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Low-cost measurement of face mask efficacy for filtering expelled droplets during speech

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Science Advances  02 Sep 2020:
Vol. 6, no. 36, eabd3083
DOI: 10.1126/sciadv.abd3083

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  • RE:

    In Fig. 3A of the published manuscript we show the relative droplet count for each mask, obtained by counting the number of emitted particles (corrected for the appearance of the same particle in successive video frames) relative to the no-mask reference. A quick visualization (shown in some news reports) can be obtained from a maximum intensity projection over the duration of the experiment, but the processing steps outlined in the manuscript are required for quantification.

    Fig. 3 displays the number of particles, irrespective of their size. As detailed in our work, we cannot accurately determine the size of very small particles. In the supplementary materials we show a qualitative size distribution of the larger emitted particles, constrained by our image resolution. Even in this limited size range we see a relative shift of particle size to smaller sizes for some masks, particularly our neck gaiter.

    There is an ongoing debate about the contribution of particles originating from the mask itself, generated during speaking. From our studies (including several follow-up experiments) we believe that for our masks the contribution of mask fragments were minor. However, for practical mask use this distinction of origin is not important since viruses can be contained in (or attached to) the particles in either case.

    Competing Interests: None declared.
  • RE: Low-cost measurement of face mask efficacy

    I found this research previously with pictures of most if not all of the face masks being tested showing the particles in the air. I've seen a few videos online that briefly show this but it's not complete showing each mask, and it's not images like it was originally. Is it possible to get the pictures showing the snapshots of the fleece gaiter where it showed more particles than the baseline?

    I ask because I specifically remember seeing obvious signs of the extra particles for the fleece gaiter being the fibers of the material being released into the air, and not particles from the face or mouth being split into smaller particles. I'd read the paper and seen the pictures before it was picked up by mainstream media. I thought the error being reported in the news would be corrected.

    Competing Interests: None declared.
  • RE: Collective response to comments on "Low-cost measurement of face mask efficacy for filtering expelled droplets during speech"
    • Martin Fischer, Duke University
    • Other Contributors:
      • Warren Warren, Duke University

    We appreciate the interest that many people have shown in our work, both here and in emails.  Many comments have led us to put some clarifications in the final online version (for example, while “neck gaiter” and “neck fleece” are both terms in common use, the latter term is confusing to many people, so the final version uses “gaiter” only). 

    We would like to address the comments in the eLetters in broad sweep.

    1. As the title indicates, our work focused on developing a simple technique for mask evaluation that can be replicated at other labs, rather than a comprehensive mask test. As we stated in the paper, our work was a preliminary study that included a set of masks worn by one person, and a subset of masks worn by four persons.  We did not do a systematic study involving many masks, speakers, and wear conditions - that would have delayed publication by many months. We expect that there are variations of performance for different styles and different materials, as well as different users wearing identical masks. More detailed studies are needed to make specific use recommendations, hence we avoided rank ordering masks other than for clarity in the graphs.

    2. We detect particles of size greater than about 0.5 µm, but cannot reliably determine the size of the smallest particles. In addition, our measurement makes no attempt at discriminating particles transmitted through the mask from particl...

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    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech

    No doubt the authors are aware of the importance of encouraging the use of face masks while at the same time publishing research on which masks might be more or less effective. One significant limitation in trying to interpret these results is the difficulty in determining the types of masks/fabrics that were tested. For example, four of the types of masks listed are “knitted, cotton, fleece, and bandana.” These four terms refer to four distinct categories that are not mutually exclusive. Knitted is a type of fabric, cotton is a fabric content, fleece is a type of fabric construction and bandana refers to a shape. One could easily construct a fleece bandana out of knitted cotton!

    Further, the authors focus on “neck fleece” as being particularly ineffective. Many if not most neck gaiters (which the authors term “neck fleece”) are not made of fleece but of some type of polyester jersey microfiber. Was the “neck fleece” that was tested actually fleece, or some other fabric? Within the category of neck gaiters, the variability of available fabrics and potential effectiveness of each (partly depending on density of the weave) makes the information contained confusing.

    While the authors say “… we do not attempt a comprehensive survey of all possible mask designs…. We merely demonstrated our method on a variety of commonly available masks and mask alternatives”, in future testing more detail and accuracy describing fabric construction, including density of the weav...

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    Competing Interests: None declared.
  • RE: Valved masks

    I was surprised at how well the valved N95 masks performed, in the chart, it appears to reduce particles by about 85%. compared to no mask, and is in the middle of the pack, 7 masks are worse, some considerably worse, 7 better.

    Nevertheless, the article states:

    "... the performance of the valved N95 mask is likely affected by the exhalation valve, which opens for strong outwards airflow. While the valve does not compromise the protection of the wearer, it can decrease protection of persons surrounding the wearer. In comparison, the performance of the fitted, non-valved N95 mask was far superior."

    Quoted in the Washington Post, one of the authors went further:

    “Those relief valves are fantastic if what you want to do is protect yourself from the outside world because air doesn’t come in through them,” Warren said. “If what you’re trying to do in this pandemic is protect the outside world from you, it completely defeats the purpose."

    Seems to me that the results in the article don't support these statements. Yes, a valved mask is inferior to an unvalved, well fitting 95 mask. But it's a lot better than nothing, and better than many masks. Moreover, a comfortable mask is more likely to be worn -- how many times do you see people wearing a surgical mask with their nose hanging out?

    I own several masks with valves that appear to be designed to direct the airflow down, presumably to avoid fogging glasses,...

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    Competing Interests: None declared.
  • RE: statements of exhaust valved masks
    • Warren Wartell, Product Developer / Regional Director for House of X, House of X

    Hello. Very interesting and compelling research and findings! Several recent media reports have cited this publication, and some mask policy is being shaped partially based on these findings. I am particularly focused on presumptions made regarding "valved" masks, and also the this study's apparent singular focus on emissions only toward the front of the speaker, and not the sides of speakers face. In the Discussion section of this publication, the author's make several definitive statements about the performance of the valved N95 mask "Furthermore, the performance of the valved N95 mask is likely affected by the exhalation valve, which opens for strong outwards airflow. While the valve does not compromise the protection of the wearer, it can decrease protection of persons surrounding the wearer. In comparison, the performance of the fitted, non-valved N95 mask was far superior." Some valved masks incorporate a filter in the exhaust valves - such as the xMask by Also, a non optimal fit allowing gaps at the wearer's nose, cheeks or chin would seemingly allow extensive expelled droplets to exhaust - though not necessarily to the front of the speaker. The typical surgical mask specifically is notoriously loose fitting. It would sure seem this is a huge liabaility for moisture exhaust out the sides of the wearer - potentially an enormously greater risk in a side-by-side passenger situation in an airplane or public transpo...

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    Competing Interests: I have participating in research and development of the valved xMask product, which incorporates a removable filter for the exhaust valve.
  • RE: Efficiency of fleece gaiters

    Do you have any data on fleece gaiters being folded over to create multiple layers?
    Your results indicate that fleece gaiters increase the particle count and decrease particle size, but do they affect particle velocity in any way?

    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech

    Your article is very confusing in its description of the material of the neck gaiter used in the test. In the photo, it looks like a polyester-spandex material, but in the article you call it a "fleece" neck gaiter. There is a huge difference between fleece material and polyester-spandex. "Fleece" material is typically thicker and fuzzier. Neck gaiters for winter are often made of "fleece" material, but neck gaiters for spring, summer and fall are made of lighter weight polyester-spandex material. You really should get a materials expert to provide you with the correct description of the material you used in the tests and then update your article.

    There's too much confusing information about this material, especially when articles in newspapers, such as the Washington Post, refer to it as "stretchy polyester-spandex", not fleece.

    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech
    • Steve Price, Consultant/Asset manger, Terra Property Analytics

    I think is is a good article and timely and I think the authors do a good job laying out their methodology and the scope of their findings. But I'm shocked at how it has been spread in the national press and how broad generalizations are being made that the paper itself (and the authors as well I assume) would not support.
    I think the take away from this study is just how desperate people are for information and opinions on masks.
    Reading the actual study, the authors are very clear that this is a low-budget experimental set up with very few observations. I do not think they really intended it to have a national audience or to have the results generalized to cover public mask wearing. Clearly their finding that a neck gator produces more smaller particles than no mask at all is interesting. But there are a couple of barriers to generalizing the results that the press miss:
    what is being measured are particles 1.4 inches out from the mask/speaker. It does not tell you anything about what happens a foot from the mask or 3 feet from the mask, which is where it probably matters for mask use in public.
    It only measures the number of particles, not how fast they are going or how far they can go.
    They only had 4 different people as test subjects
    The statistics for the bandanna and neckgator versus no mask show huge variations in the standard deviation of the measurements. So given very few observations it is unlikely there is much statisti...

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    Competing Interests: None declared.
  • RE: Neck Fleece

    What material is the "neck fleece", and how many layers were worn over the speaker's mouth?

    Competing Interests: None declared.
  • Interesting Methodological Article, Uncertain Relevance to COVID-19
    • John Murphy, Adjunct Professor, University of Toronto, Dalla Lana School of Public Health

    While methodologically interesting, it appears that the technique employed by the authors likely reflected air concentrations of relatively large sized droplets (e.g. ca. 100 um) that penetrated the various fabrics. This appears to be the point made by Figure S5, and would explain the very superior performance of the N95 (valve-free). It would be worth determining whether the configuration of the chamber used in the experimental set-up results in passage into the chamber of droplets that escape around the mask seal. The results for the surgical mask suggest that might not be the case. Growing evidence suggests the small respiratory droplet fractions (i.e. << 5 um) expelled by breathing, vocalizing or coughing play an important role in COVID-19 transmission. If the methodology used in this demonstration trial was insensitive to this small droplet fraction, then no conclusions can be drawn from the results with respect to the efficacy of the various common fabric masks in reduction of COVID-19 transmission risk. That requires clarification as there would seem to be considerable potential for misinterpretation of the reported results.

    Competing Interests: None declared.
  • suggestion

    Good job and very interesting finding regarding neck fleece. It would be great to repeat the tests after e.g. 1 hour wearing these face masks. It would be more similar to the normal life. We can suppose, that after 1 hour shopping will be all masks saturated with humidity and physical properties will substantially change.

    Competing Interests: None declared.
  • RE: Masks Used

    I wish the graph showing droplet dispersion matched the pictured masks (1-13). The textual description used in each mask is not very helpful in identifying which masks performed poorly...most notedly, which is the one that actually heightens the amount of particles?! I see that a neck "buff" was used in the testing, but do not know which textual description matches that item. I hope the results are updated so people can more easily identify the items.

    Competing Interests: None declared.
  • RE:

    I went to this article because I was hoping to see how effective (or not) the popular mask that has a round "filter" attached to one side. From what I understand, the filter is actually a one-way valve that closes when one breathes in, but opens when they exhale. That means it's purpose is to protect the wearer from breathing in dust, and presumably germs, while protecting people near the wearer not at all. Since the point of wearing a mask during this pandemic is to protect the populace, not the wearer, I think there are a lot of people wearing this type of mask who think they are being socially responsible, but actually are not. Could you perhaps add this type of mask to your study? Or do a separate study on it?

    Competing Interests: None declared.
  • RE: figures and captions

    I'm confused. Fig. 2 numbers the various masks, but Fig 3 describes them with words. Is there another table that correlates these two?

    Competing Interests: None declared.
  • RE: Valved N95

    Please provide the NIOSH approval number for the "Valved N95" device pictured as Exhibit #2 in Figure #2.

    Given the ear loop configuration, it is likely this device is not actually a NIOSH approved N95 device.

    To my knowledge, NIOSH approved N95 devices, such as the 3M respirator shown as Exhibit #14 in Figured #2, are configured with head straps.

    Competing Interests: None declared.
  • RE: Gaiters and droplets

    Nice study.
    Does the performance of the gaiter depend on (a) the tightness of the weave of the fabric and/or (b) the number of layers of fabric? Would wearing two gaiters (or stitching two of them together) solve the problems of the one gaiter? Is the comparison in the study between multi-layer masks and single layer gaiters (and bandanas)?


    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech
    • Katherine Kreis, RN-Clinical Instructor, University of Texas at Arlington - College of Nursing and Health Innovation

    The article is very informative. I particularly appreciate the means by which it encourages research at an accessible level.

    One suggestion to clarify which mask you're referring to, would be to add the numeric value associated with the mask in fig 1 to the labels in fig 2. The findings are intriguing, yet further information is needed on the materials and styles of each mask tested.

    For example, several masks appear substantially similar (4, 6, 7, 9, & 10), so it would be of interest to note the type of material and number of layers (including any filter layers). This information represents one possible confounding factor, since the fleece is presumably only one layer, while the cotton masks may have more than one layer. Overall, this is a very interesting read and establishes a need for further investigation into the quality of mask materials and design.

    Competing Interests: None declared.
  • RE: Does test setup measure only in front of face?

    Thank you for doing this research. I'd like to see a photograph of the test setup. If I understand how it operates, it only measures the droplets expelled through the front of the mask and passing before the camera. But since most masks do not adhere to the face or fit tightly, most of the air exhaled escapes around the sides of the mask (as anyone who wears glasses can tell you). That means that most of the viral particles spread by an infected person would float into the air even if the mask were 100 percent efficient at trapping particles that pass through it. Can you address this concern? Best, --Dan Littman

    Competing Interests: None declared.
  • RE: gaiter material

    Castelli cycling made a gaiter that uses Soft Flex ( their proprietary material) which feels like a synthetic jersey material. No fleece. I would like to know if this fails your test, because I suspect that you’ll need to further classify gaiters instead of a general statement.
    Thank you.

    Competing Interests: None declared.
  • RE: "fleece" material of neck gaiter
    • Heather Walder, Lecturer, Archaeologist, University of Wisconsin - La Crosse

    Fischer et al. have provided a detailed and thorough analysis of mask efficacy, but a simple piece of information is missing from Table 1. Although the material is identified for some masks, such as #5 ("Poly/Cotton"), it is not identified in Table 1, or elsewhere, for several face coverings, including #11 ("Fleece"). In the table, the description of "Gaiter type neck fleece" is worded such that it indicates the shape of the face covering (a "neck fleece") but not the material. In Figure 1, the garment shown as #11 does not appear to be made of the material commonly referred to as "fleece" (a synthetic, cold-weather fabric, a kind of man-made wool, such as the brand "Polar Fleece").

    Rather, #11 appears to be a stretchy, synthetic fabric. "Buff" brand neck gaiters are made of 95% Repreve® Polyester, 5% Elastane ( ).
    The difference between "fleece" and a stretchy polyester synthetic blend seems critical to the apparent increase in droplet count, even relative to no mask, observed in this experiment.

    We suggest that Table 1 be amended to include the type or composition of the material tested for all face coverings. Anecdotally, people engaging in outdoor activities such as running and bicyc...

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    Competing Interests: None declared.
  • RE: Fleece mask

    Can you describe the fleece mask? Is it like polar fleece that is warm and designed to allow warm, wet breath to penetrate? Or is it like the sun gators used for fishing and which many people use this summer.

    Competing Interests: None declared.
  • RE: Valved N95 Mask

    It is not clear from the article if mask #2 ("Valved N95") had its filter media installed correctly or not. This type of mask, fresh out of its package, will likely not have its filter media installed. Without it, it will perform similar to or worse than other cloth masks. So it would be good to know if any mask with replaceable filters had them installed correctly or not, and what filter media was used.

    Note that it is easy to accidentally fold the filter media during installation which will greatly reduce the performance of the mask.

    Competing Interests: None declared.
  • RE: Low Cost Filtering of Facemask Efficacy

    Thank you for your very interesting study of a very practical and urgent issue to help contain the pandemic . Your study nicely addresses the efficacy of a number of materials as masks. However, whether the position of the wearer’s face allows leakage around the edges of the masks to be included in the measurements is unclear to me. My concern about so many masks, from surgical to homemade, including several styles I have made, is the fit and air/droplet/aerosol escape around the mask edges as they are usually worn. I hope that you will be able to address measurement of leakage in further studies. Also useful would be measurements of transmission when the wearer covers only the mouth but not the nose, as is often seen in public places.

    Competing Interests: None declared.
  • RE: Low Cost measurment of facemasks, etc

    This was a good study, but to be more useful, It would have been better to show each of the 14 masks on the graph so that we could match them up with the picture of masks. I have no idea what mask 1 or mask 2 refers too. A chart on ranking masks along with pictures would be helpful to lay people desperate to protect themselves. All I can tell from here is that N95 and surgical masks are good and fleece is worse than nothing.

    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech

    Dear Madams and Sirs,

    I read your report with great interest, particularly since my "mask" of choice is a collection of neck gaiters. Needless to say, the results pertaining to neck gaiters were very surprising. However, upon further reflection, I am not surprised, yet concerned that the results will be misconstrued.

    The results of the study have already been picked up by several media outlets who state that neck gaiters are actually worse than not wearing any masks at all. I very much beg to differ and hope that Duke will issue some clarifying language.

    Your study essentially tested the droplet count and size at about 15 cm (roughly 7 inches) from the subject's mouth. However, nowhere in the study is there any mention about the viability or distance of travel of these particles beyond the light sheet in the experiment.

    I do not dispute the higher efficacy of certain types of masks over neck gaiters. However, I contend that the distance of travel of droplets when wearing a neck gaiter will be significantly less than when not wearing any face covering at all. So, will there be a higher droplet count using a neck gaiter than without one? Apparently. Will these droplets travel farther when wearing a neck gaiter than without one? I propose that they do not, and I believe that this should be mentioned as a caveat in the discussion area of the study report.

    Very sincerely,

    Nico Lacchini

    Competing Interests: None declared.
  • RE: evaluation of 14 masks needs supplementary labels

    Excellent work!
    problem : pictures of 14 masks are identified only by number.
    versus Key chart showing results are identified only by Names such as cotton2.
    Not clear how to match the 1-14 # with the names in the results.
    Suggestion : label each picture with both their # and their experiment name.
    Thank you.

    Competing Interests: None declared.
  • RE: neck fleece

    I've been using neck gaiters, but they're made of a material called "ice silk." Is that the same as the neck fleeces you are talking about?

    Also, have you tested cloth masks with a PM2.5 filter? I've been using those as well.

    Additional comments and questions:

    The first has to do with a distinction between water droplets and viruses. Water molecules are smaller than viral particles. To what extent does measuring the number of droplets reflect the number of viral particles? I’ve seen other research indicating the cotton blocks half of viral particles. Also, if the fleece fractures the water droplets, it may actually be blocking more viral particles than the other fabrics, rather than less.
    Also, the research of Lydia Bourouiba tied the distance that water droplets fly to their initial horizontal velocity upon leaving the mouth or nose. If the masks slow the horizontal velocity of the droplets, they may fall more quickly. I saw a video by a user called “Madison masks“ on YouTube that showed videos of droplets going through masks. It did seem that masks slowed the horizontal velocity of the droplets substantially.

    Competing Interests: None declared.
  • RE: Relationship between Masks in Fig 1 and results in Fig 3

    There is no clear relationship with the masks listed in Fig 1 (numbers from 1 to 14) to the masks performances showed on Fig 3 (masks names).

    Can you include this relationship in a table or directly in Fig 3 labels?

    Competing Interests: None declared.
  • RE: Face mask test

    Interesting article. It is assumed that it was published for web consumption. The information flow lacks a needed feature. The masks are identified by number in Figure 2. That helps. The results are displayed in figure 3A by name/material. You must then locate Table 1 to associate the mask number to the mask name/material. This is not always simple in the web format. How about adding the mask numbers along with the names in Figure 3A?

    Competing Interests: None declared.
  • RE: Clarifying information in figures 2 and 3

    Please match the masks shown in Figure 2 with the outcomes in Figure 3. It’s not clear what masks shown are effective and what masks are not.

    Thank you.

    Competing Interests: None declared.
  • RE: Bandana

    Which brand and country of origin was the bandana used in this study? I've noticed a dramatic difference in the USA and China variants of the same brand (Hav-a-Hank) and would like to know which was tested.

    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy for filtering expelled droplets during speech

    Greetings! In your study you conclude that fleece neck gaiters perform worse than no face covering at all. Would you attribute that to the gaiter style or the fleece fabric? Do you imagine that a gaiter made of the same materials as a higher-performing mask might also provide better protection? Thanks for any clarification you might be able to provide.

    Competing Interests: None declared.
  • RE: Low-cost measurement of facemask efficacy...
    • Elaine B. St. John, Assoc. Professor Pediatrics (retired), University of Alabama at Birmingham

    This is good information and nicely done; but, difficult to put together. I assembled the information into a graphic that is easily understood by the lay public. While as scientists we may be loathe to put qualitative labels on outcomes, this is not merely opinion but based on your data. We don't need to be reticent to disseminate information if it's the best we have in this difficult situation.

    Elaine B. St. John, MD

    Competing Interests: None declared.
  • RE: So can you rank these? Lots of info but vague results

    Hello, this article needs to rank the masks shown and also show n95. Lots of work to not provide a ranking of any sort.

    Competing Interests: None declared.
  • RE: Mask types


    As a consumer how are we to determine if a particular mask we see on the internet matches the mask type you are using? You label one mask as 'gaiter type neck fleece', but I'm looking at neck masks made from polyester. Are they in the swath of polypropylene category since they aren't made of fleece?

    Competing Interests: None declared.

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