Modeling Aerial Transmission of Pathogens (Including the SARS-CoV-2 Virus) through Aerosol Emissions from E-Cigarettes
Round 1
Reviewer 1 Report
The science in this study is highly interesting and unexplored in research on exposition of aerosol generated by electronic cigarettes. Additionally, the approach developed (modelisation) is also innovative and reflect a lack of experimental data. Modelisation provides numerical data and presented results should be considered with a great interest. However, these results are still available online in another published paper (Aerial transmission of SARS-CoV-2 virus (and pathogens in general) through environmental e-cigarette aerosol). This point shall be clarified. Otherwise, my recommandation is accepted after minor revisions.
Comments for author File: 
Comments.pdf
Author Response
24 June 2021
Dear Editors
We are resubmitting the revised version of our paper entitled “Modeling aerial transmission of pathogens (including theSARS-CoV-2 virus) through aerosol emissions from e-cigarettes”.
We would like to thank the referees for their valuable comments recommendations as we believe that fulfilling them will greatly enhance the quality of the paper, hopefully leading to its publication in Applied Sciences.
The revised manuscript contains an extra section (5.3) presenting and discussing pictorial evidence from videos and frame photographs of two vapers vaping. The photographs are grouped in three new figures in section 5.3 and the four short videos are contained in the new Supplementary Material. Minor modifications of the main text are highlighted in bold letters.
In what follows we address the specific points raised by the referees:
The title of this work should indicate that it’s a “review” paper.
Our response. We would prefer to keep this work as a research article. The type of theoretical modeling (given the absence of data) required reviewing a wide multidisciplinary literature (on vaping, respiratory parameters, respiratory droplet emissions, jet hydrodynamics). However, the purpose of the paper was to research the specific issue of the plausibility and scope of pathogen transport through vaping exhalations. However, if the paper is accepted for publication we would not object if there is an editorial decision to publish it as a review.
This paper presents tables, results and paragraph extracted from a previously published paper of the authors (Aerial transmission of SARS-CoV-2 virus (and pathogens in general) through environmental e-cigarette aerosol) published online in November 2020. Situation shall be clarified.
Our response. We have produced two other related papers with some common content, but as we mention in the introduction of the revised manuscript, these papers are distinct and independent from this one.
One of these papers is devoted to a risk evaluation of vaping exhalations as a transmission factor (in comparison with other respiratory activities), considering the specific infective parameters of the SARS-CoV-2 virus and various ventilation regimes and indoor scenarios. This paper is currently under review in another journal, but its preprint version can be found in https://osf.io/8vxag In the previous manuscript it was cited as reference [23] but the citation was incorrect (it referred to the preprint version of the present paper). The citation has now been corrected
The other paper was published by IJERPH (it is reference [24]). It mentions some of the results of the present paper and of the one mentioned above, but most of its contents address material not found in either paper, for example a review of the literature on the relation between COVID-19 with smoking and with vaping, as well as on the effects of home containment policies on smokers and vapers and various issues of face mask wearing.
I suggest providing pictures of vapers exhaling. For experts coming from fluid mechanics, jet flow and modelisation are common subjects but in vaping researches they are quite new. Pictures would be an interesting adding in order to complete explication of Figure 1 with real explication. They will also visually complete modelisation results (distance of projection, speed). A short experiment could be to film a vaper and to collect some pictures at different times to complete with an example Figures 1 & 2.
Our response. We thank the referee for this very valuable suggestion whose fulfillment greatly enhances the paper. We have added new Supplementary Material with 4 short videos of two vapers vaping. Photographs of the appropriate frames were grouped into three new figures. This material is discussed in a new section (5.3) as pictorial evidence to corroborate the theoretical jet/puff model presented in sections 4 and 5.2
Another global remark that could be make is within the approach developed. Authors have clearly explain the simplifications make for their modelisation. Vaping products and behaviours use by consumer represents billions of combinations. Consequently, a global conclusion could not be done. In mechanics sciences also in fluids, equations are often considered with characteristic dimensions in order to simplify problems. In my sense, authors clearly provide characteristic dimensions by discussing vaping styles and demographics in section 2. So, I would not use the term “oversimplification” in the discussion but just “simplification”.
Our response. We fully agree and have in the revised manuscript replaced “oversimplification” with “simplification” of vaping styles.
Additionally, through the manuscript, there is a strong nomenclature used. I would suggest to add a nomenclature section to simplify the reading comprehension.
Our response. We have added a full nomenclature list
It would also avoid confusion between the flow considered. Indeed, authors describe the particle flow within the main flow that has a particle Reynolds number below 1 (under Stokes conditions) and the main flow that is the exhalation (under transition/turbulent conditions). At least it must be specified somewhere that the subscript “p” is used for notation linked to the particles.
Our response. We have added the subindex “p” to the particle Reynolds number to distinguish it from the fluid Reynolds number (marked with the subindex “f”). We are also distinguishing between ECA and respiratory droplets by the subindices “p” and “r”
Page 2 point 2: For experts out of fluid mechanics, “Stokes regime” are unknown. This is a particular flow regime and it should be defined
Our response. In the introduction we describe the notion of ECA particles (droplets) in the Stokes regime as being subjected to forces overwhelmingly related to the fluid viscosity with inertial forces being negligible in comparison. In section 2.3 we discuss the consequences of the Stokes regime in terms of rapid relaxation times of these particles settling in the respiratory flow. The added texts are highlighted in bold type.
Page 2 point 5: This consideration is interesting for direct exposure to adult bystanders. What about child bystanders that could be exposed through the radial dispersion of the jet? Authors may discuss this point because they don’t provide modelisation data of the radial speed
Our response. We now address in more detail the radial spread of the jet/puff system in sections 4, 5.2 and 5.3. Amendments to the text appear in bold face. We argue that under our assumption of a steady jet/puffsystem the evolution is very collimated directionally, at least in the starting jet stage (this is corroborated by the videos and photographs). The possibility of a child bystander being exposed would arise not from the radial dispersion (which is small), but from vapers vaping with a a downward trust of the directional jet. We now comment on this possibility, suggesting in section 6.2.1 that children (or pets) could be exposed in locations where adults of normal height would not be. However, the fact that the jet/puff system is visible (before its dispersion) is a useful safety feature protecting bystanders against direct exposure.
Page 8: Label of the cited table does not work “are listed in Table ??” and “(see Table ??)” and same observation in page 11 “From the values in Tables 1 and ??”.
Our response. This has been corrected. These were references to a table placed as Supplementary Material SM(2).
Page 12-13 section 4: Theoretical model is well presented but I will suggest finishing this section by a paragraph describing the numerical method and software used. In modelisation, numerical method is important because it provides calculation convergence and precisions giving sense to the results.
Our response. Our model is analytical, so we did not integrate numerically the Navier-Stokes and conservation equations. The model is based on analytic solutions of these dynamical equations under the assumption of an incompressible fluid under a statistically steady evolution, an assumption compatible with axial symmetry and self similar dependence. We simply plotted these analytic solutions with the software package Maple. We added in section 5.2 a brief paragraph explaining this. Also, we mention that the Maple worksheets we used are available by email request to the corresponding author.
To finalize, we would like to thank again the referees for their valuable comments and suggestions, hoping that they will find the revised version acceptable for publication
Roberto A Sussman
Eliana Golberstein
Riccardo Polosa
Author Response File: 
Author Response.docx
Reviewer 2 Report
The title of this work should indicate that it's a "review" paper. The majority of the content is based on the intensive related references, and the authors had a good discussion and proposed their thinking on the ECA modeling for both MTL and DTL which is helpful to understand the aerial transmission of pathogens through vaping.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
