Development and Validation of a Methodology to Measure Exhaled Carbon Dioxide (CO₂) and Control Indoor Air Renewal

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors

The research article entitled “Development and validation of a methodology to measure exhaled carbon dioxide (CO₂) and control indoor air renewal” has an interesting topic and a well-organized structure. To improve the paper, some minor suggestions listed below should be considered:

1- General results obtained from the study should be added to the abstract section.

2-   The subtitle number for the analysis phase has to be corrected.

3-  The location of CO2 meter #10 should be added to Figure 2a.

4-  There is a complexity about the location of CO2 meters in Classroom 1. Figure 2a shows that there is only CO2 meter#2 in Classroom 1, but in Figure 2b, Classroom 1 includes 10 CO2 meters. It should be explained better.

5-  In sections 2.3 and 2.4, the photographs should be presented from different cultural and commercial spaces.

6- Some characteristics of the infected person evaluated in all scenarios, including age and gender, should be presented in the methodology section since they are closely associated with exposure to air pollutants. In line 291, “The values in scenarios with an infected student are less restrictive due to the reduced generation of aerosols due to respiratory activity” should be rearranged considering the characteristics of infected people examined in the present study.

7-  A discussion section should be added to compare the current results with those of previous related studies.

8- In conclusion, the general results obtained from the study should be highlighted for two cases instead of a methodological explanation.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The manuscript is written in good quality English, but it should be checked in terms of wrong spelling, double words, subscript, etc. 

Author Response

The research article entitled “Development and validation of a methodology to measure exhaled carbon dioxide (CO₂) and control indoor air renewal” has an interesting topic and a well-organized structure. To improve the paper, some minor suggestions listed below should be considered:

We sincerely appreciate your good opinion of our manuscript. Your suggestions definitively and substantially strengthen the manuscript. Thank you for recommending our work.

1- General results obtained from the study should be added to the abstract section.

We appreciate this observation. We added “Results indicate the feasibility of swiftly implementing measures to enhance shared air renewal, with the immediate opening of doors and windows being the most direct solution. The proposed methodology is practical and has the potential to mitigate the risk of aerosol transmission of respiratory diseases. Consequently, we anticipate that this work will contribute to establishing methodological foundations for CO₂ measurement as a valuable, standardized, and reliable tool.” in lines 18-23. We have also reduced the abstract to fit the 200 word maximum.

2-   The subtitle number for the analysis phase has to be corrected.

We’re sorry for this mistake, which is now updated.

3-  The location of CO₂ meter #10 should be added to Figure 2a.

CO₂ meter #10 was placed outside the building to determine the increase of CO₂ in the indoor air. Therefore, it is not depicted in Figure 2a. We have added "CO₂ meter #10 was placed outside the building to determine the increase in CO2 (∆CO₂) relative to the outside air measurement." on lines 269-270 to clarify this point.

4-  There is a complexity about the location of CO₂ meters in Classroom 1. Figure 2a shows that there is only CO₂ meter #2 in Classroom 1, but in Figure 2b, Classroom 1 includes 10 CO2 meters. It should be explained better.

The reason for this is that we used "Classroom 1" as the study space. Therefore, to study ventilation patterns and see how to improve air quality, in some experiments we placed all the meters in this space. We have added "To analyze ventilation patterns and the effectiveness of corrective measures, we deployed the 10 meters in Classroom 1, as illustrated in Figure 2b." on lines 264-266 for ease of understanding.

5-  In sections 2.3 and 2.4, the photographs should be presented from different cultural and commercial spaces.

We agree with the author. Unfortunately, businesses do not allow us to publish representative photographs of the spaces due to confidentiality issues.

6- Some characteristics of the infected person evaluated in all scenarios, including age and gender, should be presented in the methodology section since they are closely associated with exposure to air pollutants. In line 291, “The values in scenarios with an infected student are less restrictive due to the reduced generation of aerosols due to respiratory activity” should be rearranged considering the characteristics of infected people examined in the present study.

We value this comment. In each scenario we have added the age of the individual used for the calculations. However, the sex of the individual was not considered, so this information is not specified in the manuscript. We added this information in lines 165-171.

7-  A discussion section should be added to compare the current results with those of previous related studies.

We have added the discussion section and readapted the content of the conclusions. We believe that this aspect has improved the conclusion of the article.

8- In conclusion, the general results obtained from the study should be highlighted for two cases instead of a methodological explanation.

We have rewritten the conclusion and believe it is now more appropriate. We very much acknowledge the reviewer.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript, " Development and validation of a methodology to measure exhaled carbon dioxide (CO2) and control indoor air renewal” proposes a method for assessing air renewal and its relation to SARS-CoV-2 transmission. The study's concept is intriguing, and the methodology and results add significantly to the scientific literature. However, some issues need addressing before publication, including the need for English editing for improved clarity and coherence.

  

Line 34: Please correct "Covid-19" to "COVID-19." The same correction should be applied consistently throughout the entire manuscript

 Lines 33-32: The paragraph is unclear and repetitive. I recommend summarizing the content to enhance clarity and coherence.

 Line 46: The term "operating conditions" in the sentence is unclear. I suggest revising the sentence to provide a more explicit explanation or context for what the authors mean by "operating conditions."

 Line 52: This article presents a multidisciplinary research approach that, in my view, holds relevance across various fields. To enhance the comprehensibility of the study, I recommend that the authors provide a clearer explanation of metabolic CO2 and elaborate on how it can be distinguished from regular CO2. This clarification will contribute to a better understanding of the research for a broader audience.

Line 67-70: The authors should exercise caution when discussing the link between COVID-19 transmission and CO2 quantification. Although it has been mentioned that CO2 serves as a proxy, the explanation appears inadequate. The two referenced studies explicitly indicate that CO2 measurements were employed to regulate the occupancy of a specific space during the COVID-19 pandemic. A greater number of individuals in confined spaces can lead to reduced personal space and an increased likelihood of encountering someone who is COVID-19 positive. These factors amplify the risk of viral transmission. Therefore, the authors must elaborate on these points to better convey the connection between CO2 levels, occupancy, and the potential for COVID-19 spread.

 Line 71: What do the authors mean by “SARS-CoV-2 is mainly spread over time and distance”?

 Lines 71-72:  It appears that the authors intended to refer to genome copies/L, as indicated in the cited papers. It is crucial to correct this and explicitly state that these studies quantified RNA and not actual viral particles, let alone infectious particles. Clarifying this distinction is important for precision and accuracy in conveying the methodology and findings of the cited research.

 Lines 73-75: I recommend that the authors refer to the review with the DOI 10.1016/j.scitotenv.2021.149231

 Lines 77-78: Please explain why it is difficult to distinguish between super spreading events and non-spreading events.

 Lines 84-90: Same comment as before. All these efforts were made to limit the number of people in a given space.

Section 2.2. This section is important, however, in my opinion it is too long. Streamlining and summarizing the content could enhance readability and improve the overall flow of the text.

 Results and Discussion section: I recommend that the authors enhance the discussion by comparing their study's findings to broader research that has detected and quantified SARS-CoV-2 RNA in both indoor and outdoor spaces. Additionally, it would be valuable to establish more connections to public health. This study's significance extends beyond the context of the COVID-19 pandemic, and the authors should emphasize this aspect. In general, there is room for improvement in the discussion, which, in my opinion, currently appears limited in its scope and depth.

Comments on the Quality of English Language

English revision is necessary, as there are issues present throughout the manuscript. While I acknowledge that the identified problems may not be severe, enhancing them would positively impact the article's readability.

Author Response

The manuscript, " Development and validation of a methodology to measure exhaled carbon dioxide (CO2) and control indoor air renewal” proposes a method for assessing air renewal and its relation to SARS-CoV-2 transmission. The study's concept is intriguing, and the methodology and results add significantly to the scientific literature. However, some issues need addressing before publication, including the need for English editing for improved clarity and coherence.

We appreciate Reviewer #2 for the time and critical reading of the manuscript. We very much value the positive insights on the contributions of the manuscript and his/her comments and suggestions.

Line 34: Please correct "Covid-19" to "COVID-19." The same correction should be applied consistently throughout the entire manuscript.

We appreciate this observation. It is now updated.

Lines 33-32: The paragraph is unclear and repetitive. I recommend summarizing the content to enhance clarity and coherence.

We agree with the reviewer. We have rewritten this paragraph to clarify the concepts (lines 28-38).

Line 46: The term "operating conditions" in the sentence is unclear. I suggest revising the sentence to provide a more explicit explanation or context for what the authors mean by "operating conditions."

We have replaced this term with "actual conditions", referring to use in real environments (line 37-38). We appreciate this suggestion.

Line 52: This article presents a multidisciplinary research approach that, in my view, holds relevance across various fields. To enhance the comprehensibility of the study, I recommend that the authors provide a clearer explanation of metabolic CO2 and elaborate on how it can be distinguished from regular CO2. This clarification will contribute to a better understanding of the research for a broader audience.

The reviewer is right. We have added “(CO₂ produced by human respiratory activity)” to clarify this concept in line 44.

Line 67-70: The authors should exercise caution when discussing the link between COVID-19 transmission and CO2 quantification. Although it has been mentioned that CO2 serves as a proxy, the explanation appears inadequate. The two referenced studies explicitly indicate that CO2 measurements were employed to regulate the occupancy of a specific space during the COVID-19 pandemic. A greater number of individuals in confined spaces can lead to reduced personal space and an increased likelihood of encountering someone who is COVID-19 positive. These factors amplify the risk of viral transmission. Therefore, the authors must elaborate on these points to better convey the connection between CO2 levels, occupancy, and the potential for COVID-19 spread.

We have rewritten this sentence to make it more understandable. "Nonetheless, CO₂ concentration thresholds have been proposed to reduce COVID-19 airborne transmission (without considering other contagion routes), typically ranging between 700 and 1,000 ppm, irrespective of the activity. This is equivalent to an increase (metabolic CO₂) of 300 to 600 ppm compared to fresh air." (lines 58-61). Thus, we emphasize that the regulation of CO₂ levels has been performed only for the airborne route.

 Line 71: What do the authors mean by “SARS-CoV-2 is mainly spread over time and distance”?

We refer to the fact that SARS-CoV-2 is efficiently transmitted through the air, reaching long distances and for long periods of time. We have replaced the original sentence with: "SARS-CoV-2 is highly efficient in airborne transmission, covering long distances and durations" (lines 62-63). We hope this is now better understood.

Lines 71-72:  It appears that the authors intended to refer to genome copies/L, as indicated in the cited papers. It is crucial to correct this and explicitly state that these studies quantified RNA and not actual viral particles, let alone infectious particles. Clarifying this distinction is important for precision and accuracy in conveying the methodology and findings of the cited research.

Agreed. We have added "RNA" in this sentence for better understanding in line 63.

 Lines 73-75: I recommend that the authors refer to the review with the DOI 10.1016/j.scitotenv.2021.149231

We have added the reference to consolidate this statement (line 66).

 Lines 77-78: Please explain why it is difficult to distinguish between super spreading events and non-spreading events.

At present, it is not possible to differentiate potential super-propagation events because it is not possible to detect individuals that are going to propagate massively. We have rewritten this sentence to: “The distinction between superspreaders and non-spreaders remains challenging due to the unknown propagation capacity of individuals” in lines 68-69.

 Lines 84-90: Same comment as before. All these efforts were made to limit the number of people in a given space.

We have removed some lines from this paragraph to use it only to give examples of investments in this technology/method. We hope that the reviewer now agrees.

Section 2.2. This section is important, however, in my opinion it is too long. Streamlining and summarizing the content could enhance readability and improve the overall flow of the text.

We have removed some sentences to make the text lighter. We hope the reviewer agrees with these changes.

Results and Discussion section: I recommend that the authors enhance the discussion by comparing their study's findings to broader research that has detected and quantified SARS-CoV-2 RNA in both indoor and outdoor spaces. Additionally, it would be valuable to establish more connections to public health. This study's significance extends beyond the context of the COVID-19 pandemic, and the authors should emphasize this aspect. In general, there is room for improvement in the discussion, which, in my opinion, currently appears limited in its scope and depth.

We have created a new "Discussion" section adding the points we consider most important. We hope that this part has been improved. We very much acknowledge the reviewer.