Why an epidemiologist says wearing masks in indoor public spaces should be mandatory

Hamilton's board of health may vote on Friday to make the wearing of masks mandatory inside indoor public spaces. Dr. Catherine Clase, an associate professor of medicine at McMaster University and a nephrologist at St. Joseph's Healthcare Hamilton, explains below why she thinks it's a good idea.

Dr. Clase spoke with us Thursday July 9 at noon live on Facebook about the science behind how effective masks are against COVID-19. She also studied the effectiveness of masks in fighting disease over 100 years.

Read the edited and abridged transcript below or hit play above and watch the entire interview with the CBC'sConrad Collaco.

Dr. Catherine Clase, nephrologist, epidemiologist

Should masks be mandatory in public indoor spaces?

My feelings about this and my thoughts about this have evolved over the last couple of weeks. Initially, I was very hopeful that voluntary mask-wearing would be adequate and would get mask use up to where it needs to be. But, I think that for some people the fact that it's voluntary is misinterpreted as a lack of a clear signal. And what that means is that I think it will probably be simpler and effective for it to become mandatory. I'm hoping that will be the decision.

How effective are masks?

We're very convinced that they are effective for preventing transmission of a whole variety of pathogens.If we're managing somebody with COVID and we're doing something that's definitely going to generate what we call aerosols tiny little particles then we want N95masks and we have enough of those. And the reason we have enough of those is because we're not using it for other purposes. In the hospital we're wearing surgical maskswhich are the same as a medical mask. It's a disposable mask that is often blue in colour. When youlook at the material that those are made from that material is going to filter at least 95 percent of the aerosolsized particles.I would say my best guess is the cloth mask is giving you a50 percent reduction, that makes it half your risk.

That's really well worth having but there's something even better. When we look at naturalistic studiesthat were done from the 60s and 70s, we know that in those studies, looking at mouthbacteria as the marker in those studies when you look at all the bacteria it's 99 percent prevented from getting into the environment and about 90 percent of those that are in the aerosol sized particles. So, when you look at that kind of naturalistic experiment where you're looking at mouthbacteria contaminating the environment we're probably up in the 90s rather than the 50s. So my mask is preventing more than 90 percent contamination of the environment and your mask is halving the risk beyond that. So for those wearing masks we really have some synergy going.

What about people with physical limitations that would prevent them from wearing a mask?

This is one of those issues where we absolutely have to be kind and compassionate to each other. I think there is a kind of Canadian consciousness which is fairly rules-abiding. So, I think if we have mandatory mask requirements for those who can tolerate it. What I'm hoping is that Canadians will recognize that those who are not wearing masks are probably not wearing them because they can't. And we need to be kind about that.

And if all of us who can wear a mask wear a mask we'll be up above 95 percent. We'll we'll be fine and we'll be protecting the people who can't wear a mask. If you have to go to the grocery store and you can't wear a mask, that's very scary if nobody else is wearing one. But if you're the only one there without a mask I think that's absolutely fine.

Single use disposable masks or reusable masks?

When you think about it, if every Canadian is going to need two disposable masks a weekoverthe next year, that's three billion disposable masks. So, that doesn't sound feasible. The other thing that we need to do is what we set out to do at the beginning of the pandemic which is to keep the disposable masks for the people who need them, those health care workers andother high risk occupations like funeral directors and police officers.

We need to use reusable PPE and whether that's a mask that you buy from somebody or whether it's a mask that you make yourself,I think we need to make and have these masks be reusable and that means that they need to be made of cloth and then that comes down tojust being aware of how wedo that. We want the mask to be at least two layers, maybe three layers see what you can tolerate. And we want to we want to make sure that we wash that mask every time that we use it. So, you have to take it off temporarily. You're going to fold it dirty side to dirty side and put it in a brown paper bag and then when you put it back on you're going to do hand hygiene. You're going to consider the outside surface of the mask contaminated and if you do touch it inadvertently you're going to do hand hygiene. And then, at the end of the day, when you're back home and you're in your safe space you're going to put the mask in the washing machine and wash it on a warm or hot cycle.

What precautions you would like to see in the classroom?Should in-person school return in the fall?

I think we don't know what best to do with classrooms. There's some limited evidence that children don't transmitas much as adults, just exactly the opposite of influenza. So, it is possible that it will be reasonable to return to classrooms in the fall. But I would say that children in classrooms, if they can tolerate masks, should be wearing masks because we know that they are likely to be impulsive andhave difficulty maintaining their social distance and they're going to be sharing the same space.

I think what's really important is that we go slowly and that we allow ourselves time to see what's happening with every step that we take. And every time we make a change and we do some kind of easing it takes about a month before you really see that coming to school infection rate. So we need to go slowly whatever we do.

How should a mandatory masks policy be enforced?

Again, I think very gradually and with kindness and discretion. So,I think that the most important thing is to give people some time to get used to the idea. Some time to see that compliance is generally risingif you're wondering whether you really need to do this. I think if those of us who are completely convinced make sure that we adopt it 100 percent then I think that it would become a societal norm. And obviously any enforcement has to take into account the people who can't wear masks as well as the people who are reluctant to wear masks. We also need to recognize that some peoplemay need a little time to get themselves organized with masks.

What do you say to the people who are angry and don't want to be brought in to the public response requiring mandatory masks.

I would say that I completely sympathize with that and, in fact, if you'd asked me a week or 10 days ago I was really not convinced that masks needed to be made mandatory... but in April mask wearing was at 41 percent and in the latest YouGov poll from, I think the 11th of June, mask wearing in Canada was at 58 percent. This is people self-reporting what they usually do. That's just not high enough. So, I would say I'm very sympathetic to people who would have preferred it not to be mandatory. I would have preferred it not to be mandatory as well. But it doesn't look as if we're getting thereas a society and I think we really do need to get up into that 95percent range.