and Mail

For months, Samira Mokenan has avoided doing her laundry on the weekends when the laundry room of her building is busiest. She and her family used the stairs to their sixth-floor apartment, instead of taking the elevator. And whenever they went outside, she would line her young children in a row, sandwiched between her and her husband, so the little ones never touched the doors.

But since receiving a public-health notice of a COVID-19 outbreak in her building this month, Ms. Mokenan said she and her four children have stayed inside their unit, taking greater pains to avoid contact with other tenants, common areas and shared surfaces. Her husband is the only one who leaves home.

“We’re living in a horror,” Ms. Mokenan said, explaining when she considers how many of her neighbours have had COVID-19, “You imagine: How many units? Which floor? It’s just scary, you know?”

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Her building, the Rebecca Towers, in Hamilton, where 110 COVID-19 cases including one death have been reported, is among a number of outbreaks in high-rises, and experts are trying to figure out how residents have been contracting the virus. In some of these outbreaks, local health officials have said it’s not about the buildings themselves, but rather social interactions and transmission among members of the same household. Still, some experts suggest the virus may spread between units in older buildings through ventilation or air transfer.

Energy engineer David Elfstrom, who contributed to the investigation of a North Bay apartment outbreak, said he believes more apartment dwellers may be sickened from the migration of aerosolized virus through corridors and other units than public-health officials recognize.

Officials aren’t always able to keep up with contact tracing and identifying multiple cases within buildings, he said, and they aren’t always investigating building ventilation and airflow as a source of transmission.

“They’re following the idea that, well, if there’s a physical separation between units, then they’re not going to look,” Mr. Elfstrom said.

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Findings that suggest COVID-19 can be spread through air ducts and airflow have been documented in building outbreaks in other parts of the world. David Fisman, an epidemiologist at the University of Toronto’s Dalla Lana School of Public Health, pointed to a study in the International Journal of Infectious Diseases that found a COVID-19 outbreak occurred along two vertical lines of a South Korea apartment building, each connected by an air duct.

He also noted media reports that guests of an Australian quarantine hotel were infected with a genetically identical SARS-CoV-2 virus, despite never coming in close contact.

None of these observations make sense without aerosol transmission of the virus, which in animal experiments have been shown to be able to “turn corners” in air ducts, Dr. Fisman said.

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At Rebecca Towers, resident Nicole Morris said a friend in the building and her husband both caught COVID-19 but don’t know how. The couple had a health care worker who visited them, but they rarely left home themselves, she said. The friend is now in hospital, and the husband died.

Ms. Morris said many have offered different ideas about how her neighbours have been getting sick, including that air quality in the building is “quite a big concern.”

The potential for transmission between units should come as no surprise, said James Scott, professor of occupational and environmental health at the Dalla Lana School of Public Health.

A group of community volunteers who raised over $14,000 in four days were at Rebecca Towers on Saturday morning delivering fresh groceries, dry goods, pet supplies, and PPE to residents.

Peter Power/The Globe and Mail

During the 2003 SARS outbreak, early cases at a Hong Kong housing estate were tied to fecal aerosol transmission – that is, infected sewage particles in the air – via air movement in the building’s soil stacks, he said, and the current coronavirus is a “very close cousin.”

Mr. Elfstrom said since Canadian apartments don’t have floor drains like those in parts of Asia, this mode of fecal aerosol transmission, which happens when U-traps dry out, is unlikely to be widespread here. A more likely route is through air transfer in older buildings, he said.

In newer apartment buildings, which require firestopping to prevent the spread of fire, there are usually individual ventilation systems or exhaust systems in each suite, Mr. Elfstrom said. But those constructed in the 1970s and earlier tend to have a centralized system, typically located on the roof, that delivers air down through corridors and under the doors of each suite through positive pressure, he said. Air can also move between units through holes around plumbing in cabinets and under sinks, he added.

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At Rebecca Towers, however, Hamilton Public Health Services said an engineer found the ventilation system was in working order and capable of preventing cross-contamination between suites. The plumbing stacks were also sealed off from the units, it said.

In an e-mail, it said in two other apartment outbreaks in the city, COVID-19 was acquired from members of the same household in two-thirds of the cases, while those among the other third had no known exposure, which it said is consistent with trends seen in the community.

Meanwhile, in a London, Ont., apartment outbreak in December, where some 46 cases were reported, Alex Summers, associate medical officer of health with the Middlesex-London Health Unit, said officials did not see any evidence that residents had caught the virus through heating and ventilation systems; rather they seemed to have done so through social interactions.

“Transmission appeared to be driven in the same way that COVID consistently has been transmitted, which is close contact of individuals, particularly within their social environment,” he said.

Nevertheless, Dr. Scott recommended keeping two windows open, with a box fan in front of one of them, blowing air into the unit. This builds positive pressure in the unit to resist air movement coming from the corridor, he said. He also suggested using a standalone air purifier and closing toilet lids before flushing, which reduces aerosol generation within a unit.

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