Open this photo in gallery A man wears face mask as he waits for transit in Calgary on March 18, 2020. Jeff McIntosh/The Canadian Press

In the coming days – perhaps hours – the U.S. Centers for Disease Control and Prevention is expected to recommend that all Americans wear face coverings in public to help reduce the spread of coronavirus.

This change in policy by the largest public-health agency in the world, and bolstered by the U.S. coronavirus task force, will mark a seismic shift in expert opinion in the Western world.

Outside Asia, where mask-wearing is the norm, the accepted wisdom has long been that masks aren’t that useful for ordinary folk and should be reserved for medical personnel and first responders.

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So should we all rush out and buy masks?

Absolutely not.

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First, we have to consider the fine print. The CDC will not recommend that the general public wear surgical masks or N95 masks with sensitive filters. What they are considering telling people is to make their own makeshift face coverings with bandannas, scarves and the like.

Surgical masks are still in desperately short supply and must be reserved for medical personnel and high-risk patients such as those who are immunocompromised.

There is loud and growing grassroots movement promoting mask-wearing. It’s driven largely by a fearful public’s desire to have some control over their fate, and that’s understandable. If you have an old T-shirt, some elastic and a sewing machine, have at it.

It seems self-evident that covering your mouth, whether it’s by coughing into your elbow or tying a bandana around your mouth, is better than not covering your mouth.

But there are some subtleties to consider.

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Coronavirus, like other respiratory viruses, is spread by droplets, the spittle that people express when they cough, sneeze or talk.

Those viral droplets can infect us by direct hits – being physically close to someone who coughs.

Technical arguments about cough droplet ballistics aside, if you practise social distancing – staying at least two metres from other people – a mask provides little additional protection.

We can’t make the mistake of believing that makeshift masks will block all pathogens, some of which can only be measured in microns. They won’t.

Even if you avoid all people who cough and sneeze, you can still pick up coronavirus from things you touch, and then risk infection by putting fingers in your mouth or nose.

The downside of a mask is that it can actually lead you to touch your face more often, as you adjust it and pull it on and off to eat.

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In other words, it can give you a false sense of security. Public-health officials have been driving home that message for weeks.

"There is no specific evidence to suggest that the wearing of masks by the mass population has any potential benefit,” Dr. Mike Ryan, executive director of the WHO health emergencies program, said on Monday.

But, as an international group of researchers wrote recently in the medical journal The Lancet: “There is an essential distinction between absence of evidence and evidence of absence.”

What they mean is that, while evidence that face masks provide effective protection against respiratory infections is scant, it doesn’t mean there is any harm in people wearing them – as long as they don’t pilfer from health workers.

If you’re going to wear a mask – and every indication is this is going to be increasingly common in the weeks to come, especially as the number of cases soars in the U.S. – then you have to wear it properly.

A mask needs to cover both the mouth and the nose. It needs to be snug, so that there’s a seal. The biggest mistake people make is having a loose-fitting mask.

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At the same time, it can’t be suffocating. You have to be able to breathe normally.

It’s also important that a homemade mask be moisture-resistant, washable and washed regularly. That’s why most do-it-yourselfers are using cotton T-shirts.

And, as always, be wary of stuff sold by online hucksters, especially masks that purport to be N95 standard.

Finally, wearing a mask may be helpful when you go outside, especially to places such as the grocery store or pharmacy. But it can’t be an excuse to ignore measures such as social distancing and self-isolation.

Mask-wearing must be complementary, not a substitute, to other actions.

In short, the modified pandemic advice is essentially: a mask if you feel it necessary, but not necessarily a mask.

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