Are face masks useful against the Covid-19 epidemic? As obvious, it depends on various factors: the WHO says there is no scientific evidence of a benefit from the generalised use of masks by "healthy people in the community." But it is also stated that masks are beneficial when used in special conditions. Here, I tell you what I found about the scientific literature, but first I'll discuss how people tend to approach the problem according to their political views.
This is a modified version of a post that appeared in Italian on "Effetto Cassandra"
The Covid-19 epidemic is winding down in Italy, just like everywhere in Europe. The data indicate that the average mortality in most countries has been below average during the past three weeks, at least, and the curve shows no signs of picking up again. In Italy, there are still a few scattered cases, mostly in Northern Italy, but the numbers are minuscule. On all counts, the epidemic is over.
As a consequence, the use of face masks is not mandatory anymore in most regions of Italy, except inside shops, buses, and other crowded places. I expected that people walking in the streets would gladly shed their masks and enjoy the fresh air of these last weeks of spring, but many didn't. An informal measurement that I made during the past few days shows that if you take a walk in Florence you'll see a good 30% of the people still wearing masks tightly fitted on their faces. Another 30%-40% wear the mask loose on their face, leaving free the nose and often also the mouth. The rest don't wear masks at all.
Curiously, wearing or not wearing the mask seems to be linked to political opinions. In the US, it has been noted that the use of masks is more widespread in "blue states" than in "red states," those supporting Trump. Even more surprisingly, the mortality in red states is lower than in blue states (of course, that's not necessarily linked to the use of masks).
In Italy, things seem to be similar. I have no statistical data, but from what I read and what I see, I can tell you that left-oriented people are in favor of masks, whereas right-oriented ones are not. As it happens more or less everywhere, every issue discussed on social media soon becomes political with two opposite and incompatible opinions. It also happened with masks and extreme opinions are the rule. I received a comment to a post of mine that went as "If we don't wear masks we'll all die." Then, a well-known right-wing member of the Italian parliament, Mr. Vittorio Sgarbi, aired an emotional speech in which he maintained that "Masks are not only useless, they are harmful!"
For a discussion of why most people just can't approach this issue from a rational/data-oriented viewpoint, you can see this post by Chuck Pezhensky. Here, I am trying to do the currently undoable: examine the subject from a rational viewpoint. This approach has already caused someone to shout at me on Facebook "you are an abhorrent negationist" when I stated that things were not so bad as described in the media. But so is life and let me try to tell you the results of a search I made about masks on scientific literature. Just a quick disclaimer: I don't pretend to be a medical doctor, but I do pretend to be able to examine and analyze data. It is my job (*).
To begin with, what is the main route for the transmission of these viruses? On this point, you can read a recent review (in Italian) that summarizes what we know. To go more in-depth in the matter, you can read a hugely interesting article by Shaman and others. It is not very recent (2010) but it provides a complete description of how respiratory diseases diffuse and about their seasonal nature.
From these and other articles and documents, we can conclude that the main vector of the epidemic is the cloud of "droplets" emitted when infected people breath, cough, or sneeze. Instead, the so-called "fomites", particles that stand on solid surfaces and infect by contact, play a marginal role. But it seems logical that diseases affecting the respiratory system are transmitted mainly through the respiratory tract.
At this point, we need to understand how and under what conditions these "droplets" spread and reach other people. Here, the story becomes fascinating. It is clear that the droplets produced by sneezing, those visible to the naked eye, are basically harmless since they quickly fall to the ground. If that was the only problem, we wouldn't need masks at all. But smaller particles, those below a micron (millionth of a meter), are much more effective as vectors. The term "aerosol" is used for a suspension of these particles. You can find a very interesting clip on this matter at this link.
The measurements show how these particles remain airborne for a long time indoors and spread even at a distance of several meters. In practice, an aerosol may saturate a closed space and that makes social distancing useless. Not only that, but their persistence depends on the absolute humidity of the environment. In dry environments (a typical indoor condition), droplets partially evaporate, become even smaller, and the virus remains airborne for longer times. The opposite happens in humid environments.
These characteristics explain why influenza is a typical seasonal disease, as Shaman and others explain well. In summer, the absolute humidity is higher, and therefore the particles forming aerosols may grow in size and fall to the ground. It helps that people keep the windows open longer, which makes the aerosols for ventilation disappear quickly. The ultraviolet rays of the sun also help a lot, but of course only outdoors.
At this point, the question is whether the masks block the emission of aerosols. The answer is " in part, yes." One problem is that there are so many types of masks and that people often don't wear them correctly. But we can say that, in general, any mask -- even a simple damp cloth -- will block at least a fraction of the small droplets that generate aerosols -- you can see that effect here.
So, is wearing mask sufficient to have an effect on the diffusion of the epidemic? It seems that the answer is positive. A recent study by Leffer et al. shows that there exists a correlation between the use of face masks and the speed of diffusion of the Covid-19 epidemic. That's to be taken with some caution because it is difficult to disentangle the many parameters affecting the diffusion pattern, but it makes sense. Also the recent study by Chu et al. confirms that face masks reduce the transmission of the virus.
On the basis of these data, I would infer that the place where it is easiest to get infected is at home, where you often find yourself in poorly ventilated and dry rooms, especially in winter. From that, you could deduct that locking people in their apartments may not have been a good idea and, indeed, Leffer et al. find no evidence that the lockdown had any effect in slowing down the spread of the epidemic. But for the moment we do not have enough data on this point.
Now we can summarize.
- The virus is transmitted mainly as an aerosol in indoor and poorly ventilated areas, in winter. In these conditions, social distancing is of little use.
- In these environments, masks can help a lot, but it would be better to ventilate rooms as much as possible and try to keep them moist. It is even better to stay outdoors as much as possible, exposing yourself to the sun.
- In conditions where there is no crowding and there is no evidence of the presence of infected people, masks are not needed. This is what the WHO says in their recommendations.
- Most people will ignore the above points ad behave according to their political orientation.
And this is what I found. If you know more than me or have different data, please feel free to comment. In science, nothing is ever carved in stone, you can and you should change your mind when new data become available.
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(*) About the right to speak on scientific matters, let me cite again Chuck Pezhensky:
"One of the reasons I fervently believe our current society in the U.S. is collapsing is the loss of noblesse oblige — the idea that those of us that are better off in some definable way should help those who are less fortunate. I view my role as a full professor as one where I am supposed to think about complex and complicated things for the common good, just like a rich person is supposed to build housing developments for the poor."
