Canada's chief public health officer now says that wearing a non-medical mask can help stop the spread of COVID-19.

Dr. Theresa Tam, the top doctor at the Public Health Agency of Canada, said today that Canadians can use non-medical masks in tandem with social distancing measures to limit the transmission of the deadly virus when out grocery shopping or at a pharmacy.

The recommendation represents an about-face for the public health officer — who until now has resisted the idea of non-health care professionals wearing masks.

"Wearing a non-medical mask is an additional measure that you can take to protect others around you," Tam said — while warning that a non-medical mask doesn't necessarily protect the person wearing it.

"A non-medical mask can reduce the chance of your respiratory droplets coming into contact with others or landing on surfaces," Tam said. "The science is not certain but we we need to do everything that we can and it seems a sensible thing to do."

Watch: Dr. Theresa Tam discusses non-medical mask use

Canada's Chief Public Health Officer Dr Theresa Tam spoke with reporters on Parliament Hill on Monday 2:21

Tam has long maintained that masks should be reserved for the sick among us, and for doctors and nurses working in hospitals, in order to preserve diminishing stockpiles of personal protective equipment.

'Emerging information'

Beyond supply concerns, Tam also has questioned the usefulness of masks for the general population in the past.

"It can sometimes make it worse, if the person puts their finger in their eye or touches their face under their mask," Tam said in January during a teleconference with reporters.

She has also said wearing a mask could give people a "false sense of security" that could encourage wearers to relax physical distancing.

On March 28, Tam said "most people haven't learned how to use masks" and "there is no need to use a mask for well people."

Speaking to reporters on March 30, Tam said that "putting a mask on an asymptomatic person is not beneficial."

She reiterated her concerns about the "potential negative aspects of wearing masks" — namely that they don't protect the wearer's eyes and could encourage people to touch their faces more as they fidget with the mask.

On Monday, however, Tam said wearing a mask is an added layer of protection that can help prevent pre-symptomatic and asymptomatic people from inadvertently infecting others with COVID-19. She said the policy change comes in response to "emerging information" from the science and medical community.

Watch: What you need to know about wearing a mask

Dr. Samir Gupta provides crucial information about wearing a mask during the COVID-19 pandemic and what to think about before putting one on. 2:30

Tam said new research on patients aboard the Diamond Princess cruise ship — as many as 712 people who were aboard the vessel contracted the virus — and a recently published report out of Singapore were behind the policy change.

"We are very rapidly trying to integrate that later science," she said of the new studies that suggest people who have yet to develop symptoms can still have high viral loads that can be transmitted.

She said Canadians shouldn't wear medical-grade masks like the N95, as those supplies have to be reserved for medical professionals.

She said Canadians could use t-shirts or cotton sheets and elastic bands to create makeshift masks at home.

The U.S. Centers for Disease Control and Protection (CDC) announced a similar policy shift over the weekend, saying that "new evidence" had led it to recommend that people wear cloth face coverings in public areas.

Watch: Dr Tam on making non-medical masks and who should wear them:

Canada's Chief Public Health Officer Dr Theresa Tam spoke with reporters on Parliament Hill, Monday. 2:22

"We now know from recent studies that a significant portion of individuals with coronavirus lack symptoms ("asymptomatic") and that even those who eventually develop symptoms ("pre-symptomatic") can transmit the virus to others before showing symptoms," the agency said in a statement on its website.

"This means that the virus can spread between people interacting in close proximity — for example, speaking, coughing, or sneezing — even if those people are not exhibiting symptoms."

The CDC is recommending masks in settings where "other social distancing measures are difficult to maintain," such as grocery stores and pharmacies.

Both the Canada and U.S. health agencies have recommended people avoid crowded places and keep a distance of at least two arms' lengths — approximately two metres — from others as much as possible.

WATCH | Can cloth masks protect you from COVID-19? Two doctors weigh in:

Doctors answer your questions about the coronavirus, including whether people should be wearing homemade or cloth masks when they are outside. 3:30

The Public Health Agency of Canada warned in a note on its website Monday that wearing a mask alone will not stop COVID-19.

"It is important to understand that non-medical masks have limitations and need to be used safely. You must consistently and strictly adhere to good hygiene and public health measures, including frequent hand washing and physical (social) distancing," the agency said.

If you choose to use a non-medical face mask, the agency recommends you wash your hands before putting it on and after taking it off.

Face masks can become contaminated on the outside or when they're touched by hands. The agency recommends that people avoid touching their face masks while wearing them and change their cloth masks as soon as they get damp or soiled.