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Chief Public Health Officer Theresa Tam responds to a question during a news conference in Ottawa, on April 6, 2020.

Adrian Wyld/The Canadian Press

Canada’s Chief Public Health Officer has new advice for Canadians: Wear a face mask to help cut down the spread of the novel coronavirus when you are in situations where you can’t always maintain proper physical distance from others.

The aim is to prevent transmission by people who are unknowingly infected with the virus.

Dr. Theresa Tam said this would help in scenarios such as shopping in the grocery store or riding public transit.

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“Canadians can take this additional measure [in] instances where social distancing is difficult to maintain,” she told a briefing in Ottawa on Monday.

She advised people not to use medical face masks because this gear should be reserved for health-care professionals.

“A non-medical mask can reduce the chance of your respiratory droplets coming into contact with others or land[ing] on surfaces,” Dr. Tam said.

She offered advice on how to fashion a non-medical mask, saying Canadians could use a cotton shirt for instance, combined with rubber bands.

“We’re talking about things that are readily available in people’s homes,” she said. “Things like cotton shirts, sheets, bandanas.”

“If you can get a cotton material like a T-shirt, you cut it up, you fold it and put elastic bands around it – this is a non-medical facial covering.”

This new direction from Canada comes several days after a similar message from U.S. authorities.

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Dr. Tam said this measure is now being advised to help cut down on transmission by Canadians who are infected by the virus but are asymptomatic or presymptomatic.

“Our collective scientific knowledge of COVID-19 continues to grow,” she said.

“Now that more countries have had a larger number of cases … it is clear that transmission of the virus is happening more often than previously recognized from infected people right before they develop symptoms,” she said. “This is called presymptomatic transmission.”

She said there is also evidence that people who never develop symptoms, the asymptomatic, are also able to transmit the virus.

The announcement Monday represents a new decision from medical experts in Canada based on the latest studies.

“With this emerging information, the special advisory committee on COVID-19 has come to a consensus that wearing a non-medical mask, even if you have no symptoms, is an additional measure that you can take to protect others around you in situations where physical distancing is difficult to maintain,” Dr. Tam said.

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Public-health officials were pressed Monday to explain why they’re only advising makeshift masks now, a number of weeks after Canada began fighting the pandemic.

Dr. Tam said the evidence concerning presymptomatic and asymptomatic illnesses only came to light in the last 10 days or so.

“These are very recent studies … so we very rapidly try to integrate that latest science,” she told reporters.

Dr. Tam said public-health officials in Canada still don’t know exactly how important transmission from presymptomatic or asymptomatic cases is in driving this outbreak.

Study of presymptomatic cases shows "just before they develop symptoms, [people] have high viral loads,” she said. “And so that tells us the possibility is there they can transmit.”

The Chief Public Health Officer emphasized this new advice does not confer better protection on the mask user, but is advised to protect others.

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She said this advice is a “permissive statement” offered to help reduce transmission and does not constitute an official recommendation. She said more information would be made available on the federal government’s website.

Canada’s advice comes three days after the U.S. Centers for Disease Control and Prevention recommended that Americans should wear a cloth face mask when in public.

Mario Possamai, who was a senior adviser to the Ontario government’s SARS Commission, said evidence has been mounting that countries where wearing masks in public is universal – such as China, Hong Kong and South Korea – have been more effective in containing the pandemic. He said the new advice is overdue.

He says, however, that concurrent with Dr. Tam’s announcement, there should be robust public messaging and training from Ottawa to demonstrate how to wear masks safely because, as Dr. Tam has herself pointed out, people could jeopardize their health by self-contamination if they don’t know how to safely wear face coverings.

“You can’t issue a strong signal as Dr. Tam has that, in effect, endorses a practice like public masking without telling people how to do so safely,” Mr. Possami said.

“We in Canada have never been in a situation since the Spanish flu where there is a recommendation for the universal wearing of masks.”

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The federal government has been talking to American officials after reports that a shipment of millions of N95 masks bound for Ontario was held at the U.S.-Canada border, Prime Minister Justin Trudeau says. The Canadian Press

Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters.

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