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Caroline Duchaine has spent months hunting for traces of microscopic COVID-19 particles in far-flung corners of infected patients’ rooms, hoping to answer critical questions about whether the coronavirus could pose an airborne transmission risk.

The findings, which Dr. Duchaine and her colleagues expect to publish soon, will help researchers understand how far viral particles may travel from a patient. But they won’t reveal whether those particles contain enough viral material to make people sick.

That question is much more difficult to answer. It’s also at the heart of a years-long debate in the scientific and medical community about whether certain respiratory viruses are capable of remaining suspended in the air for hours at a time, spreading to people who are nowhere near the infected patient. It’s an important issue with significant implications for public-health strategies designed to prevent transmission of COVID-19.

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The debate flared up again this week, after a group of 239 scientists from around the world, including Dr. Duchaine and nine other Canadians, signed a letter asking the World Health Organization and other health bodies to recognize the potential for airborne spread.

In response to the letter, the WHO said this week it may be possible for airborne transmission to occur under certain conditions and that it would be releasing a new briefing document on the subject.

New WHO guidance calls for more evidence on airborne transmission of coronavirus

The term “airborne” doesn’t simply mean that a virus can travel through the air through coughs, sneezes or regular conversation. That type of spread is called droplet transmission and it’s how the vast majority of people become infected with COVID-19.

Airborne transmission refers to a process under which viral particles become aerosolized, meaning they are small and light enough to be suspended in the air for hours. Measles, for instance, is a classic example of an airborne illness. It’s so contagious that if an unvaccinated individual enters the room where an infected person was an hour or two earlier, that person can get it.

The Public Health Agency of Canada and provincial health authorities are focused on preventing the droplet-based spread of COVID-19. That’s why people are being urged to stay two metres apart and wear masks if they can’t.

But should they be putting more resources into stopping the potential airborne transmission of COVID-19? The experts who signed the letter, published Monday in the journal Clinical Infectious Diseases, say yes – to a degree. Media coverage of the letter in The New York Times and other publications says the scientists believe the potential risks are great and that the WHO is trying to play down the potential of COVID-19 airborne transmission.

Dr. Duchaine framed the argument much differently. She said health authorities in Canada are doing a “great job” and that she agrees with their guidance: Most of COVID-19 transmission is droplet-based, she said, but it’s possible, under certain circumstances, for the virus to become airborne. So she hopes health officials start to think about how to improve ventilation in schools, offices and other public buildings.

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“We all knew there would be places with zero ventilation and lots of positive patients,” Dr. Duchaine said. “Air may be one part of that [transmission].”

David Miller, a professor and toxicologist at Carleton University in Ottawa, said the point of the letter isn’t to change the classification of COVID-19 to an airborne disease, which would require health care workers to use different personal protective equipment when caring for infected patients. It’s simply to highlight the importance of improving airflow in buildings and ensure other measures, such as masks, are used.

“This really refers to the open workplace … going back into office buildings and other structures,” he said.

Several infectious disease experts who spoke to The Globe and Mail agree with that assessment: It’s possible for airborne transmission to occur, but the risk is small. However, it’s also a good idea to consider how to improve ventilation indoors.

Isaac Bogoch, an infectious diseases physician at Toronto General Hospital, said with any virus there’s a spectrum of transmission risk. With COVID-19, most of the risk falls on the droplet end of the spectrum, but there is a chance airborne transmission may occur under certain circumstances.

But some of them are concerned that discussion of the letter is venturing from a measured conversation about airflow in buildings to a polarizing discussion creating unnecessary fear in the public about the ability of COVID-19 to spread through the air.

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Many of the signatories are experts with backgrounds in engineering, chemistry and the environment and they point to evidence of studies that demonstrate the airborne presence. Members of Canada’s infectious disease community take issue with the letter, saying there’s a major difference between laboratory studies that demonstrate a theoretical possibility of airborne spread and the real world, where months of experience with COVID-19 have proven the majority of cases occur through close contact with an infected person.

“Although we recognize airborne transmission can occur, the fact of the matter, the epidemiology of this disease, tells us it is not a major contributor to transmission of the virus around the world,” said Gerald Evans, chair of the infectious diseases division at Queen’s University in Kingston.

Dr. Evans said that after The New York Times article on the letter was published last weekend, he had to reassure nervous hospital employees that the current precautions in place are enough to protect them from COVID-19.

Janine McCready, an infectious diseases physician at Toronto’s Michael Garron Hospital, said she agrees that it’s important to think about better ventilation of public buildings and encouraging people to wear masks when they’re in enclosed spaces.

But public-health authorities should not classify COVID-19 as an airborne disease, she said, because it’s not what the evidence shows. Dr. McCready pointed out that she has had close contact with about 150 COVID-19 patients while wearing protective equipment designed to prevent droplet-based spread, including a surgical mask. She hasn’t contracted the illness.

“If those precautions didn’t work, we would see vastly higher numbers in our health care workers,” Dr. McCready said. “We have epidemiological real-world experience.”

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If airborne transmission occurred readily, the number of people who catch the disease from an infected individual would be much higher, Dr. Evans said. On average, COVID-19 patients will spread the illness to two or three others. With measles, individuals can infect up to 18 people, he said.

Airborne transmission of COVID-19 would also mean there would be many more cases around the world, such as among airline passengers. A study published this week in the journal Travel Medicine and Infectious Disease provides more evidence that close contact is behind most new cases. The study looked at a January flight from Singapore to Hanghzou, China, that was carrying 16 passengers who were later diagnosed with COVID-19. Based on questionnaires answered by the passengers, the researchers found that only one person became infected on the plane: a man who was seated near four infected individuals. He wasn’t wearing his face mask properly.

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