What the research says about fabric facemasks and SARS-CoV-2 particle transmission
Most of us have picked up a cloth mask or two since the start of the pandemic. Companies have put their logos on cotton masks, social justice warriors have screen-printed political messages on them, and fashionistas have coordinated their outfits with them. Face masks are not only a stylish statement, but, more importantly, a public health measure and a sign that we are all in this pandemic together, caring for ourselves and each other.
Sources agree that face masks slow the transmission of SARS-CoV-2 and other viruses when used properly, and just about every study has confirmed that N-95, KN-95, and surgical masks are far better at protecting against virus transmission than cloth masks.
What sources don’t seem to agree upon, however, is on the merit of cloth masks – namely, whether wearing a cloth mask is really any better than wearing no mask at all.
Let’s take a deep (masked) breath together and look at the data on cloth masks:
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 outbreak.1 1,607 hospital care workers were assigned to wear a cloth mask, a surgical mask, or no mask at all during every shift they worked over a period of four weeks.
The surgical masks protected the hospital workers against illness much more effectively than the cloth masks: The rates of respiratory illness and flu-like illness symptoms, as well as laboratory-confirmed viral illness rates, were much higher in the cloth mask group than for the surgical mask group. Laboratory tests demonstrated that the penetration of particles through the cloth masks was very high – 97% – whereas medical masks had a particle penetration of just 44%.
The study also discovered that the healthcare workers who wore cloth masks had higher rates of sickness symptoms and laboratory-confirmed viral illnesses than the workers who wore no masks at all.
The authors speculate: “Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection.” (As an aside, the authors of a 2015 article on the transmission of Ebola virus raise a similar concern, demonstrating that the virus may survive on the surface of facemasks and thus present the risk of self-contamination through repeated use.2)
This data cautions against the use of cloth masks among hospital workers – and experts did pay attention to these findings in 2019 when COVID-19 escaped the lab, or the bat, or wherever it came from. Due to the sudden spike in demand for N-95’s and surgical masks at the start of the pandemic, civilians were asked to let healthcare workers have priority for the more effective masks and were instructed to make cloth masks out of bandanas and t-shirts. As supply caught up with demand, however, consumers had better access to disposable surgical masks, but many stuck with their sub-par cloth face coverings.3
This could be because cloth masks are more environmentally friendly, more affordable (because they can be reused), and/or more stylish. Unfortunately, the persisting practice of wearing cloth masks may be doing us all a disservice: The evidence that has come out since the pandemic began has only confirmed what the 2015 study showed: Cloth masks are pretty bad at preventing the spread of viral illnesses.
Recent studies on cloth masks
Most (but not all) of the recent studies on cloth masks have shown that cloth masks are better than nothing, and they do somewhat slow the spread of SARS-CoV-2 (the COVID-19 virus).
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 particles less than 300 nm (nanometers) in diameter. (Coronaviruses have a diameter of around 120 nm.4) 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 a mask’s 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 – pretty good, actually!5
Another article from May 2020 by Clase et al argues that cloth masks are far from perfect but better than nothing: There is ample evidence that many (though not all) cloth masks can reduce the droplet and aerosol transmission of viruses, 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.”6 [Emphasis added by me.]
A review by Lima et al 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 report that the type of fabric used, the number of layers included, and the frequency of washings influence efficacy.7
While this review highlighted the benefits of double-layer masks, observations made during the SARS epidemic suggest that double masking increases the risk of infection due to the effects of moisture and pathogen retention.8,9 So we have some conflicting data, here.
A September 2020 review by Sharma et al says: “Cloth face masks have limited efficacy in combating viral infection transmission.”10
Another September 2020 review (this one by Silva et al) concludes: “Evidence shows that cloth masks do not have the same protective characteristics as surgical masks, indicating an increased risk of infection due to humidity, diffusion of fluids, virus retention, and improper preparation… cloth masks could be proposed as a last resort.”11
This could be very well why Faheem Younus, MD, Chief of Infectious Diseases at the University of Maryland, has advised ditching cloth masks in favor of more effective masks,12 and why it has been argued that cloth masks should not be mandated for healthcare workers.1,13
It’s also important to consider fit: a study published this summer (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 the nose.14
One study suggests cloths masks may be worse than no mask at all.
A study published in the journal Nature in September of 2020 directly compared the efficacy of different types of masks.15 The authors recruited 10 volunteers (six male and four female) between the ages of 18 and 45 years. The volunteers were asked to to breathe, talk, cough, and move their jaws as they sat in front of an aerodynamic particle sizer, which objectively counted the number of particles that passed from each person’s face during the activities. Each volunteer 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 cloth masks were made from brand new, unwashed 100% cotton t-shirts.)
Consistent with previous studies on the matter, the data from this experiment shows that surgical masks and KN-95 respirators substantially reduce the number of particles emitted from breathing, talking, and coughing – even without fit testing. In other words: Surgical masks and KN-95 respirators work.
The cloth masks didn’t quite make the cut, however: Brand new cotton masks either did nothing at all or spread even more particles as compared to mask-free conditions.
Let’s take a closer look at the data from the four different activities:
The surgical masks, KN-95 respirators, and even the homemade paper towel mask reduced the transmission of aerosolized particles emitted when the volunteers breathed in front of the particle counter; the cloth masks didn’t.
Wearing a single layer cotton mask while breathing yielded a significant increase in particle emission rate as compared to wearing no mask. (In fact, two of the participants, the single layer t-shirt mask was associated with a 384% increase in particle transmission – almost a four-fold increase.)
Wearing a double-layer cotton mask while breathing had no significant effect on the particle emission rate, meaning the double-layer mask was equal to wearing no mask at all.
Next the researchers asked the participants to talk. KN-95 respirators, surgical masks, and even the homemade paper towel mask all significantly decreased the outward emission of particles during speech.
“The homemade cloth masks,” in contrast, “yielded either no change or a significant increase in emission rate during speech compared to no mask.” [Emphasis added by me]
Wearing a single layer mask emitted a median value of 16.37 particles per second, as compared to 2.77 particles per second with no mask at all.
When considering just the smallest particle sizes emitted, the single layer cotton mask increased the emission of particles sized 0.3 – 0.5 µm by almost 600%. The double layer cotton mask had no significant effect, meaning it was about as protective as going mask-free.
With respect to coughing, both the single and double layer cloth masks yielded a significant increase in particle emission as compared to mask-free conditions.
The cotton masks were associated with an emission rate of 49.2 particles per second (or per cough), whereas wearing no mask yielded an emission rate of 36.1 particles per second (or per cough).
For the fourth activity, the participants were asked to move their jaws with their mouths closed while breathing through their noses, as if they were chewing gum. As compared to no mask, the surgical and KN-95 masks did not significantly change the particle emission during this activity, surprisingly. (This calls into question whether the practice of pulling a mask down to bite into food and lifting it back up to cover the face while one chews does anything but annoy the person who is eating.)
All of the homemade mask types – the single-layer paper mask, the single-layer cloth mask, and the double-layer cloth mask – substantially increased the particle emission rate during jaw movement. The biggest culprit was the single layer cloth mask: Whereas the particle emission rate with no mask was 0.12 particles per second, with the unwashed single layer cloth mask it jumped to 1.72 particles per second.
Limitations of the study
Due to the design of the equipment used, only the particles emitted through the front of the masks were measured. The data reported in the study may thus underestimate the particle transmission counts, as they do not include any particles that might escape out of the sides of the masks. In other words, the masks might perform even worse than these data might suggest, though the data do allow us to compare one mask type to another.
Another limitation of this data is that it focuses on cloth masks made from brand new cotton t-shirts, but not personally laundered cotton masks. To account for this, the researchers asked four participants 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.” Furthermore, the authors report: “manual rubbing of a washed double-layer cotton mask aerosolized slightly more particles than the [new] unwashed mask.”
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!
The above study was published roughly a year ago in the journal Nature, and the findings are quite shocking. It is unfortunate that these findings did not receive more attention and news coverage; this information could have helped us choose more effective masks and in turn save lives. While the data reflects poorly on cloth masks, it confirms the fact that KN-95’s and surgical masks work.
In short: choose any other mask besides cloth
Most (if not all) studies on the topic of face masks agree that surgical masks and N-95 and KN-95 respirators far outperform cloth masks (with the respirators being the best option).
The studies comparing cloth masks to no mask at all, however, are varied in their conclusions. Most studies say that cloth masks are better than nothing. A smaller percent of studies suggest that going mask-free may be equal to or possibly even safer than wearing a cloth mask.
Fortunately, disposable surgical masks are widely available now and can be found in most supermarkets and pharmacies. The other good news is that based on the information we have available, it seems that the risk of COVID-19 transmission is much lower outdoors,16,17 making masks are more important in indoors settings or in poorly ventilated areas.18–20
If your only choice is a cloth mask
If your only face mask option is a cloth mask, here is some advice from a recent paper co-authored by epidemiologist Dr. Abrar A. Chuchtai:13
- Use cloth masks as a last resort, only when other masks types are not available
- Use a fabric with a high thread count (fine weave)
- Choose hybrid materials like cotton-flannel, cotton-silk, or cotton-chiffon
- Choose water-resistant fabrics when possible
- If you’re limited to t-shirt material, cotton blends might be better than pure cotton
- 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 allow for a better fit than ear loops
- Wash cloth masks every day with hot water and soap and let them dry completely between uses
- Have at least two masks per person to allow for proper laundering between uses
What this doctor thinks
Humans have never had a pandemic quite like this one. We are all doing the best we can with the information available to us – and that information is changing every week. As of this writing, however, I am urging my patients, friends, and community to use only disposable KN-95’s or surgical masks, and to toss the cloth masks.
- MacIntyre CR, Seale H, Dung TC, et al. A cluster randomised trial of cloth masks compared with medical masks in healthcare workers. BMJ Open. 2015;5(4):e006577. doi:10.1136/bmjopen-2014-006577
- Osterholm MT, Moore KA, Kelley NS, et al. Transmission of Ebola viruses: What we know and what we do not know. MBio. 2015;6(2). doi:10.1128/mBio.00137-15
- Tufekci Z, Howard J. Why Aren’t We Wearing Better Masks? The Atlantic. 2021. https://www.theatlantic.com/health/archive/2021/01/why-arent-we-wearing-better-masks/617656. Accessed August 20, 2021.
- Editors of Encyclopaedia Britannica. Coronavirus. In: Britannica. https://www.britannica.com/science/coronavirus-virus-group. Accessed August 20, 2021.
- Konda A, Prakash A, Moss GA, Schmoldt M, Grant GD, Guha S. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano. 2020;14(5):6339-6347. doi:10.1021/acsnano.0c03252
- Clase CM, Fu EL, Joseph M, et al. Cloth Masks May Prevent Transmission of COVID-19: An Evidence-Based, Risk-Based Approach. Ann Intern Med. 2020;173(6):489-491. doi:10.7326/M20-2567
- Lima MM de S, Cavalcante FML, Macêdo TS, Galindo-Neto NM, Caetano JÁ, Barros LM. Cloth face masks to prevent covid-19 and other respiratory infections. Rev Lat Am Enfermagem. 2020;28:1-8. doi:10.1590/1518-8345.4537.3353
- Li Y, Wong T, Chung J, et al. In vivo protective performance of N95 respirator and surgical facemask. Am J Ind Med. 2006;49(12):1056-1065. doi:10.1002/ajim.20395
- Szarpak L, Smereka J, Filipiak KJ, Ladny JR, Jaguszewski M. Cloth masks versus medical masks for COVID-19 protection. Cardiol J. 2020;27(2):218-219. doi:10.5603/CJ.a2020.0054
- Sharma S, Mishra M, Mudgal S. Efficacy of cloth face mask in prevention of novel coronavirus infection transmission: A systematic review and meta-analysis. J Educ Health Promot. 2020;9(1). doi:10.4103/jehp.jehp_533_20
- Silva AC de OE, Almeida AM de, Freire MEM, Nogueira J de A, Gir E, Nogueira WP. Cloth masks as respiratory protections in the COVID-19 pandemic period: evidence gaps. Rev Bras Enferm. 2020;73(suppl 2):e20200239. doi:10.1590/0034-7167-2020-0239
- India Today Web Desk. Covid ‘airborne’: Use N95 or KN95 masks, keep alternating every 24 hours, says Infectious Diseases expert . India Today. https://www.indiatoday.in/coronavirus-outbreak/story/which-mask-best-covid-corona-lancet-report-infectious-diseases-expert-dr-faheem-younus-1792226-2021-04-18. Published 2021. Accessed August 20, 2021.
- Chughtaita AA, Seale H, MacIntyre CR. Effectiveness of Cloth Masks for Protection against Severe Acute Respiratory Syndrome Coronavirus 2. Emerg Infect Dis. 2020;26(10). doi:10.3201/EID2610.200948
- Shah Y, Kurelek JW, Peterson SD, Yarusevych S. Experimental investigation of indoor aerosol dispersion and accumulation in the context of COVID-19: Effects of masks and ventilation. Phys Fluids. 2021;33(7):073315. doi:10.1063/5.0057100
- Asadi S, Cappa CD, Barreda S, Wexler AS, Bouvier NM, Ristenpart WD. Efficacy of masks and face coverings in controlling outward aerosol particle emission from expiratory activities. Sci Rep. 2020;10(1):1-13. doi:10.1038/s41598-020-72798-7
- Bulfone TC, Malekinejad M, Rutherford GW, Razani N. Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses: A Systematic Review. J Infect Dis. 2021;223(4):550-561. doi:10.1093/infdis/jiaa742
- Leclerc QJ, Fuller NM, Knight LE, Funk S, Knight GM. What settings have been linked to SARS-CoV-2 transmission clusters? Wellcome Open Res. 2020;5. doi:10.12688/wellcomeopenres.15889.2
- Tufekci Z. We Need to Talk About Ventilation. The Atlantic. 2020. https://www.theatlantic.com/health/archive/2020/07/why-arent-we-talking-more-about-airborne-transmission/614737/. Accessed August 20, 2021.
- Are Outdoor Mask Mandates Still Necessary? The Atlantic. 2021. https://www.theatlantic.com/ideas/archive/2021/04/are-outdoor-mask-mandates-still-necessary/618626/. Accessed August 20, 2021.
- Kory P. Masks! – Clearing Up the Confusion. Front Line COVID-19 Critical Care Alliance. https://covid19criticalcare.com/guide-for-this-website/masks-clearing-up-the-confusion/. Published 2020. Accessed August 20, 2021.