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. 2021 Apr 22;16(4):e0250432. doi: 10.1371/journal.pone.0250432

Effects of distancing and pattern of breathing on the filtering capability of commercial and custom-made facial masks: An in-vitro study

Lorenzo Ball 1, Stefano Alberti 2,*, Claudio Belfortini 2, Chiara Almondo 1, Chiara Robba 1, Denise Battaglini 1, Carlo Cravero 3, Paolo Pelosi 1, Valentina Caratto 2, Maurizio Ferretti 2
Editor: Francesco Di Gennaro4
PMCID: PMC8062003  PMID: 33886642

Abstract

Background

Since the beginning of the COVID-19 pandemics, masking policies have been advocated. While masks are known to prevent transmission towards other individuals, it is unclear if different types of facial masks can protect the user from inhalation. The present study compares in-vitro different commercial and custom-made facial masks at different distances and breathing patterns.

Methods

Masks were placed on a head mannequin connected to a lung simulator, using a collecting filter placed after the mannequin airway. Certified, commercial and custom-made masks were tested at three different distances between the emitter and the mannequin: 40 cm, 80 cm and 120 cm. Two patterns of breathing were used, simulating normal and polypneic respiration. A solution of methylene blue was nebulized with a jet nebulizer and different mask-distance-breathing pattern combinations were tested. The primary endpoint was the inhaled fraction, defined as the amount of methylene blue detected with spectrophotometry expressed as percent of the amount detected in a reference condition of zero distance and no mask.

Findings

We observed a significant effect of distance (p < 0.001), pattern of breathing (p = 0.040) and type of mask (p < 0.001) on inhaled fraction. All masks resulted in lower inhaled fraction compared to breathing without mask (p < 0.001 in all comparisons), ranging from 41.1% ± 0.3% obtained with a cotton mask at 40 cm distance with polypneic pattern to <1% for certified FFP3 and the combination of FFP2 + surgical mask at all distances and both breathing pattern conditions.

Discussion

Distance, type of device and breathing pattern resulted in highly variable inhaled fraction. While the use of all types of masks resulted relevantly less inhalation compared to distancing alone, only high-grade certified devices (FFP3 and the combination of FFP2 + surgical mask) ensured negligible inhaled fraction in all conditions.

Introduction

Since December 2019, the pandemic of coronavirus disease (COVID-19), sustained by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) had a dramatic impact on healthcare systems as well as people’s lifestyle and social behavior [1]. This disease is transmitted through several routes, but the respiratory route is the principal one. Half of patients 2019-nCoV RNA was detected, thus concluding that saliva droplets and water aerosol may actually bring living virus [2]. In fact, the disease is mainly transmitted through the emission of droplets during coughing and sneezing, while the airborne route is more debated [3]. The distinction between these two routes is based on the size of emitted particles, with a cut-off diameter commonly set at 5 μm: droplets which are larger than 5 μm contain a large amount of virus but can spread for a shorter distance, while aerosols of smaller size could remain in the ambient air in suspension resulting in contagious aerosols [3, 4]. Also normal speaking may cause airborne virus transmission in confined environments, where small droplets likely play a major role [5].

Shortage of personal protection equipment (PPE) has heavily affected the healthcare systems in the early phases of the pandemics and have led to the public use of a variety of solutions which are generally of unknown efficiency [6]. Masks reduce the spread and transmission of respiratory particles and droplets potentially containing viruses and their use in the general population has been adopted in most countries, even though still under debate [7, 8]. In general, while the gold standard for personal protection is represented by certified respirators [9], their high cost and low availability brought interest towards cheaper alternatives such as surgical masks as well as custom-made protection devices, especially for non-healthcare workers and general population; despite the characterization of their efficacy could represent a significant knowledge for medical professionals working in areas with a low but still real risk of contamination. The National Institute for Occupational Safety and Health defined N95 and N99 standards in the United States and the European Committee for Standardization specified with the EN 143 regulation standards for type 1, 2 and 3 filtering face pieces (FFP1, FFP2 and FFP3). While there is an undoubted role of facial masks as mean to reduce the spread of aerosols and droplets from unaware infected patients, it is unclear whether non-PPE devices can offer some degree of protection to healthy individuals [10, 11].

We aimed to compare the filtering capabilities of commercial and custom-made facial masks in an in-vitro model of mixed airborne-droplet aerial transmission. We hypothesized that the distance from the emitter and the pattern of breathing could affect the protection offered to a model of spontaneously breathing adult by the different types of investigated devices.

Methods

Experimental model

As illustrated in Fig 1, the emitter was simulated using a Hudson Micromist (Teleflex, US) small volume jet nebulizer operated at 7 L/m driving flow. Under these conditions, the nebulizer generates a distribution of drops with a median size of 5 μm and an asymmetric distribution [12] thus covering the range of both airborne and droplet transmission. As marker, we nebulized a solution of 3.44 g/L of methylene blue.

Fig 1. Experimental setting.

Fig 1

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The receiver was simulated with a pneumatic lung simulator (Dimar, Mirandola, Italy) connected to a head mannequin to simulate an exposed individual, protected with different masks. We simulated two respiratory patterns: normal breathing with 550 mL tidal volume and respiratory rate of 14 min-1, and mildly polypneic pattern with 550 mL tidal volume and respiratory rate of 20 min-1.

The emitter was placed in front of the receiver at three distances: 40, 80 and 120 cm. Between the receiver and the lung simulator (Fig 1), a custom-made filter with disposable cotton pads, as described in a previous study [13], was placed to intercept the nebulized methylene blue. The collecting filter was placed after the mannequin airways, to estimate the amount of nebulized particles inhaled by this model of spontaneously breathing subject. The nebulizer was operated for 40 min loaded with 16 ml of methylene blue solution refilling the nebulizer to its maximum capacity during two brief interruptions of nebulization lasting <10 seconds, and each mask-distance-breathing pattern combination was measured three times, in replicate. The experimental time of 40 min was chosen to simulate a scenario of prolonged person-to-person exposure in a closed environment, such as occurs in several social settings. The nebulization time was also titrated in order to achieve sufficient sensitivity for the measurement of inhaled fraction at the precision level of 1%.

As detailed in Table 1, we tested certified PPEs, commercially available non-PPE masks and custom-made devices and compared them to the effect of distancing alone, without mask. Fig 2 shows the tested masks, non-PPEs and custom-made devices in all examined configurations (i.e., alone and their combinations): Surgical mask, FFP1, FFP2, FFP3, FFP2 + Surgical mask, Malpositioned FFP2 (uncovered nose), Cotton mask, Dusting Cloth mask and Cotton + Dusting mask.

Table 1. Detailed description of the different types of tested devices, according to their certification, commercial name and manufacturer.

Short name Type of device Description Commercial name Manufacturer
Surgical mask Commercial Multilayer surgical mask Non-woven face mask Sinomedic, Serbia
FFP1* Commercial, certified PPE certified as FFP1 Shell Mask XMASC Icoguanti S.p.A., Italy
FFP2* Commercial, certified PPE certified as FFP2 Aura 9320+ 3M, Minnesota, US
FFP3* Commercial, certified PPE certified as FFP3 2737 FFP3 RD GVS Filter Technology, UK
FFP2* + Surgical mask Combination of two devices FFP2 mask plus a covering surgical mask
Malpositioned (uncovered nose and covered mouth) FFP2* Commercial, certified, malpositioned FFP2 placed not optimizing fitting and leaving a small space between nose and mask
Cotton mask Commercial, not certified Two-layers cotton plus non-woven tissue mask
Dusting cloth mask Custom-made Two layers of dusting cloth shaped as face mask Swiffer Procter & Gamble, US
Cotton + dusting mask Combination of two devices Cotton mask plus a covering dusting mask
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*FFP stands for Filtering Factor Protection of type 1, 2 and 3, according to EN 143 regulations standards, as depicted in Introduction paragraph.

Fig 2. Tested devices.

Fig 2

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Inhaled fraction measurement

The amount of methylene blue deposed on the cotton pads was analyzed with ultraviolet-visible spectrophotometry (Lambda 35, Perkin Elmer, US), operated at a wavelength of 664 nm, corresponding to the minimum transmittance of methylene blue [14]. We compared the amount detected at each mask-distance-breathing pattern combination with the amount detected with the emitter placed directly in contact with the mannequin airway opening, considered as reference (100% inhaled fraction). A blank cotton pad was used as reference for 0% inhaled fraction. The filtering capability was expressed as the percent inhaled fraction of methylene blue compared to zero-distance and no mask with the two patterns of breathing. We assessed the minimum amount of detectable inhaled fraction (i.e., sensitivity) in a calibration experiment in which nebulization time was decreased in 1 s steps. The minimum nebulization time required to detect a non-zero transmittance was 22 s, corresponding to 0.92% of the nebulization time used in our experimental runs. We therefore assumed a sensitivity of 1%; when the measured inhaled fraction was below this threshold, we assumed an inhaled fraction of 1%.

Statistical analysis

The filtering capability was reported as percent inhaled fraction compared to distancing alone and aggregated as average ± standard deviation. We used a linear model to investigate the effects of distance, device and pattern of breathing on inhaled fraction. We used the Dunnett test for post-hoc multiple comparisons. All analyses were performed with SPSS 25.0 (IBM, Chicago, Illinois). Statistical significance was considered for two–tailed p < 0.05.

Results

Report from Fig 3 shows the percent inhaled fraction at 40 cm, 80 cm and 120 cm with all the tested devices and the two patterns of breathing and compared to unmasked distancing alone. At the linear model modeling inhaled fraction as function of the following parameters, we observed a significant effect of distance (p < 0.001), pattern of breathing (p = 0.040) and type of mask (p < 0.001) on the inhaled fraction. All masks resulted in an inhaled fraction lower than that without mask (p < 0.001 in all pairwise post-hoc comparisons). However, the inhaled fraction was highly variable among devices, with the highest value of 41.1% ± 0.3% obtained with a cotton mask at 40 cm distance with polypneic pattern and only certified FFP3 and the combination of FFP2 + surgical mask ensuring inhaled fraction below 1% at all distance and breathing pattern conditions. At any distance, all devices had higher filtering capability than distancing alone and the magnitude of differences amongst devices was higher at closer distances. The polypneic pattern of breathing increased the inhaled fraction with all devices except FFP1, FFP2 and FFP3 masks at 40 cm and 80 cm, while at 120 cm we did not observe differences between patterns except for the surgical mask. Also, incorrect positioning of the mask decreased the mask’s performances (Figs 3 and 4). The dependence of the percent inhaled fraction as a function of the distance and breathing pattern is shown in Fig 4.

Fig 3. Effects of different distances and pattern of breathing on the inhaled fraction with the tested masks during normal (white bars) and polypneic (gray bars) pattern of breathing.

Fig 3

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The inhaled fraction is expressed as percent of the amount inhaled without mask and at zero distance from the emitter. Bars represent means, error bars the standard deviation. The horizontal gray bar represents the sensitivity limit of our technique. *Significant difference compared to distancing alone (p<0.001), °Significant difference compared to distance of 40 cm, § significant difference compared to normal pattern of breathing at same distance (p<0.05).

Fig 4. Effect of distance from the emitter on inhaled fraction with different masks during normal (A) and polypneic (B) pattern of breathing.

Fig 4

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*Significant difference compared to distancing alone (p<0.001).

Discussion

The main findings of this study are that: 1) all tested masks reduced the inhaled fraction compared to distancing alone, but their efficiency was highly variable; 2) the differences between devices were more pronounced at shorter distances; 3) a polypneic breathing pattern led to an increase in inhaled fraction in most tested conditions.

The determination of masks’ efficacy is a complex topic that still represents an active field of research. In the present paper, we developed an inexpensive method to test the filtering capabilities of several face masks, simulating different scenarios and factors affecting their efficiency. In a recent in-vitro study, authors demonstrated that several custom-made masks approached the performances of surgical masks [6]. However, in that study authors focused on the masks’ capability of filtering expelled droplets, thus the efficacy of masks in protecting other individuals, while in this study we focused on the inhaled fraction, i.e. the ability of masks to protect the individual from an unknown, unmasked subject.

We investigated the effect of different type of devices considering distance and type of pattern of breathing as factors modifying mask efficacy. The jet nebulizer guaranteed a distribution of nebulized particles covering the size range of both droplet and airborne transmission models. While the main route of transmission of COVID-19 seems to be related to larger droplets, smaller drops emitted during normal speaking can survive in suspension in the air, resulting in airborne transmission [15]. Direct airborne transmission due to coughing or sneezing is a complex phenomenon: bigger particles may remain in the surrounding air for several minutes prior to dropping to the ground; while smaller particles tend to remain into the air, thus favoring long-distance diffusion. In case of absent ambient ventilation, these particles will slowly and steadily diffuse throughout the space, remaining in the air for many hours [16]. These mechanisms should be carefully accounted for in this pandemic frame, as they play a crucial role for understanding the airborne transmission of infectious diseases [17, 18].

Considering a short distance as in the 40 cm experimental setting, a typical distance between an infected patient and a healthcare professional or occurring during close social interaction, the only PPE that reduced inhaled fraction to below 1% were only FFP2, FFP3 and FFP2+surgical mask. Moreover, incorrect positioning of the FFP2 mask resulted in a relevant decrease of its filtering capability. Custom made devices based on cotton, dusting cloth and their combinations offered low protection but comparable to that obtained with a surgical or FFP1 mask. Distance alone markedly reduced the inhaled fraction also without masks, thus underlining how physical distancing intrinsically increase the protective effect of all other measures. In fact, at 120 cm from the emitting source, both breathing patterns resulted in an inhaled fraction below 5% also without mask and any type of mask lowered this value below the sensitivity threshold of our technique with a quiet pattern of breathing. On the other hand, at the same distance but with a polypneic pattern, only FFP2, FFP3 and dusting cloth custom-made devices ensured inhaled fraction below 1%. However, also in the worst studied scenario, thus with 40 cm distance and polypneic breathing, also cheap non-certified and custom-made devices reduced the inhaled fraction to 26%-41%, depending on the device, compared to more than 60% obtained without mask. Whether these values are sufficient to actually reduce the risk of infection is difficult to ascertain, there is rising consensus on the fact that also reduction of initial inhaled viral load could be beneficial in reducing the severity of the disease [19].

This in-vitro study adds to the current knowledge as it explores the hypothesis that, in addition to preventing dispersion of droplets, non-certified devices might anyway offer a low but significant reduction of the inhaled fraction. Previous epidemiologic investigations have suggested a strong relationship between public masking and pandemic control, reducing the growth of the epidemic curve [19]. Our findings suggest that this might also be related to some degree of individual protection, in addition to the known ability of protecting others through reduction of emitted droplets.

This study has limitations that should be addressed. First, it is an in-vitro study, thus we were not able to conclude on actual potential of preventing infections. However, we simulated a distribution of droplets similar to that achieved in the real world, and we tested realistic scenarios and different devices. Second, our technique had a sensitivity of around 1% thus, we cannot conclude on comparisons made between efficient devices, however this sensitivity was sufficient to highlight the differences observed with all the devices commonly used in the general population. Third, we did not investigate the effects of additional factors and devices, such as face shields. However, while of proven efficacy in the healthcare setting [20], their use in the general population is very limited.

Conclusions

Distance, type of device and breathing pattern affected the protective efficacy of masks. While only high-grade certified devices ensured negligible inhaled fraction in all conditions, the use of all types of masks resulted less inhalation compared to distancing alone.

Supporting information

S1 Table. Experimental dataset of percentage inhaled fraction as a function of type of device, breathing pattern and distance.

(DOCX)

Click here for additional data file. (19KB, docx)

Data Availability

All relevant data are within the paper and its Supporting Information files.

Funding Statement

The authors received no specific funding for this work.

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Decision Letter 0

Francesco Di Gennaro

8 Mar 2021

PONE-D-20-40104

Effects of distancing and pattern of breathing on the filtering capability of commercial and custom-made facial masks: an “in-vitro” study.

PLOS ONE

Dear Dr. Alberti,

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We are also concerned that the conclusions of your study are overstated, and that they are not directly supported by the results of the experiments. PLOS ONE's fourth publication  criterion (http://journals.plos.org/plosone/s/criteria-for-publication#loc-4) states that the data presented in PLOS ONE manuscripts must support the conclusions drawn. We would like you to clarify that you are not claiming this method reduces infection rates, just the amount of aerosol particulates inhaled. As such, we ask you to revise the language used in your Discussion and Conclusions sections, to ensure that all statements are supported by the results of the study.

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Reviewer #2: Yes

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Reviewer #2: Yes

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Reviewer #1: It is a scientifically and methodologically well-articulated study. I found it very relevant to the current global pandemic situation. I recommend acceptance for publication after a minor revision is done. I hope the following clarity concerns will help the authors to improve their manuscript.

Specific comments

1. I suggest ‘physical or social distancing or just simply distance’ to include as a keyword.

2. Page6, 2nd Paragraph

• The nebulizer was operated for 40 minutes and a refill was possible. But it is not clear how long was the exposure time for each test? The test was replicated three times? Was any difference recorded among each test?

• Does exposure time(Length of exposure influenced the percentage of inhalation fraction?

3. Page9, the 1st paragraph of discussion the phrase “ 1) all investigated variables resulted significant (p < 0.001)” need to be clarified. Does it mean all investigated materials resulted in a significant reduction of an inhaled fraction? The result showed that Cotton masks at a 40cm distance with polypneic pattern only reduce to 41.1% ± 0.3%. Can we say this is significant as compared to certified FFP3 or FFP2 + surgical mask which reduce to lower than 1%?

4. Page12, as a limitation, it is mentioned that the sensitivity of the technique for this study is 1%. Can we say this technique is valid or acceptable with this very low sensitivity rate? If I am not mistaken how sensitivity is used in this case, a 1% sensitivity rare makes your methodology unacceptable. Please revised or clarify how the sensitivity of the technique was measured and used in your methodology.

Reviewer #2: First of all it was interesting area of study and I would like to thank the authors. Then I have minor comments regarding your manuscript;

Generally it is better to see your English, some re-arrangements of your reporting and clarity of your statements for every reader.

Please change INTERPRETATION to Discussion in your abstract section.

In result section please start with Report from figure 3 shows…

The pattern of breathing had a negative impact on the filtering capacity: compared to normal breathing... Also, incorrect positioning of the mask decreased the mask’s performances. “Statements that are discussing should be taken to discussion section

°Significant difference compared to distance of 40 cm, § what are the signs at first and end of statements?

“The main findings of this study are that: 1) all investigated variables resulted significant (p < 0.001); 2) all tested masks reduced the inhaled fraction compared to distancing alone; 3) distance decreased the inhaled fraction; 4) the differences between devices were more pronounced at shorter distances; 5) polypneic breathing pattern led to a decrease in most masks’ efficacy; 6) findings clarified the limitations/advantages of using masks both for individual and other protection.” Take this statement to finding section.

[17], [18] your citation should better like this [17, 18].

Some of your statements are not easy to understand for all readers please revise all of your write up…“Distance alone markedly reduced the inhaled fraction also without masks, thus underlining how physical distancing policies intrinsically increase the protective effect of all other measures.”

“Worst experimental condition” what does this mean? Is there such experiment?

**********

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Reviewer #1: Yes: Serebe Gebrie

Reviewer #2: Yes: THOMAS AYALEW ABEBE

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Attachment

Submitted filename: PONE-D-20-40104_reviewerTA.pdf

Click here for additional data file. (1.1MB, pdf)
PLoS One. 2021 Apr 22;16(4):e0250432. doi: 10.1371/journal.pone.0250432.r002

Author response to Decision Letter 0

26 Mar 2021

PONE-D-20-40104

Effects of distancing and pattern of breathing on the filtering capability of commercial and custom-made facial masks: an “in-vitro” study.

PLOS ONE

Dear Dr. Alberti,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Reply: We are thankful to the Editor for giving us the opportunity to revise our manuscript. We have modified the manuscript according to the reviewers’ and Editor’s additional comments and we hope you will find the revised manuscript suitable for publication on PLOS ONE.

We also had some further concerns, which should be addressed in the revised manuscript. PLOS specifies that experiments, statistics, and other analyses are performed to a high technical standard; sample sizes are large enough to produce robust results; and methods are described in sufficient detail to allow another researcher to reproduce the experiment (http://journals.plos.org/plosone/s/criteria-for-publication#loc-3). We are concerned that you have not provided sufficiently detailed methodology for the study to be reproducible. Nor is the rationale clear for some of their methodological choices -- i.e., why do you choose a 40-minute spray time, and is this realistic in terms of real-world scenarios? We also note that you do not specify the amount of time that you pause to refill, nor why you designed the experiment where a refill is necessary.

Reply: We agree that part of the methods was unclear and poorly described. We better clarified the experimental setting in the revised version of the manuscript. The choice of 40 minutes of nebulization was to simulate a prolonged person-to-person exposure and also conditioned by the necessity of achieving a sufficient sensitivity to detect inhaled fractions below 1%. The chosen study design required two brief stops of nebulization (<10 seconds) over 40 minutes, we judged these suspensions of nebulization irrelevant to the findings of our study. We hope that these aspects are now better clarified in the revised manuscript.

We are also concerned that the conclusions of your study are overstated, and that they are not directly supported by the results of the experiments. PLOS ONE's fourth publication criterion (http://journals.plos.org/plosone/s/criteria-for-publication#loc-4) states that the data presented in PLOS ONE manuscripts must support the conclusions drawn. We would like you to clarify that you are not claiming this method reduces infection rates, just the amount of aerosol particulates inhaled. As such, we ask you to revise the language used in your Discussion and Conclusions sections, to ensure that all statements are supported by the results of the study.

Reply: Thank you for pointing this out. We recognize that the original submitted manuscript contained parts that tended to over-interpret our findings. We have extensively revised the abstract and the discussion in order to avoid such over-interpretation.

Reply to Reviewer #1 comments

Reviewer #1: It is a scientifically and methodologically well-articulated study. I found it very relevant to the current global pandemic situation. I recommend acceptance for publication after a minor revision is done. I hope the following clarity concerns will help the authors to improve their manuscript.

Reply: we thank the reviewer for their positive and constructive comments.

Specific comments

1. I suggest ‘physical or social distancing or just simply distance’ to include as a keyword.

Reply: thank you for your suggestion, we added the suggested keyword.

2. Page6, 2nd Paragraph

• The nebulizer was operated for 40 minutes and a refill was possible. But it is not clear how long was the exposure time for each test? The test was replicated three times? Was any difference recorded among each test?

Reply: Thank you for pointing out this lack of clarity. As also requested by the Editor, we better clarified the experimental design in particular concerning the nebulization time. As better stressed in the revised manuscript, each measurement was performed in triplicate. Reproducibility was good, as highlighted by the standard deviations illustrated in the figures.

• Does exposure time(Length of exposure influenced the percentage of inhalation fraction?

Reply: as now better clarified in the revised manuscript, the exposure time was constantly kept at 40 minutes, therefore no effect of time could be investigated.

3. Page9, the 1st paragraph of discussion the phrase “ 1) all investigated variables resulted significant (p < 0.001)” need to be clarified. Does it mean all investigated materials resulted in a significant reduction of an inhaled fraction? The result showed that Cotton masks at a 40cm distance with polypneic pattern only reduce to 41.1% ± 0.3%. Can we say this is significant as compared to certified FFP3 or FFP2 + surgical mask which reduce to lower than 1%?

Reply. We recognize that the reporting of statistic was unclear. We now better clarify that the first p-values refer to the linear model using device, distance and pattern of breathing as co-factors. The following analyses refer to the Dunn post-hoc tests. We also agree that the clinical relevance of 41% protection compared to FFP3 with inhaled fraction <1% deserves better discussion. We restructured the discussion and results to better reflect this reasoning.

4. Page12, as a limitation, it is mentioned that the sensitivity of the technique for this study is 1%. Can we say this technique is valid or acceptable with this very low sensitivity rate? If I am not mistaken how sensitivity is used in this case, a 1% sensitivity rare makes your methodology unacceptable. Please revised or clarify how the sensitivity of the technique was measured and used in your methodology.

Reply: Thank you for raising this point. Sensitivity of 0.92% (rounded to 1%) is defined as the smallest amount of nebulized particles that we were able to detect, defined as percent of what was detected with zero distance between receiver and emitter and no mask. While we understand the concerns of the reviewer, we believe that 1% is sufficient for the purposes of this study. In fact, this resolution allowed us to describe a wide range of values (from above 60% to the sensitivity limit) according to the type of mask used. We agree that we could not claim that a device with <1% inhaled fraction would be as protective as a certified FFP3 mask, but this would be out of the scopes of the study, and no such claims are made throughout the manuscript. We better discuss this limitation in the revised manuscript, also de-emphasizing the translation from our in vitro findings and possible implications for masking policies, which was an error of over-interpretation we acknowledge and we worked on in this revision process.

Reply to Reviewer #2 comments

Reviewer #2: First of all it was interesting area of study and I would like to thank the authors. Then I have minor comments regarding your manuscript;

Generally it is better to see your English, some re-arrangements of your reporting and clarity of your statements for every reader.

Reply: We thank the reviewer for their positive and constructive comments. We edited extensively the manuscript including revision of wording and phrasing of certain parts.

Please change INTERPRETATION to Discussion in your abstract section.

Reply: Thank you. We changed accordingly.

In result section please start with Report from figure 3 shows…

Reply: Thank you. We changed accordingly.

The pattern of breathing had a negative impact on the filtering capacity: compared to normal breathing... Also, incorrect positioning of the mask decreased the mask’s performances. “Statements that are discussing should be taken to discussion section

Reply: We thank the reviewer for underlining this important point. We extensively revised the results section, removing these statements and presenting data in a more straightforward way.

°Significant difference compared to distance of 40 cm, § what are the signs at first and end of statements?

Reply: We apologize for this lack of clarity. As per PLOS ONE editorial guidelines, figure legends are in-line with the main text. Symbols should be interpreted as legends of the corresponding figure (Figure 3).

“The main findings of this study are that: 1) all investigated variables resulted significant (p < 0.001); 2) all tested masks reduced the inhaled fraction compared to distancing alone; 3) distance decreased the inhaled fraction; 4) the differences between devices were more pronounced at shorter distances; 5) polypneic breathing pattern led to a decrease in most masks’ efficacy; 6) findings clarified the limitations/advantages of using masks both for individual and other protection.” Take this statement to finding section.

Reply: We have extensively revised the discussion, and this introductory part was shortened and now serves as introduction of the discussion section.

[17], [18] your citation should better like this [17, 18].

Reply: Thank you. We changed accordingly.

Some of your statements are not easy to understand for all readers please revise all of your write up…“Distance alone markedly reduced the inhaled fraction also without masks, thus underlining how physical distancing policies intrinsically increase the protective effect of all other measures.”

“Worst experimental condition” what does this mean? Is there such experiment?

Reply: We apologize for the lack of clarity. All the sections mentioned by the reviewer have been extensively revised and clarified according to the reviewer’s suggestions. We hope you will find the revised manuscript clearer to follow.

Attachment

Submitted filename: Response to reviewers.docx

Click here for additional data file. (19.7KB, docx)

Decision Letter 1

Francesco Di Gennaro

7 Apr 2021

Effects of distancing and pattern of breathing on the filtering capability of commercial and custom-made facial masks: 

an “in-vitro” study.

PONE-D-20-40104R1

Dear Dr. Alberti

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Francesco Di Gennaro

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

congratulations

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: All my comments and concerns are addressed. I see the authors have done lots of improvement on the method, results, and discussion. Now, I feel it is scientifically acceptable for publication.

Reviewer #2: Thank you for considering my comments and improved your manuscript apropriately. I found Authors revision sound for publication.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Serebe Gebrie

Reviewer #2: Yes: Thomas Ayalew Abebe

Acceptance letter

Francesco Di Gennaro

13 Apr 2021

PONE-D-20-40104R1

Effects of distancing and pattern of breathing on the filtering capability of commercial and custom-made facial masks: an “in-vitro” study.

Dear Dr. Alberti:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Francesco Di Gennaro

Academic Editor

PLOS ONE

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Table. Experimental dataset of percentage inhaled fraction as a function of type of device, breathing pattern and distance.

    (DOCX)

    Click here for additional data file. (19KB, docx)
    Attachment

    Submitted filename: PONE-D-20-40104_reviewerTA.pdf

    Click here for additional data file. (1.1MB, pdf)
    Attachment

    Submitted filename: Response to reviewers.docx

    Click here for additional data file. (19.7KB, docx)

    Data Availability Statement

    All relevant data are within the paper and its Supporting Information files.

    Articles from PLoS ONE are provided here courtesy of PLOS

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