MUST SEE: 'Masks-for-all for COVID-19 Not Based on Sound Data'

Dr. Brosseau is a national expert on respiratory protection and infectious diseases and professor (retired), University of Illinois at Chicago.
Dr. Sietsema is also an expert on respiratory protection and an assistant professor at the University of Illinois at Chicago.


Editor’s Note: The authors added the following statement on Jul 16.

The authors and CIDRAP have received requests in recent weeks to remove this article from the CIDRAP website. Reasons have included: (1) we don’t truly know that cloth masks (face coverings) are not effective, since the data are so limited, (2) wearing a cloth mask or face covering is better than doing nothing, (3) the article is being used by individuals and groups to support non-mask wearing where mandated and (4) there are now many modeling studies suggesting that cloth masks or face coverings could be effective at flattening the curve and preventing many cases of infection. Fauci: The Bernie Mado... Ortleb, Charles Buy New $5.99 (as of 02:55 EDT - Details)

If the data are limited, how can we say face coverings are likely not effective?

We agree that the data supporting the effectiveness of a cloth mask or face covering are very limited. We do, however, have data from laboratory studies that indicate cloth masks or face coverings offer very low filter collection efficiency for the smaller inhalable particles we believe are largely responsible for transmission, particularly from pre- or asymptomatic individuals who are not coughing or sneezing. At the time we wrote this article, we were unable to locate any well-performed studies of cloth mask leakage when worn on the face—either inward or outward leakage. As far as we know, these data are still lacking.

The guidelines from the Centers for Disease Control and Prevention (CDC) for face coverings initially did not have any citations for studies of cloth material efficiency or fit, but some references have been added since the guidelines were first posted. We reviewed these and found that many employ very crude, non-standardized methods (Anfinrud 2020, Davies 2013, Konda 2020, Aydin 2020, Ma 2020) or are not relevant to cloth face coverings because they evaluate respirators or surgical masks (Leung 2020, Johnson 2009, Green 2012).

The CDC failed to reference the National Academies of Sciences Rapid Expert Consultation on the Effectiveness of Fabric Masks for the COVID-19 Pandemic (NAS 2020), which concludes, “The evidence from…laboratory filtration studies suggests that such fabric masks may reduce the transmission of larger respiratory droplets. There is little evidence regarding the transmission of small aerosolized particulates of the size potentially exhaled by asymptomatic or presymptomatic individuals with COVID-19.” As well, the CDC neglected to mention a well-done study of cloth material filter performance by Rengasamy et al (2014), which we reviewed in our article.

Is wearing a face covering better than nothing?

Wearing a cloth mask or face covering could be better than doing nothing, but we simply don’t know at this point. We have observed an evolution in the messaging around cloth masks, from an initial understanding that they should not be seen as a replacement for physical distancing to more recent messaging that suggests cloth masks are equivalent to physical distancing. And while everyone appears to understand that this messaging suggests that a cloth mask is appropriate only for source control (ie, to protect others from infection), recent CDC and other guidance recommending their use by workers seems to imply that they offer some type of personal protection.

We know of workplaces in which employees are told they cannot wear respirators for the hazardous environments they work in, but instead need to wear a cloth mask or face covering. These are dangerous and inappropriate applications that greatly exceed the initial purpose of a cloth mask. We are concerned that many people do not understand the very limited degree of protection a cloth mask or face covering likely offers as source control for people located nearby.

Do we support cloth mask wearing where mandated?

Despite the current limited scientific data detailing their effectiveness, we support the wearing of face coverings by the public when mandated and when in close contact with people whose infection status they don’t know. However, we also encourage everyone to continue to limit their time spent indoors near potentially infectious people and to not count on or expect a cloth mask or face covering to protect them or the people around them. The pandemic is not over and will not likely be over for some time. As states and local jurisdictions reopen, we encourage people to continue to assess and limit their risks. Cloth masks and face coverings likely do not offer the same degree of protection as physical distancing, isolation, or limiting personal contact time. The Case Against Masks... Heckenlively, Kent Buy New $12.99 (as of 04:52 EDT - Details)

Will face coverings ‘flatten the curve’ and stop the pandemic?

We have reviewed the many modeling studies that purport to demonstrate that cloth masks or face coverings have the potential for flattening the curve or significantly decrease the number of cases. These studies fail to recognize several important facts:

  • The filter performance of a cloth material does not directly translate or represent its performance on an individual, because it neglects the understanding of fit.
  • Cloth masks or coverings come in a variety of shapes, sizes, and materials and are not made according to any standards.
  • Transmission is not simply a function of short random interactions between individuals, but rather a function of particle concentration in the air and the time exposed to that concentration.
  • A cloth mask or face covering does very little to prevent the emission or inhalation of small particles. As discussed in an earlier CIDRAP commentary and more recently by Morawska and Milton (2020) in an open letter to WHO signed by 239 scientists, inhalation of small infectious particles is not only biologically plausible, but the epidemiology supports it as an important mode of transmission for SARS-CoV-2, the virus that causes COVID-19.

In summary, though we support mask wearing by the general public, we continue to conclude that cloth masks and face coverings are likely to have limited impact on lowering COVID-19 transmission, because they have minimal ability to prevent the emission of small particles, offer limited personal protection with respect to small particle inhalation, and should not be recommended as a replacement for physical distancing or reducing time in enclosed spaces with many potentially infectious people. We are very concerned about messaging that suggests cloth masks or face coverings can replace physical distancing. We also worry that the public doesn’t understand the limitations of cloth masks and face coverings when we observe how many people wear their mask under their nose or even under their mouth, remove their masks when talking to someone nearby, or fail to practice physical distancing when wearing a mask.


Anfinrud P, Stadnytskyi V, Bax CE, et al. Visualizing speech-generated oral fluid droplets with laser light scattering. N Engl J Med 2020 (published online Apr 15)

Davies A, Thompson KA, Giri K, et al. Testing the efficacy of homemade masks: would they protect in an influenza pandemic? Disaster Med Public Health Prep 2013 Aug;7(4):413-8

Green CF, Davidson CS, Panlilio AL, et al. Effectiveness of selected surgical masks in arresting vegetative cells and endospores when worn by simulated contagious patients. Infect Control Hosp Epidemiol 2012 May;33(5):487‐94

Johnson DF, Druce JD, Birch C, et al. A quantitative assessment of the efficacy of surgical and N95 masks to filter influenza virus in patients with acute influenza infection. Clin Infect Dis 2009 Jul 15;49(2):275-7

Konda A, Prakash A, Moss GA, et al. Aerosol filtration efficiency of common fabrics used in respiratory cloth masks. ACS Nano. 2020 (published online Apr 24)

Leung NHL, Chu DKW, Shiu EYC, et al. Respiratory virus shedding in exhaled breath and efficacy of face masksNat Med 2020 (published online Apr 3)

Ma QX, Shan H, Zhang HL, et al. Potential utilities of mask-wearing and instant hand hygiene for fighting SARS-CoV-2. J Med Virol 2020 (published online Mar 31)

Morawska L, Milton DK. It is time to address airborne transmission of COVID-19. Clin Infect Dis 2020 (published online Jul 6)

National Academies of Sciences, Engineering, and Medicine. 2020. Rapid expert consultation on the effectiveness of fabric masks for the COVID-19 pandemic. Washington, DC, National Academies Press. Apr 8, 2020

Rengasamy S, Eimer B, Szalajda J. A quantitative assessment of the total inward leakage of NaCl aerosol representing submicron-size bioaerosol through N95 filtering facepiece respirators and surgical masks. J Occup Environ Hyg 2014 May 9;11(6):388-96

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