CDC and other government orgs discourage regular citizens from using face masks at work and outside: “CDC does not recommend the routine use of respirators outside of workplace settings (in the community).” However, people still buy them en masse and we are currently experiencing a shortage. It has gotten so bad that folks are starting to manufacture their own with the old sewing machine and some cloth.
So let’s see who’s right and who’s less so and should we leave those sewing machines alone. Using data of course. It turns out that there are a few scientific, peer reviewed articles looking into the effectiveness of masks in preventing illnesses and particle passage. There are individual studies and also meta-analyses – studies of studies (a sort of a ring to rule them all) – which combine data from many individual studies to make more robust conclusions. To get a quick read into a subject, I like to look at meta-analyses when they are available and supplement with other studies. Let’s dive in.
Too long, didn’t read: 1) Medical masks and especially respirators like N95, are effective at reducing the risk of infection for healthcare workers 2) Cloth masks are a lot less effective than medical masks and even less so than N95 respirators, but they can still block *some* particles 3) If you are making cloth masks at home, go for ticker, layered materials like sweatshirts or towels because they block more particles. And don’t forget to wear masks when you are sick to protect others.
Are (any) masks effective at preventing disease?
First, what type of masks do we have? Medical or surgical masks (like on the photo of not me above), the ones you see most often and the ones that are made out of polypropylene and other plastics, help protect us from saliva, sneezes and other large droplets [1]. They are not specifically designed to protect people from airborn infection. That is the job of the coveted N95 respirators. They are highly regulated and have to be fitted to the users face [1]. The 95 in N95 stands for the 95% of particles these respirators are suppose to filter out, in case you were wondering. And then there are cloth masks that were used in the early days of medicine and that are currently used in areas where other options are not available.
An effectiveness of preventive measures or remedies is measured in RRs – risk ratios (or relative risks). Risk ratios are used to compare the incidence of a diseases in a group that receives a treatment to that that does not. So for example if there are 50% of those who did not wear a mask and got sick and 25% of those who wore a mask and got sick, we get an RR of 25/50 = 0.5 which means a reduced risk of getting the disease for those who wore masks. The lower this ratio the more effective the treatment.
RR in this case is 25/50 = 0.5.
Image source
A 2017 meta-analysis in the journal of Clinical Infectious Diseases [1] looked at the effectiveness of masks against respiratory infections in healthcare professionals. Combining results from multiple studies they found that overall masks and respirators reduce the risks to get respiratory illness (RR = 0.59) and influenza-like illness (RR = 0.34). N95 respirators essentially doubled the protection in comparison to regular masks for respiratory illness (RR = 0.47 for N95s vs regular masks) and bacterial infections (RR = 0.46). Regular masks were as effective as N95 for viral infection and influenza-like illness.
N95 respirators were found superior in another large study [2], where mask types and wearing practices were compared in terms of their protective abilities in 3.5K healthcare workers. Four groups were compared on protection from various infections: 1) those were asked use medical masks continuously at all times 2) those who wore N95 respirators at all times 3) those who wore N95 respirators only while doing high risk procedures and 4) the control group that was not instructed on mask use and continued to do what they usually do. This control group was chosen from hospitals where mask/respirator use is low. As you can see in the chart below, continuous use of N95 provided the greatest protection while medical masks provided the least protection in comparison to controls (but were still better than controls).
How about cloth masks?
There are few studies looking at cloth masks out there, and only one randomized control study I could find that looked at actual infection rates. This large study from the University of New South Wales [3] compared the protective effect of cloth and medical masks in 1.6K hospital workers. They compared a group that was asked to wear medical masks continuously, a group that was asked to wear cloth masks continuously and a control group that just continued to do what they usually do. The study found that cloth masks performed the worst out of the three conditions (see figure below) and medical masks performed the best. The RR of cloth masks in comparison to medical masks was 13.2 (!) for influenza-like illnesses which means 13 times greater risk of infection.
So what we learn from this study is that cloth masks are not as effective as medical masks. But are they effective in comparison to wearing nothing?
We get a clue from another study [4] that looked at how many particles of various sizes cloth masks and masks improvised from household cloth materials can block. This study is simply cool because they blasted particle jets into hoodies, t-shirts, towels and scarfs and caught them on the other side to see how many passed. Maybe if I were doing these experiments I’d still be in academia. The other cool part is that the particles in the experiment were 75 ± 20 nm in diameter, which is similar to the COVID-19 partical size of 70–90 nm [5]. So what did they find? The most effective method at blocking these particles were N95 masks of course. As promised they blocked ~95% of stream. Surprisingly, commercial cloth masks performed not better than scarves from Walmart, and at best blocked 26% of the particles. The best sweatshirts showed the highest protection among all cloth options (Hanes brand, and this is not product placement, this is science). All the towel brands performed similarly, blocking around 40% of the particles. Ford Perfect was right, a towel is the most important item you can carry. T-shirts on the other hand are too thin and only block 14% at best.
So what can we conclude? Cloth masks are much less effective than medical masks and respirators, but if there are no alternatives cloth masks might help block some of the airborn particles. Thicker, layered materials seem to protect better than thinner once, which makes a lot of sense. To make better conclusions, what we need is a study that compares the rate of infection in people wearing cloth masks to those not wearing protective gear. However, a controlled randomized trial like that would be near to impossible to run because of the ethical issue of knowingly exposing people to disease.
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Mind the data 
