After going through over 100 research papers, here’s what I’ve found.
In August 2021, Senator Rand Paul of Kentucky said that cloth masks don’t prevent infection from SARS-CoV-2 (that causes Covid-19) in a YouTube video. But Youtube banished the video and suspended Mr. Paul for a week. But, at the concerns of many, could he actually be right?
With my curiosity piqued, I first googled the effectiveness of cloth masks against SARS-CoV-2, especially the more transmissible Delta variant. Due to the lack of concrete info, I went to PubMed, a biomedical literature database, and looked through over 100 indexed papers about cloth masks.
Face masks in brief
We commonly use three types of face masks: N95s, surgical masks, and cloth masks. (For simplicity, all masks mentioned hereon are face masks.)
N95s (or KN95s) are tight-fitted and filter about 95% of particles down to 0.3 microns in size. (Airborne SARS-CoV-2 are aerosolized particles of about 5 microns.) It’s made of four layers of materials, usually polypropylene, as well as electrets that trap particles electrostatically. Its production is strictly regulated, making it less readily available for the general public.
Surgical masks are loose-fitted, three-layered materials (usually polypropylene) that are widely available. While its production has to meet certain standards, they are not as strict as N95s. Surgical masks are intended to stop respiratory droplets (larger than aerosols), but they can also stop aerosols if worn without side gaps:
Cloth masks can be made from various fabrics — such as cotton, silk, or nylon — without having to meet any national standards. Although cloth masks are reusable, their efficacy may decrease with repeated washing.
As the science on cloth masks is more confusing, this article will focus on cloth masks with comparisons to N95s or surgical masks whenever appropriate.
Published systematic reviews on masks and respiratory viruses (pre-Covid-19)
While mounting observational data support the usefulness of masks against respiratory viruses like SARS-CoV-2, I’ll not discuss them here. After all, the rule is that observational studies only show associations, not causation. (As observational studies don’t do randomization, they cannot account for every single factor that may have influenced the results.)
To confirm cause-and-effect, we need randomized controlled trials (RCTs). Data from several RCTs can be pooled together for a meta-analysis to derive an overall outcome. To ensure no cherry-picking of studies, meta-analyses are also systematic reviews, where certain keywords are searched in literature databases to find all relevant studies regardless of their results.
A meta-analysis (published August 2021) of 16 RCTs covering 17,048 participants found that wearing N95s and surgical masks decreased the risk of respiratory infections by 33% and 17%, respectively, compared to no mask. Surprisingly, wearing double-layered cloth masks increased infection risk instead by a staggering 380% versus no mask. But none of those 16 RCTs were related to SARS-CoV-2; most were about influenza. Although this meta-analysis was published this month, its last literature search was in July 2020. So, this paper isn’t very updated, but it still warns about the possible harm of wearing double-layered cloth masks amidst influenza season.
Another meta-analysis (published February 2021) of 11 RCTs (last searched in August 2020) found an 11% reduction in influenza-like illnesses in those who wore masks, but this result did not reach statistical significance. The authors noted that the included RCTs were of low quality, so their results may change if better-designed RCTs were available. Moreover, this meta-analysis did not differentiate between surgical or cloth masks. If cloth masks were to promote influenza infection instead — as shown in the above meta-analysis — this meta-analysis can be misleading.
One systematic review (published May 2021) synthesized data from 21 RCTs available as of April 2020 about masks and respiratory viral infections. Due to huge variations in methods between studies, a meta-analysis was not suitable. That said, this review still offers a few noteworthy points:
- 3 RCTs found N95s were more beneficial than surgical masks, and another 3 RCTs found no effects. For masks versus usual practice, 2 RCTs reported beneficial effects, 8 RCTs reported probably beneficial, 3 RCTs reported no effects, and 1 RCT was inconclusive (see bubble plot below).
- 6 RCTs reported participants in the mask group were more likely to experience adverse events of headaches, skin irritation, worsening acne, shortness of breath, and other respiratory difficulties.
- The review authors also emphasized an RCT — the same one identified by the two meta-analyses above — finding that cloth maks (double-layered) increased the risk of influenza-like illness by 6.6-fold compared to usual practice among healthcare workers.
Based on these latest systematic reviews, we can see that masks do protect against respiratory viruses that come before Covid-19, but they are not foolproof. Wearing masks may not be pleasant either, with some people experiencing headaches and problems with breathing and skin condition. But aside from cloth masks (double-layered), we can be assured that N95s and surgical masks don’t promote respiratory viral infections.
Face masks and SARS-CoV-2 infection
To date, there’s only one RCT in Denmark on the topic of masks and SARS-CoV-2 infection. This RCT (published November 2020) randomly assigned adults who spent >3 hours outside the home into either mask (FDA-approved surgical masks with 98% filtration rate) or control group. Each group has about 3,000 participants. One month later, 1.8% and 2.1% of participants from the mask and control group, respectively, got infected with SARS-CoV-2, with the differences being non-statistically significant.
As you might expect, this RCT has been misused to say that there’s no point wearing masks during the Covid-19 pandemic. The fact is that this RCT’s results must also be interpreted in light of its limitations.
For one, all participants practiced physical distancing and hand hygiene. SARS-CoV-2 was also not spreading widely during this study period. Only 46% of participants wore the mask as recommended (47% predominantly as and 7% not as recommended). Thus, the more appropriate conclusion is: asking people to wear masks doesn’t add any more benefits if other prevention measures are practiced during the early pandemic in Denmark.
A newly released RCT (unpublished) randomized 342,126 adults in Bangladesh into the control (no intervention) or mask group, where participants received free masks and info on masks’ importance. At week-10, proper mask-wearing was 42% in the mask group and 13% in control group. And the prevalence of Covid-like symptoms was 7.6% in the mask group and 8.6% in control group— translating to a 10% decrease after adjusting for baseline factors. Thus, “a 30% increase in mask-wearing led to a 10% drop in Covid,” the lead researcher said.
More importantly, this effect was mainly driven by surgical masks (with 95% filtration efficiency). Non-woven, triple-layered cloth masks (with 37% filtration efficiency) only provided a small benefit — 30–80% lesser than surgical masks —that nearly misses statistical significance (P = .048; P > .05 means no statistically significant effects).
Filtration efficiency of face masks
While the production of N95s and surgical masks are standardized, cloth masks can be manufactured in various ways without proper regulations. So, the types of cloth masks that studies have studied must also be noted.
One brilliant review (published February 2021) searched the literature semi-systematically for studies on the filtration efficiency of mask materials (last searched in January 2021). This paper then created a well-designed plot of the filtration efficiency data of multi-layered masks from multiple studies:
Averaging these data points, the mean filtration efficiency of an N95 is ~91%; surgical mask ~62%; air filter mask ~85%; non-woven cloth ~58%; microfiber ~41%; bedsheet ~29%; towel ~21%; paper ~37%; interfacing ~32%; cotton/synthetic mix ~18%; synthetic knit ~15%; cotton knit ~16%; cotton woven ~15%; chiffon ~15%; quilt fabric ~26%; silk ~8%; other natural fiber ~32%; other materials ~20%; non-woven + clothing (cotton) ~66%; non-woven + quilt fabric ~60%; non-woven + towel ~47%; non-woven + other natural fiber ~90%; paper + quilt fabric ~65%; paper + other material ~48%; synthetic knit + towel ~34%; synthetic knit + clothing (cotton) ~21%; at least three fabric types combined ~57%. (These numbers are written in order from left to right of the plot above, per the black dots.)
Based on their findings, the review authors then provided a few takeaways:
- Muti-layered microfiber and non-woven materials have higher filtration efficiencies than other types of materials.
- Cotton (knit or woven), synthetic knit, chiffon, quilt fabric, quilt batting, flannel, fleece, and interfacing usually have filtration efficiencies of <25%, even with multi-layers.
- Single-layered fabrics have low filtration efficiencies of <25%. (This paper has also produced a similar plot for single-layered cloth masks of various fabrics, with mean filtration efficiencies of mostly <25%.)
- Washing decreased the filtration efficiency of non-woven material, but not wool felt. Dampening decreased the filtration efficiency of denim, but not quilting cotton, cotton flannel, and dense polyester.
Studies newer than January 2020 that were not included in this review have also reported similar patterns: aerosol/particle filtration efficiencies of N95s are greater than surgical masks, which are in turn greater than cloth masks. Here are a few more graphs that convey more than texts:
Is cloth mask still a wise choice?
Unless it’s a really effective cloth mask made of non-woven, multi-layered microfiber plus/or cotton with a filtration efficiency of 90% (as shown in the colorful plot above), the answer is no.
“The filtration, effectiveness, fit, and performance of cloth masks are inferior to those of medical masks and respirators,” a 2020 review concluded after reviewing the cloth mask literature. “Cloth masks are a more suitable option for community use when medical masks are unavailable.”
But there are other researchers who think cloth masks can be helpful if used properly. Another research review stated that “Cloth masks can offer substantial filtration, in some cases equivalent to some medical masks. This knowledge can be used to create evidence-informed cloth masks to mitigate transmissibility of viruses such as COVID19.”
Still, we can’t deny that cloth masks are generally inferior to N95s and surgical masks by design. During the early pandemic, cloth mask is recommended for community use due to shortages in N95s and surgical masks. The rationale is that some mask is better than no mask, which remains true to this day. So, in situations where N95s or surgical masks are unavailable, cloth masks can be self-made, washed, and re-used.
(N95s’ and surgical masks’ waste also create environmental problems, unlike cloth masks, but I’ll not discuss them here.)
How masks are worn is also equally, if not more, critical. Even surgical masks (and N95s) designed with high filtration efficiency are unhelpful if it’s loosely fitted or side gaps are too wide (see the CDC’s image above). To this end, Christian L’Orange, a professor of mechanical engineering, has made a good point: “If wearing a slightly less efficient mask means that they wear the mask correctly, that’s probably the better mask for them.”
We must consider the indoor conditions too. Indoors with poor ventilation and/or many people are high-risk areas for catching SARS-CoV-2 and other aerosolized pathogens. (Ventilation is the process of displacing indoor air with clean outdoor air.) Wearing cloth masks in such situations isn’t exactly wise, unless worn on top of surgical masks.
(Unless within close proximity for long periods, SARS-CoV-2 infection rarely happens outdoors with fresh air that rapidly dissipates aerosols. The WHO, however, continues to recommend wearing masks outdoors if physical distance couldn’t be maintained.)
Some countries don’t even like us using cloth masks. France has banned the use of homemade and some shop-purchased cloth masks during the pandemic, in response to the more contagious SARS-CoV-2 variants. Some countries like Austria and Germany have mandated surgical and FFP2 (filtering facepiece) masks in certain indoor settings.
All in all, in light of the spread of SARS-CoV-2 variants of concern — such as Delta that’s more transmissible and vaccine-resistant — we should choose and wear our masks more carefully. Recent evidence has also suggested that SARS-CoV-2’s evolution is leaning towards airborne capacity, further stressing that we should be warier of what we breathe in. This is especially important for those with risk factors for severe Covid-19 — old age, male sex, multiple medical conditions, and no vaccination (because vaccines still prevent Covid-19 hospitalization and death despite infection).
It’s not only SARS-CoV-2. Cloth mask’s use isn’t even advised as a protective means against other aerosolized pathogens — namely, Mycobacterium tuberculosis (the bacterium that causes tuberculosis), Measles morbillivirus (the measles virus), and varicella-zoster virus (the chickenpox virus). SARS-CoV-2 shouldn't be any different, especially when the Delta variant is estimated to be as contagious as chickenpox.
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