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A man wears ski goggles, gloves and clothing while taking the subway during the global outbreak of coronavirus disease (COVID-19) in Toronto on March 31, 2020.CHRIS HELGREN/Reuters

Anyone who lives in Canada should have an intuitive sense of why the battle to reduce the spread of COVID-19 is so dependent on everyone keeping their distance from each other.

Just picture a group of people standing at a bus stop on a typical winter morning, every breath forming a puff of fog in the frigid air. Now move the scene indoors and imagine those same overlapping puffs made visible as people exhale and speak in close proximity. For a respiratory virus, the situation presents a golden opportunity to jump from one host to the next.

“Because you breathe all the time ... the amount of virus that gets in the air is overwhelmingly greater from you breathing than from coughing or sneezing," said Allison McGeer, an infectious disease researcher at Mount Sinai Hospital in Toronto.

This reality is why the U.S. is now reportedly considering a recommendation that people cover their mouths with masks, scarves or bandanas when they enter stores and other settings where they may be in close proximity with others. The point of the covering is not to keep the virus out, but to help keep it in, or at least keep it close. (The effectiveness of the strategy may vary greatly depending on the situation.)

What Dr. McGeer and her colleagues would like to know is how long that exhaled virus survives in the air and how easily it can infect others.

Growing evidence that people who are not even aware they are infected can transmit the virus highlights the importance of the two-metre rule for physical distance. The wide berth eliminates the possibility of making direct contact with someone who is infected. It is also out of range of liquid droplets that fly outward like tiny virus-containing cannonballs when someone coughs, sneezes or expels saliva in the act of speaking.

What is less certain is what happens when the virus becomes truly airborne and travels even farther. In that case, a droplet might be small enough for the water it contains to evaporate in mid-air, leaving behind a microscopic residue called an aerosol, which is light enough to be buoyed by air currents.

Last week, researchers at the University of Nebraska Medical Center posted results that showed they were able to detect airborne traces of COVID-19 genetic material at a distance greater than two metres from the source. However, the work has not been peer reviewed, and the researchers stressed that they have not demonstrated whether the virus can remain viable under such conditions.

A study published last month in the New England Journal of Medicine suggested that a virus could survive after hitching a ride on an aerosol, but it is not clear how likely this is outside a laboratory setting.

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“It may happen but it’s certainly not the main route of infection that the virus will use to spread,” said Caroline Duchaine, a professor at Laval University who specializes in biological aerosols.

The reason, she said, is that as it dries out in the air, the virus can lose its spiky, protective envelope. Its genetic contents may endure, but they are no longer packaged in a form that allows it to infect a cell. She added that if COVID-19 were primarily spread through the air, its infection pattern would more closely resemble measles, a highly contagious virus that is known to survive in aerosol form.

Other researchers say that the jury is still out on how much the new coronavirus can spread through aerosols.

On Wednesday, a German-led research team presented evidence from nine patients who developed mild cases of COVID-19. The patients had upper respiratory infections and shed large quantities of virus during the first week. The study, published in the journal Nature, may help explain why the COVID-19 pandemic is playing out differently than the SARS outbreak of 2003, despite the similarities between the two viruses.

“I assume a high potential of this virus to spread via aerosols,” said Clemens-Martin Wendtner, a co-author on the study and director of infectious diseases and tropical medicine at Klinikum Schwabing, a hospital in Munich.

“We could detect virus in patients’ rooms," he added. "Right now virus culture experiments are ongoing to prove or disprove that virus is infectious.”

In Canada, Dr. Duchaine is teaming up with Dr. McGeer and other collaborators to study how the virus fares in airborne form, particularly in a hospital setting, such as when a patient with a severe case of COVID-19 is being intubated. But the results, which the researchers hope to have in hand within a few weeks, may also shed light on the question of whether masks may be a useful safeguard for reducing transmission after the initial wave of the pandemic peaks.

Dr. McGeer stressed that because of limited supplies in Canada and the urgent need for protective gear in hospitals, any discussion of mask use (apart from what people may fashion for themselves) should prioritize those working with infected patients.

“We just don’t have a good projection of what’s going to happen in May,” she said. “It would be really awful if we all used up masks in the community now and didn’t have any left for health-care workers."

Christopher Mio and Meghan Hoople found themselves jobless and wanting to help in the wake of COVID-19 isolation in Toronto. After flyering their neighbourhood with a free-of-charge offer, they received an outpouring of support and requests from people in need.

The Globe and Mail

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