Source control: Why face masks remain crucial to the fight against COVID-19

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Wearing a face mask in public has gone from eyebrow-raising to commonplace in Canada in the span of just 18 months – and, despite the wide availability of vaccines, remains a key tool in the fight against COVID-19.

But how, exactly, did health experts go from advising against mask-wearing at the outset of the pandemic to the current pro-masking guidelines?

James Scott

James Scott, a professor in the occupational and environmental health division at the University of Toronto’s Dalla Lana School of Public Health, says mask-wearing outside health-care environments – including industrial and military settings – was previously driven mainly by the need for personal protection (Hence, the name “personal protective equipment,” or PPE).

“That was really the thinking that underpinned the very early public-health statements that people don’t need to wear masks [during the COVID-19 pandemic],” says Scott, an expert in masks who is cross-appointed to the departments of laboratory medicine and pathobiology and clinical pharmacology and toxicology at the Temerty Faculty of Medicine.

“We had been – since the early 20th century – locked into this idea that what masks do is protect the people wearing them.”

As it turned out, masks proved to be a key method of COVID-19 “source control” by preventing people infected with COVID-19 from emitting potential infectious droplets that could infect others – a relatively new concept in public health circles.

Today, Scott says the evidence around masks’ effectiveness at reducing COVID-19 transmission is indisputable.

“Masks as source control are, by far, the most effective means of reducing transmission – through the air – of the virus that causes COVID-19,” he says.

While masks have long been used for source control in health-care settings – such as when a patient suspected of having a respiratory virus is masked in the hospital waiting room to protect others – the pandemic marks the first time masks have been deployed for this purpose in the wider community.

Susy Hota

“Particularly in North America, it’s not been standard practice to see people outside wearing masks, although it’s a little more common in some other parts of the world,” says Susy Hota, an associate professor in the department of medicine at U of T’s Temerty Faculty of Medicine and a medical director of infection prevention and control at University Health Network.

“We never applied it here prior to the pandemic because there was no scientific evidence that it was useful or necessary.”

Hota adds that the very first COVID-19 cases in Canada were acquired through travel. So, masks were only used for source control when people had symptoms of COVID-19 or were identified as high-risk due to their travel history.

But everything changed when it became clear the virus was infiltrating the community, and that people could transmit it despite being asymptomatic.

“This is when community use of masking became important,” says Hota. “It started in hospitals in March 2020 before becoming a part of municipal orders, and masking mandates in indoor spaces began to occur.”

That includes post-secondary institutions across Canada. At U of T, where campuses and classrooms are once again stirring to life, mask-wearing remains a key part of the university’s 12-step plan for a safe return right alongside a vaccination requirement.

U of T’s face mask policy requires that masks cover the nose, mouth and chin without gaps and should be made of at least two layers of tightly woven fabric and, if possible, a middle filter layer. Masks must be worn in all indoor university spaces including classrooms, labs, meeting rooms, office spaces, cafeterias, washrooms and common areas.

Certain learning and teaching environments may be exempt from mask requirements if instructors recommend an exception on the basis that masks would impair the effectiveness of the learning activity – for example, in musical instruction and drama performance.

Susan McCahan, U of T’s vice-provost of academic programs and innovations in undergraduate education, says mask-wearing is crucial to ensuring a smooth and safe return to in-person classes, labs and other academic activities.

“Teaching and learning is always a collaborative enterprise and especially so this fall,” McCahan says. “Wearing masks is an important step so we can collectively have a safe and productive return to campus.”

As scientific evidence around masks and mask-wearing has evolved over the course of the pandemic, so too have public health recommendations and U of T’s own policies. For example, in May 2020, U of T began offering two-layered non-medical masks to all students, faculty, staff and librarians. But these masks are no longer being distributed as they were made of a polyester material that doesn’t meet updated standards for fiber and thread count.

So, what makes a mask effective per the latest evidence and standards?

“You need something that’s a couple of layers of textile,” Scott says. “It needs to have a fairly tightly-woven or non-woven couple of layers. And the face piece needs to fit your face well.”

While textile masks don’t have the filtration capabilities of medical-grade N95 or KN95 masks, Scott says they’re still effective because they make it difficult for droplets to escape and spread because of a phenomenon known as impaction.

“It’s like if you take a carton of eggs and stand a few metres away from a chain link fence and throw the eggs one by one at the fence,” he says. “Even if the egg can fit through a hole in the chain link fence, you’ll find that you end up smashing a lot of eggs on the fence.

“So even if you have a fairly inefficient filter on your mask, you still get a lot of particle-capture by the fibres in that filter.”

Scott adds that while respiratory droplets are relatively large when first emitted, they become much smaller once they hit the air due to evaporation. Masks, he says, reduce the number of smaller, more transmissible particles by preventing the droplets from hitting the air in the first place.

“That particle – while still big and liquid – will hit the inside of the mask, and it doesn’t go on to be one of the fine particles that travels further. That’s why masks as source control are so effective.”

A third filter layer made from materials like polypropylene can provide incremental benefits, Scott says, since it can increase the amount of real estate for particles to land on while still maintaining low air resistance for comfortable breathing.

Hota notes that multi-layered cloth masks are also recommended for the general public because they’re easy to obtain and comfortable to wear, boosting compliance.

“We’ve seen across the board that doing something on a large scale is better than trying to apply a more effective type of mask on a smaller scale because people may not be able to tolerate something that’s less comfortable or may not be available everywhere,” Hota says. “That’s why masking mandates have allowed for different kinds of masks.”

That said, Hota stresses that it’s important that masks have multiple layers, the right type of fabric and weave and a proper fit.

“In terms of mask policies, it’s all about trying to make it as feasible as possible while using data and science to inform it,” she says.

The data and science also explain why mask-wearing continues to be important even as more Canadians are vaccinated and many institutions, including U of T, implement vaccine requirements.  

Ashleigh Tuite

“No single public health measure is perfect, including vaccination,” says Ashleigh Tuite, an infectious disease epidemiologist and assistant professor at the Dalla Lana School of Public Health.

“It’s not 100 per cent perfect at preventing infection. So, we need to have additional precautions in place in order to prevent transmission from happening. And masks are a piece of that puzzle in terms of providing additional protection.”

Tuite notes there are already several examples in the scientific literature of transmission by people who were either fully vaccinated or who recently tested negative for COVID-19. She cites an outbreak in Provincetown, Mass. – linked to Fourth of July events and celebrations – where around three-quarters of the cases were among fully vaccinated individuals. The outbreak prompted the U.S. Centers for Disease Control and Prevention to update its mask guidelines to recommend that vaccinated people wear masks indoors, according to CNN.

“It was a highly vaccinated population, but people still got infected – and those people could potentially go on to transmit to others,” says Tuite. “As much as the vaccines prevent against severe illness, you can potentially be infectious and spread infection to those who are unvaccinated or those who are vaccinated but are potentially immunocompromised so didn’t mount as strong of an immune response.”

Tuite points to the rise in cases and hospitalizations among children in parts of the U.S. have coincided with a pushback against mask-wearing. “Part of that could be prevented by mask-wearing,” she says. “So, the community derives a benefit from everybody wearing masks.

“It really is a matter of community-mindedness.”

Tuite says that while she recognizes that people may be tired of adhering to measures like mask-wearing, they’re a means of potentially protecting against more restrictive measures.

“The reality is that masks open up a lot of freedoms for us,” she says. “The alternative is that we start seeing uncontrolled growth and start talking about lockdowns again, which nobody wants.

“So, the idea of putting on a mask in public – if that allows you to continue to go out, see your friends and go to the gym – I think that’s a relatively small burden to ask people to bear in order to keep society open.”



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