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Do Masks Protect Against COVID-19?

Do Masks Protect Against COVID-19?

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What the research says about facemasks and viral particle transmission

Most of us have picked up a cloth mask or two (or three) since the COVID-19 pandemic began. Summer memes joked about our transition away from the bikini and toward the “trikini” – consisting of matching swim trunks, top, and facemask. Companies have put their logos on cloth masks, and social justice warriors have screen-printed political messages on them. Facemasks have not only become a public health measure, but a symbol that we are all in this pandemic together, fighting to stay safe and save lives.

While many sources agree that facemasks do indeed slow the transmission of SARS-CoV-2 and other viruses, just about every study has shown that cloth masks are less effective than surgical, KN-95, and N-95 masks in filtering aerosolized particles.

Let’s take a deep (masked) breath and look at the research:

Before the pandemic: the first randomized controlled trial on cloth masks

The first randomized controlled trial (RCT) of cloth masks occurred in 2015 – six years before the COVID-19 pandemic began.1 The study compared the efficacy of cloth masks to surgical masks or no mask at all in the context of viral infections among 1,607 hospital healthcare workers who were followed over a period of four weeks. The data showed that cloth masks did not protect the healthcare workers nearly as well as medical masks.

Perhaps unsurprisingly, the rates of respiratory illness symptoms, flu-like illness symptoms, and laboratory-confirmed viral illness were much higher in the cloth mask group than in both the surgical mask group and the group that wore no mask at all.2

This study was performed in hospital care workers, and not in the general population, but still there is a compelling case for surgical masks over cloth masks.

Cloth masks are quite inferior at preventing the spread of viral illnesses.

So why then, did we so eagerly use cloth masks when the pandemic hit? Likely because of an initial shortage of surgical masks. Due to the sudden spike in demand for N-95’s and surgical masks that happened at the start of the pandemic, civilians were asked to let healthcare workers have priority and were instead instructed to make their own cloth masks out of old t-shirts. As supply caught up with demand, however, consumers had better access to disposable surgical masks, yet many stuck with their less effective cloth masks.3 Why?

This could be because cloth masks are more environmentally friendly, more affordable (because they can be reused), and/or more stylish (you can coordinate them with your outfits). But the evidence that has come out in recent months and years has only confirmed what the 2015 study showed: Cloth masks are quite inferior at preventing the spread of viral illnesses.

Recent studies on cloth masks

Some (but not all) of the recent studies on cloth masks have shown that they aren’t entirely useless. When used correctly, cloth masks might be better than nothing.

A literature review published in August of 2020 found that cloth facemasks (made of cotton, scarf, pillowcase, antimicrobial pillowcase, silk, linen, tea towel, or vacuum bag) offered “marginal/reasonable protection against particles.” The authors found that the type of fabric used, the number of layers comprising the mask, and the frequency of washings influenced the efficacy.4

Another article, published in May of 2020, argues that cloth masks are far from perfect, but that they’re better than nothing. The authors explain that there is ample evidence that many (though not all) cloth masks can reduce the droplet and aerosol transmission of any virus, including SARS-CoV-2. The authors argue that given the severity of the pandemic, “the possible benefit of a modest reduction in transmission likely outweighs the possibility of harm.”5

The gaps created by improper mask fit could drop the filtration efficacy by over 60%.

An April 2020 review of the various fabrics used in cloth masks found that their efficacy in filtering viral particles varied widely – anywhere from 5 to 80% for particle sizes of less than 300 nm (nanometers) in diameter. (Coronaviruses have a diameter of around 120 nm.6) High thread count cotton worked better than looser-weaves of the fabric. The data also implied that the gaps created by improper mask fit could drop the filtration efficacy by over 60%. The efficacy of cloth masks improved when multiple layers were used and when each layer was comprised of a different type of fabric. The filtration efficacy of hybrid multi-layer masks (such as those made of cotton and silk; cotton and chiffon; and cotton and flannel) was over 80% for filtering out particles less than 300 nm.7

While this review highlighted the benefits of double-layer masks, some conflicting observations made during the SARS epidemic suggest that double masking could increase the risk of infection due to the effects of moisture and pathogen retention.8,9

Cloth masks may not be the best option

A study published in September of 2020 directly compared the performance of different mask types.10 The authors recruited 10 volunteers (six male; four female) between 18 and 45 years of age, and asked them to breathe, talk, cough, and move their jaws. Each participant completed all four of the activities while wearing no mask or one of the different mask or respirator types: a surgical mask, an unvented KN-95 respirator, a homemade single-layer paper towel mask, a single-layer cloth mask, and a double-layer cloth mask. (Both the single- and double-layer cotton masks were made from brand new, unwashed 100% cotton t-shirts.) The participants sat in front of an aerodynamic particle sizer, which was used to objectively count the number of particles that passed from each person’s face during the activities.

Consistent with previous studies on the matter, the data from this study found that surgical masks and KN-95 respirators substantially reduced the number of particles emitted from breathing, talking, and coughing – even without fit-testing.

The cloth masks didn’t quite cut the mustard, however: Brand new cotton masks were much worse at particle filtration than surgical masks and respirators. Depending on the activity, the cloth masks either performed worse than the other mask types but better than nothing, equal to the mask-free conditions, or even worse than the mask-free conditions.

Because the participants wore cloth masks made from brand new cotton t-shirts in this study – and not personally laundered cotton masks – four participants were asked to hand wash their double-layer cotton masks with soap and water, rinse them well, and let them air dry. These four people then repeated the experiment while wearing their washed double-layer masks. This allowed the researchers to directly compare the performance of laundered masks to unwashed, brand-new ones. Curiously, they found that “handwashing the double-layer t-shirt mask with soap and water followed by air-drying yielded no significant change in the particle emission rate as compared to the original unwashed masks.”

It’s worth noting that the study did not include truly “dirty” cotton masks. It’s frightening to think what the particle emission rate might have been from a mask that had been worn daily for a week prior to testing!

Choose surgical, N-95, or KN-95 masks when available

Most (if not all) studies on the topic agree that surgical masks, KN-95 respirators, and N-95 respirators far outperform cloth masks – with the respirators offering even better protection than surgical masks.

This could be very well why Faheem Younus, MD, Chief of Infectious Diseases at the University of Maryland, has advised ditching cloth masks in preference of N-95’s or KN-95’s,12 and why it has been argued that cloth masks should not be mandated for healthcare workers.1,13

A study published in July of 2021 found that cloth masks only filter about 10% of exhaled aerosol droplets.

It’s also important to consider fit: A study published in July of 2021 found that cloth masks only filter about 10% of exhaled aerosol droplets. The remaining 90% of aerosols leak out into the ambient air unfiltered, mainly escaping through the top of the mask where it fits (poorly) over one’s nose.15

If your only choice is a cloth mask

A recent paper co-authored by epidemiologist Dr. Abrar A. Chughtai outlines the following suggestions with respect to cloth masks:13

  • Use cloth masks as a last resort, only when other mask types are not available
  • Use a fabric with a fine weave and high thread count
  • Choose hybrid materials, like cotton-silk, cotton-flannel, or cotton-chiffon
  • If limited to t-shirt material, cotton blends might be better than pure cotton
  • Choose water-resistant fabrics when possible
  • Select a mask that has at least two layers, ideally with batting between each layer
  • Make sure the mask fits properly and seals around the face; ties ensure a better fit than ear loops
  • Wash cloth masks every day with soap and hot water, and let them dry completely between uses
  • Have at least two masks per person, to allow for rotation of masks (so there is always a clean, dry mask to use while the other is being laundered)

Fortunately, disposable surgical masks can be easily found in most supermarkets and pharmacies. The other good news is that based on the information we have available, it seems that COVID-19 transmission is much less likely to occur outdoors than indoors.16,17 With colder weather around the corner pushing us back inside, however, the masks we choose to wear can make a huge difference.18–20



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