Ventilation Challenges in Costa Rican Urban Public Transport: Implications for Health and Sustainable Mobility

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors studied CO2 concentrations across public transit vehicles. The idea is interesting but there are key factors missing or not well discussed. For example, the CO2 sensors are low-cost and there is no indication as to whether they were calibrated against reference sensors. The data is shown only in aggregate form. The authors discuss the possibility that trains having more stops facilitates air exchanges and this could be easily demonstrated with CO2 timeseries figures showing the stops and whether the levels drop. There are stylistic concerns as well. Specific comments are listed below:

The citations are not in the journal format.

Line 39: The comma (",") after the dash is not necessary.

Lines 38-40: This sentence should be rewritten to improve clarity.

Figure 1: The bus and train figures are redundant as they are nearly identical. Also, the panel labels (e.g., "A)") are not in the correct format. Also, generally "a)" and "b)" panels are in the same row.

Lines 60-61: Are there citations available for these systems?

Line 62: It would be more precise to mention the internal air quality or cabin air quality instead of just "air quality".

Lines 63-6: This sentence is too long and may benefit from being divided.

Figure 2: A scale would be helpful to gauge approximate distances. Different colors (maybe red and blue?) would further differentiate bus and train routes.

Lines 136-7: Is there a reference or website to learn more about the bus specifications?

Line 143-4: This sentence is confusing - does the second option mean that the passenger can take the same train, just for one more stop? In this case why would anyone take a taxi?

Figure 3: The panel labels are not the standard format.

Line 178: Were the devices calibrated against a reference sensor?

Line 180: Consider referencing the software or device manual.

Line 185: There seems to be two sections 2.4.

Line 204: Please provide citation for R.

Line 232: It seems that the "o" in "Co" should be subscripted.

Lines 233 and 236: It seems that the "a" in "Ca" should be subscripted.

Figure 4: The "2" in "CO2" should be subscripted. It seems that 95th percentile should be higher than median but that is not reflected in a. and b. The dots are red and blue but the caption reads green and orange. Panel labels are not the standard format.

Section 3.3: To improve clarity and provide perspective, it would be useful to provide a comparison of the volume of both the buses and trains.

Lines 358-9: This is not a complete sentence.

Lines 376-80: Have there been other studies that have looked into this?

Lines 383-4: This statement is somewhat of a stretch. Previously, the authors mentioned that the trains have more frequent stops and it is likely that this "natural" ventilation is the main factor, in conjunction with the larger volume, that keeps CO2 levels lower compared to buses.

Line 431: "ait" should be "air".

Line 432: "question" should be "questions".

Lines 449-52 seems to be aligned differently.

 

 

Comments on the Quality of English Language

The English Language is relatively good, there are a few typos so it may be useful to proofread the document before the next revision.

Author Response

Reviewer 1:

(1.1) The authors studied CO2 concentrations across public transit vehicles. The idea is interesting but there are key factors missing or not well discussed. For example, the CO2 sensors are low-cost and there is no indication as to whether they were calibrated against reference sensors. The data is shown only in aggregate form. The authors discuss the possibility that trains having more stops facilitates air exchanges and this could be easily demonstrated with CO2 timeseries figures showing the stops and whether the levels drop. There are stylistic concerns as well. Specific comments are listed below:

Authors’ Response: We thank the reviewer for their review of our work and for the useful points to improve the article. We have modified the work to address the concerns raised by the reviewer.

(1.2) The citations are not in the journal format.

AR: Thank you for your comment. We have updated the citation style to match the journal's format.

(1.3) Line 39: The comma (",") after the dash is not necessary.

AR: Done. The comma has been removed.

(1.4) Lines 38-40: This sentence should be rewritten to improve clarity.

AR: Done. The sentence has been revised for more clarity.

(1.5) Figure 1: The bus and train figures are redundant as they are nearly identical. Also, the panel labels (e.g., "A)") are not in the correct format. Also, generally "a)" and "b)" panels are in the same row.

AR: Thank you for highlighting. The image in Figure 1 has been revised and simplified. The label format has been corrected, and the figures repositioned to align a) and b) in the same row. The figure caption has been amended accordingly.

(1.6) Lines 60-61: Are there citations available for these systems?

AR: The systems used in different transport jurisdictions are varied. So, while we enlist the geographical regions, we do not discuss the specific systems.

(1.7) Line 62: It would be more precise to mention the internal air quality or cabin air quality instead of just "air quality".

AR: Thank you for your comment. The word ‘cabin’ has been added for more clarity.

(1.8) Lines 63-6: This sentence is too long and may benefit from being divided.

AR: Done. The sentence has been split in two for better readability.

(1.9) Figure 2: A scale would be helpful to gauge approximate distances. Different colors (maybe red and blue?) would further differentiate bus and train routes.

AR: Thank you for your comment. The figure has been changed in the new version of the manuscript following your suggestions.

(2.0) Lines 136-7: Is there a reference or website to learn more about the bus specifications?

AR: No, unfortunately there isn’t. As mentioned in the manuscript buses in Costa Rica are owned by private companies; thus, coach type, size, and conditions vary depending on the company. A single company can run several types of buses on the same route.

(2.1) Line 143-4: This sentence is confusing - does the second option mean that the passenger can take the same train, just for one more stop? In this case why would anyone take a taxi?

AR: Thank you for your comment. For clarity the sentence has been modified in the new version of the manuscript as follows:

“The second option is to travel from Alajuela to San José (Atlántico Station) and then either take the train one more stop toward Cartago to reach UCR or take a taxi directly from the Atlantico Station.”

(2.2) Figure 3: The panel labels are not the standard format.

AR: The panels have been relabeled.

(2.3) Line 178: Were the devices calibrated against a reference sensor?

AR: We have added the following description to provide context regarding monitor calibration:

“We first performed a manual calibration with outdoor air as reference, following the procedures recommended by the manufacturer. After that, we checked the measurements of all four devices and selected the three that showed the closest agreement for this study. Prior to each usage, the monitor was calibrated against outdoor air as reference, again, following manufacturer’s recommendations.”

(2.4) Line 180: Consider referencing the software or device manual.

AR: A reference to the software manual has been added in the revised manuscript.

(2.5) Line 185: There seems to be two sections 2.4.

AR: Thank you for the observation. This has been changed in the new version of the manuscript.

(2.6) Line 204: Please provide citation for R.

AR: A reference to the software has been added in the revised manuscript.

(2.7) Line 232: It seems that the "o" in "Co" should be subscripted.

AR: Thank you for your comment. This has been changed in the new version of the manuscript. 

(2.8) Lines 233 and 236: It seems that the "a" in "Ca" should be subscripted.

AR: Thank you for your comment. This has been changed in the new version of the manuscript.

(2.9) Figure 4: The "2" in "CO2" should be subscripted. It seems that 95th percentile should be higher than median but that is not reflected in a. and b.

AR: Thanks for that sharp observation. It has been corrected. The mistake was on our side, as we swapped the names of the Y-axis while editing the figures.

(3.0) In Figure 4, the dots are red and blue but the caption reads green and orange. Panel labels are not the standard format.

AR: Thank you for your comment. This has been changed in the new version of the manuscript.

(3.1) Section 3.3: To improve clarity and provide perspective, it would be useful to provide a comparison of the volume of both the buses and trains.

AR: Thank you for the suggestion. A direct and accurate comparison of the inner volume of train and bus coaches is difficult, as different bus models and types are used by various operators on the observed routes. Nevertheless, in general terms, the cabin of a train is several times larger than that of a bus, as multiple interconnected coaches form a single train unit. For clarity, we have added an explanatory sentence in Section 3.3.

“This should be noted considering also that the passenger cabin of a train is several times larger in volume than that of a bus, as each train unit comprises multiple interconnected coaches.”

(3.2) Lines 358-9: This is not a complete sentence.

AR: Thank you for your comment. This has been changed in the new version of the manuscript.

(3.3) Lines 376-80: Have there been other studies that have looked into this?

AR: There have been prior studies which have examine the problem but for this particular situation, the recorded CO2 levels were much higher than seen in prior studies. We have made the following addition.

The maximum levels reached 5 000 ppm, which goes beyond the 1 000 ppm to 3 000 ppm levels recorded for buses in several prior studies. 

(3.4) Lines 383-4: This statement is somewhat of a stretch. Previously, the authors mentioned that the trains have more frequent stops and it is likely that this "natural" ventilation is the main factor, in conjunction with the larger volume, that keeps CO2 levels lower compared to buses.

AR: As advised, we have dropped the reference to ventilation design and reiterated the aspect of greater effective volume and more frequent stops for trains, as quoted below:

 “However, the results could be an artefact of the larger effective volume in rail transport and the more frequent stops.”

(3.5) Line 431: "ait" should be "air".

AR: Thank you for your comment. This has been changed in the new version of the manuscript.

(3.6) Line 432: "question" should be "questions".

AR: Thank you for your comment. This has been changed in the new version of the manuscript.

(3.7) Lines 449-52 seems to be aligned differently.

AR: Thank you for your comment. Alignment checked and modified to “Justified”

Reviewer 2 Report

Comments and Suggestions for Authors

This is a well-written paper which describes the present status of indoor air quality of buses and trains in Costa Rica, employing CO₂ as a key indicator for infectious health risks. Even though there are limitations in the use of CO2 as an indicator, authors clearly explained the merit of the application in introduction (2.4) and discussion (4.7) sections. The CO2 measurements were carefully done in actual public transportations. The results were clearly shown in the manuscript with reasonable discussions. So, I have no severe criticisms on this paper and I would like to recommend “acceptance” after minor revisions. Please address following points for consideration.

I am very impressed by Figure 4 which shows clear relationship between rebreathed air and tip length. This relationship suggests that the longer trip may increase the health risk mostly via a droplet infection. However, I have a question whether this relationship can be also applied to the risk via airborne-transmission or not. When the indoor air is not thoroughly stirred to homogenize the air, the rebreathing CO2 is contributed more from people nearby.  Please add a mention on this point.

Values in Y-axis are missing in Fig.4b.

Author Response

(2. 1) This is a well-written paper which describes the present status of indoor air quality of buses and trains in Costa Rica, employing CO2 as a key indicator for infectious health risks. Even though there are limitations in the use of CO2 as an indicator, authors clearly explained the merit of the application in introduction (2.4) and discussion (4.7) sections. The CO2 measurements were carefully done in actual public transportations. The results were clearly shown in the manuscript with reasonable discussions. So, I have no severe criticisms on this paper and I would like to recommend “acceptance” after minor revisions. Please address following points for consideration.

Authors’ Reply (AR): Thank you for your kind comments. We deeply appreciate them.

(2.2) I am very impressed by Figure 4 which shows clear relationship between rebreathed air and tip length. This relationship suggests that the longer trip may increase the health risk mostly via a droplet infection. However, I have a question whether this relationship can be also applied to the risk via airborne-transmission or not. When the indoor air is not thoroughly stirred to homogenize the air, the rebreathing CO2 is contributed more from people nearby.  Please add a mention on this point.

AR: This is an important point and we have added it to the discussions in Section 4.3:

“In a high-density occupancy like public transport where the air may not necessarily be homogeneous and an infectious co-passenger maybe physically close to susceptible passengers, there are added risks from close range transmission. So, the rebreathed air fraction maybe treated as a conservative expression of the overall risks.”

 

(2.3) Values in Y-axis are missing in Fig.4b

AR: Thank you for your comment. This has been changed in the new version of the Figure.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript describes an on‑site campaign that tracked in‑cabin CO₂, temperature and humidity on commuter buses and trains serving the San José metropolitan area, using those data to infer ventilation adequacy and discuss implications for rider comfort and airborne‑disease risk.

While competently executed, the work’s added scientific value is not very clear. Similar CO₂‑based audits of public transport have been published for other cities; the paper does not yet explain what new insight a Costa‑Rican case study—or its train‑versus‑bus comparison—offers beyond existing literature. A stronger positioning against prior tropical or Latin‑American studies and a clearer statement of how the findings advance ventilation or health‐risk modelling are needed to avoid the impression that this is merely another local audit that “could be done anywhere.”

Minor point:

-Line 431: “outdoor ait” should be “outdoor air”.

-Follow SI style for large numbers: “5 000 ppm”, “70 L”. 

Author Response

(3.1) The manuscript describes an on‑site campaign that tracked in‑cabin CO₂, temperature and humidity on commuter buses and trains serving the San José metropolitan area, using those data to infer ventilation adequacy and discuss implications for rider comfort and airborne‑disease risk. While competently executed, the work’s added scientific value is not very clear. Similar CO₂‑based audits of public transport have been published for other cities; the paper does not yet explain what new insight a Costa‑Rican case study—or its train‑versus‑bus comparison—offers beyond existing literature. A stronger positioning against prior tropical or Latin‑American studies and a clearer statement of how the findings advance ventilation or health‐risk modelling are needed to avoid the impression that this is merely another local audit that “could be done anywhere.”

Authors’ Reply (AR): Thank you for this observation. We have addressed this by examining the context specific needs of conducting this study (tropical location, comparative evaluation of two transportation modes, and developing as opposed to developed economies) in the Introduction (lines 78-98), Methods (lines 133-135) and Discussions (lines 384-388, lines 470-474, lines 494-500) sections.

Minor point:

-Line 431: “outdoor ait” should be “outdoor air”.

AR: This has been modified and apologies for the spelling error.

-Follow SI style for large numbers: “5 000 ppm”, “70 L”.

AR: These instances have been modified as per the reviewer’s feedback.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors addressed all my comments appropriately.