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Editor's Note
December 10, 2020

Filtration Efficiency of Face Masks Used by the Public During the COVID-19 Pandemic

Author Affiliations
  • 1Editor at Large, JAMA Internal Medicine
JAMA Intern Med. 2021;181(4):470. doi:10.1001/jamainternmed.2020.8234

In November 2020, the US was averaging more than 1 million new coronavirus disease 2019 (COVID-19) cases per week, an astounding number. To make progress against the pandemic, routine and universal use of face masks throughout society is essential.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is “transmitted predominantly by respiratory droplets generated when people cough, sneeze, sing, talk, or breathe,” and masks “reduce the emission of virus-laden droplets” and “help reduce the inhalation of these droplets by the wearer.”1 In the face of severe shortages of medical-grade masks, public health officials have recommended that the general public wear consumer-grade face masks to protect themselves against COVID-19, such as “non-valved multi-layer cloth masks.”1 However, there has been considerable discussion and debate about the types of masks that would be best, especially because the shortage of medical grade masks is not as acute as it once was (N95 masks remain in much shorter supply).

In this issue of JAMA Internal Medicine, Clapp and colleagues,2 building on their earlier evaluation of hospital face mask alternatives,3 report on the fitted filtration efficiency of consumer-grade cloth masks and medical procedure mask modifications as personal protective equipment for the public. The filtration efficiency refers to “the protection that masks offer to the wearer when exposed to others who may be infected,” and was assessed with a test aerosol of sodium chloride particles that were slightly smaller than individual SARS-CoV-2 virions.3

In a laboratory atmosphere, with all tests performed on a single individual for consistency, Clapp et al2 found variable fitted filtration efficiencies against the test aerosol among the face masks that ranged from approximately 25% to 80%. Comparatively, a National Institute for Occupational Safety and Health–approved N95 respirator had a filtration efficiency of approximately 98%. Notably, the filtration efficiency of some of the consumer-grade masks, such as a washed 2-layer nylon mask with ear loops and an aluminum nose bridge (79.0%), exceeded that of a medical-grade procedure mask with ear loops (38.5%), and a surgical mask with ties (71.5%). Even a folded cotton bandana had approximately a 50% filtration efficiency, which was better than the procedure mask with ear loops. In general, improved fit between the mask and the wearer’s face increased filtration efficiency, such as through use of an aluminum nose bridge or by tying the upper and lower ear loops of a procedure mask to each other near the mask and then tucking the side pleats of the mask in against the cheeks.

Face masks are not perfect. They are one of a combination of measures, including physical distancing, hand washing, sufficient ventilation, and the avoidance of crowds and gatherings, that can reduce transmission of SARS-CoV-2. The face mask that one person wears to reduce the potential release of droplets that contain virus complements the mask that another person wears to reduce the risk of inhaling these droplets.1 The consistent, correct, and universal wearing of face masks increases the benefit for individuals, and for all. This is neither rocket science nor a political statement. It is common sense and responsible behavior.

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Article Information

Corresponding Author: Robert Steinbrook, MD, JAMA Internal Medicine, Editorial Office, University of California, San Francisco, 505 Parnassus Ave, Ste M1180, Box 0124, San Francisco, CA 94143-0124 (robertsteinbrook@gmail.com).

Published Online: December 10, 2020. doi:10.1001/jamainternmed.2020.8234

Correction: This article was corrected on January 25, 2021, to fix the spelling of the author’s surname.

Conflict of Interest Disclosures: None reported.

US Centers for Disease Control and Prevention. Scientific brief; community use of cloth masks to control the spread of SARS-CoV-2. Accessed November 17, 2020. https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html
Clapp  PW, Sickbert-Bennett  EE, Samet  JM,  et al; US Centers for Disease Control and Prevention Epicenters Program.  Evaluation of cloth masks and modified procedure masks as personal protective equipment for the public during the COVID-19 pandemic.   JAMA Intern Med. Published online December 10, 2020. doi:10.1001/jamainternmed.2020.8168Google Scholar
Sickbert-Bennett  EE, Samet  JM, Clapp  PW,  et al.  Filtration efficiency of hospital face mask alternatives available for use during the COVID-19 pandemic.   JAMA Intern Med. 2020. Published online August 11, 2020. doi:10.1001/jamainternmed.2020.4221PubMedGoogle Scholar
4 Comments for this article
COVID-19: Masks with exhaust valves must be excluded.
Gary Ordog, MD, DABMT, DABEM | County of Los Angeles, Department of Health Services, (retired)
Good comments on masks during COVID-19, complimentary to original study. Please note that some are still wearing the facemasks that have the exhaust valve, made for ease of breathing when used as a 'dust mask.' Note that these do not provide any protection for other parties, and should not be used for virus protection. 
Pratical Mask Usage
Paul Weismantel, BS, Business (minor EE) | no affiliation
In real world use I am asked a lot whether a mask is effective and I learned a great simple test from a presentation in one of the many National Academy of Science workshops. With mask on hold your open palm about 3 inches from your mouth & blow with medium force. If you feel it on your hand replace your mask.

Just one other point, in the discussion of "droplets" as the medium of transmission it is useful to also include "aerosolized" virus given the clear effect of clouds carrying enough virus to create many indoor spreading
Efficacy of results in relation to time exposure
Alan Henderson |
What were the timing criteria for the study? For instance, were the masks worn for a time period of "x" minutes before the test began and rates were measured?

Are extended wear times for cloth masks over multiple day periods able to replicate these types of reduced transmission rates? Is there a saturation rate/level in the mask filtration at which the aerosolized contaminants penetrate freely? What increased risk does that pose to the general public ?

My concern is that studies conducted in such controlled settings  do not replicate accurate public incorporation of mask utilization. Studies
should push the limits (i.e., how long does a mask, of "x" type, filter / prevent transmission efficiency above 20%, what conditions exacerbate efficiency, washing re-usable cloth masks and efficiency over-time).

We know the hazards that COVID-19 can cause. We should be arming the public and government entities with the holistic analysis of the threat for public application, not for lab conditions with fresh equipment.
Full-scale national masking policy needed, so we can all wear better masks
KEITH SCHLESINGER, PhD History | Concerned citizen
The commitment of the UNC team to continue its research and assessment over a period of months and update its published results is very encouraging. Persistence and continuous work are the essential things needed if masking amongst the wider public is going to improve both in quality and in quantity.

It is high time for the disparate efforts of FDA, EPA, Dept. of Homeland Security DHS (through its Science & Technology Directorate) and CDC (through its National Personal Protection Technology Lab under NIOSH) to coordinate and reconcile themselves. That would permit the federal government to offer the
UNC team and others working on better masks (including the UI-Urbana team using electric cookers to decontaminate N95 type masks and the Beth Israel Deaconess Medical Centre/Harvard Medical School team using microwave ovens to do the same) the opportunity to have their work fully vetted for purposes of formulating a unified, consistent, and robust national public policy regarding mask recommendations, provisioning of supply chains, and ways of ensuring equity in accessing materials and methods.

The goal should be for the public to access the most protective masks available and to reuse them as long as it is reasonably safe and feasible to do so. Not only will this ease the supply chain issue, but it will also begin to address the enormous plastic pollution problem being generated by the use of disposable masks far beyond the boundaries of the professional medical community.

The do-it-yourself (DIY) spirit associated with the work done by the UNC team and others, combined with standard peer-reviewed science, government research support, and evidence-based policy formulation can produce the kind of certitude necessary to win over the uncertain and confused, and win back the skeptical and rebellious.

Absent this kind of determined, organised and comprehensive effort, the chaos that has largely reigned for nearly a year in the USA and some other countries will maintain its grip on significant portions of humanity. Given the slow deployment of vaccines, the inequitable distribution of those vaccines inside the USA and across the world, and the increasing liklihood that COVID-19 variants will reduce the efficacy of existing vaccines over the coming months, masking will likely remain the single most important protective technology available to the greatest number of people. The better we perform masking as a society, the healthier we will be from medical, psychological, educational and economic standpoints. Otherwise, we will pay a high and painful price in all these areas for continuing disunity and collective immaturity.