The face mask question is more complicated than it might at first seem, explains Siouxsie Wiles.

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Maybe you’ve seen the graph that says those countries where everyone wears a mask are the ones that have managed to keep Covid-19 under control? The first thing to say about that claim is that those countries also did lots of other things, too – they acted fast, with intense testing, contact tracing and quarantine.

The reality is, when it comes to wearing a face mask there are pros and cons that differ depending on where in the world you are. I’ll get to these shortly. But this is a complicated one. Even the experts can’t agree on whether everyone should be wearing one. That’s raised accusations of racism as well as given life to several conspiracy theories. Face masks have become such a heated topic over the last few weeks that I’ve personally been harassed and threatened over it.

Until now, the WHO has recommended that if you are healthy, you need only to wear a mask if you are taking care of a person with suspected Covid-19. They also recommend wearing a mask if you are coughing or sneezing. The WHO has now convened a group to review new evidence to see whether or not they should update those recommendations. Which is the right thing to do.

The different kinds of face masks

There are lots of different types of face masks. The three you have probably heard mentioned are N95 respirators, surgical masks, and cloth or home-made masks.

N95s are designed to protect the wearer from breathing in or swallowing droplets and very small particles – like bacteria and viruses. To work, N95s need to form a tight seal around your face. That means first having a “fit test” to ensure the mask forms a proper seal. If you’ve a beard, forget it. Because N95s are designed to form a tight seal they can become very uncomfortable as heat and humidity build up. As with all masks, people need training to put them on and off without contaminating themselves or the mask. Importantly, N95s are needed by healthcare workers who are caring for people with Covid-19 and have the biggest risk of getting infected. They form one part of their PPE – personal protective equipment.

Surgical masks are disposable, loose-fitting face masks that cover your nose, mouth and chin. They’re mainly used to protect the wearer from sprays and splashes, or to stop the wearer from spreading their respiratory secretions to others – like a patient during surgery. Cloth masks are homemade versions of surgical masks made out of whatever you have to hand.

Will surgical and cloth masks help stop the spread of Covid-19?

Because of what they are made of and how they fit, surgical and cloth masks are unlikely to stop you catching Covid-19. But the question everyone is asking is: do they play a role in stopping the spread of the virus? The prevention of spread has become especially important as understanding of how the virus transmits has changed.

At first it was thought the virus was spread through droplets from people with symptoms. Now it’s understood that people can shed the virus even before they have symptoms. And some people may shed the virus without ever developing symptoms. Precisely how many people do this isn’t known. There is now also a worry that people can spread the virus through talking and singing.

This is why physical distancing is so important and why the distance we need to stay away from other people has changed. First it was one metre. Now it’s two. A new study might see that revised again. Dr Lydia Bourouiba is an associate professor at the Massachusetts Institute of Technology in the US. She runs the very appropriately named Fluid Dynamics of Disease Transmission Laboratory and used a high-speed camera to film a healthy person coughing and sneezing. Some of the droplets from a vigorous sneeze travelled as far as seven or eight metres. So, if you are someone who fancies having a glass of wine with neighbours from two metres away, don’t. Do it online instead.

But back to the original question: can masks play a role in stopping the spread of Covid-19? It’s too early for studies yet about this particular coronavirus but a study on influenza has just been published in the journal Emerging Infectious Diseases. The authors looked at the data from the 10 randomised controlled trials and showed overall, wearing surgical masks didn’t do anything to reduce the spread of influenza.

On the other hand, an analysis of SARS trials, which is caused by a related coronavirus, did show some effect. Overall, one infection was prevented for every six people who wore a surgical mask. That said, hand-washing was much more effective. But it doesn’t mean we can’t do both, right?

In the last few days, another study has just been published that looked at whether surgical face masks could prevent transmission of human coronaviruses (though not Covid-19) and influenza viruses from people with symptoms. They collected the breath of 123 people who they knew had influenza (43 people) or the rhinoviruses (54 people) and coronaviruses (17 people) that cause the common cold. Some of these people were randomly allocated to wear a face mask when the researchers collected their breath. The rest didn’t wear a mask. The researchers then looked for viruses in everyone’s exhaled breath. The first thing to note is that the number of people in the study is small. With that in mind, they were only able to detect coronavirus from roughly four out of 10 people when they weren’t wearing a surgical face mask. If the Covid-19 coronavirus behaves in a similar way, then this study suggests not everyone will be shedding virus every time they breathe out. But the even better news is that they weren’t able to detect any virus when people were wearing a surgical mask.

Why I’ve been reluctant to recommend masks for everyone – at least here in New Zealand

Quite a few reasons. A bit like what happened with hand sanitiser, I was really concerned that if everyone rushed out to buy face masks then those who really needed them would go without. I was also really worried that people would feel a false sense of security and then not do all the things that will help protect them and others from Covid-19. And every time I’ve gone to the supermarket I’ve seen evidence of this.

One of the reasons you’ll see advice about mask wearing changing is partly because new evidence is coming to light, like the studies I’ve mentioned. But it’s also because there are many places where the virus is spreading exponentially, and many people are still going about their business. In this scenario, the more people who wear masks, the more likely those who are infected but don’t know it will be wearing one. And that will help capture at least some virus-laden droplets which should help reduce transmission. In other words, the pros vastly outweigh the cons.

So, if you are living in a place with widespread community transmission of Covid-19, please wear a mask. And if you are living in New Zealand, where we are all in our bubbles and aren’t seeing exponential spread of the virus? Then check out one of the excellent tutorials on making your own face mask and wear it when you go out to the supermarket or to access other essential services. That way you’ll be protecting our essential workers.

But promise me this. That’ll you’ll wash your hands before you put your mask on, and then again once you’ve got it on. That you won’t touch it while you are wearing it. And, if you do, you’ll immediately wash your hands. That you will wash it after every use and allow it to dry properly before using it again. And that you’ll keep up with regular hand-washing and physical distancing.