Can the Building Make a Difference to User’s Health in Indoor Environments? The Influence of PM_2.5 Vertical Distribution on the IAQ of a Student House over Two Periods in Milan in 2024

Round 1

Reviewer 1 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

The authors have reorganized the discussion and results sections, making the structure of the paper more reasonable and the logic clearer. 

Author Response

Thank you very much for taking the time to review this manuscript, also for your important comments in the previous reviewed version!

Comments 1: The authors have reorganized the discussion and results sections, making the structure of the paper more reasonable and the logic clearer.

Response 1: Thank you for your comments! The sections of methods and discussion are modified according to the review comment from the other reviewer, to provide the key information in a more detailed and clear expressions.

 

Author Response File: Author Response.pdf

Reviewer 2 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

Overall Assessment

This manuscript explores the impact of building characteristics on indoor air quality (IAQ), specifically the vertical distribution of PM2.5, within a student dormitory in Milan across two distinct periods (June and October 2024). Utilising low-cost sensors for PM2.5 monitoring, alongside qualitative occupant surveys, the study reports that lower floors exhibited slightly higher outdoor PM2.5 levels, whilst higher floors showed a lower indoor-to-outdoor (I/O) ratio. It also suggests that indoor sources become dominant when outdoor PM levels are low. This manuscript presents highly relevant and timely research addressing key IAQ concerns in residential environments, specifically its focus on the less-explored aspect of vertical PM2.5 distribution within buildings and its potential health implications. The study's practical monitoring approach, involving the extensive deployment of low-cost sensors, yields valuable real-world IAQ data across various building locations and orientations. Furthermore, the inclusion of a dual seasonal analysis (June and October) provides key insights into the effect of seasonal changes on PM2.5 distribution. The paper also effectively highlights the role of user behaviour, such as ventilation practices, in shaping indoor pollutant levels, offering actionable recommendations for residents.

However, to improve the quality of paper, the authors are recommended to revise the manuscript in line with the comments below:

• Low survey response rates significantly undermine the reliability of conclusions drawn from user perceptions or direct health symptom correlations; therefore, these substantial survey limitations must be explicitly acknowledged, avoiding definitive causal claims and presenting health links as potential associations for future rigorous studies.
• The analysis of "daily PM2.5 I/O ratio features" and "AP I/O differences" is primarily qualitative, lacking robust statistical validation for hypothesised relationships; consequently, appropriate statistical methods must be incorporated to quantitatively substantiate all claims, or if data is insufficient, these observations should be reframed as testable hypotheses.
• The calibration methodology for devices is insufficiently detailed, preventing reproducibility and proper assessment of data reliability; thus, comprehensive details on the calibration protocol, including specific metrics and uncertainty discussions, are required.
• The application of reference levels for CO2 and PM2.5 is at times misaligned, and the 2000 ppm CO2 "intervention level" often exceeds international IAQ guidelines; therefore, clearer justification for these comparisons and thresholds, or alignment with more stringent limits, is necessary.
• Acknowledged battery limitations impacting monitoring intervals and duration suggest that "many subtle variations" in IAQ might have been missed, potentially affecting dynamic insights; hence, the discussion must explicitly address how these technical constraints affect findings and highlight the necessity for longer, higher-frequency monitoring in future work.
• Reliance on the CO2 I/O ratio as a proxy for ventilation or window operations lacks direct validation, limiting conclusive links between specific occupant behaviours and IAQ changes; this limitation must be acknowledged, suggesting future research incorporate direct monitoring methods for window opening or actual air exchange rates.
• It is also recommended to discuss the key findings in a standalone Discussion section.

Author Response

Thank you very much for your detailed and precise comments on this work, and the problems you pointed out are clear and essential for us!

In the latest version of this manuscript, the limitations have been emphasised in the corresponding subchapter, as well as the limitation session at the end of the paper, including the low response rate, monitoring frequency and lack of tools for window operation records, etc. In the methodology chapter, the introduction of reference levels of CO2 and PM is clarified with sources, and the calibration is added with more details in the evaluation session and existing limitations. Also, the findings in the discussions are not described in a hypothetical way, as currently there are not enough data for validation, and it will be our work in the next stage.

Comments 1: Low survey response rates significantly undermine the reliability of conclusions drawn from user perceptions or direct health symptom correlations; therefore, these substantial survey limitations must be explicitly acknowledged, avoiding definitive causal claims and presenting health links as potential associations for future rigorous studies.

Response 1: The limitation of the low response rate has been highlighted in the session about the survey results as well as the limitation session at the end of the paper. Also, the discussions on the reported symptoms have been modified accordingly to emphasise that there were just hypotheses of health links due to this limitation.

Comments 2: The analysis of "daily PM2.5 I/O ratio features" and "AP I/O differences" is primarily qualitative, lacking robust statistical validation for hypothesised relationships; consequently, appropriate statistical methods must be incorporated to quantitatively substantiate all claims, or if data is insufficient, these observations should be reframed as testable hypotheses.

Response 2: Yes, at the current stage, this finding is actually in a hypothesised condition and requires more data and case studies for validation, which is lacking at the moment. The corresponding discussions and expressions are modified in hypothetical ways, and this point is added in the limitation session as a future development direction.

Comments 3: The calibration methodology for devices is insufficiently detailed, preventing reproducibility and proper assessment of data reliability; thus, comprehensive details on the calibration protocol, including specific metrics and uncertainty discussions, are required.

Response 3: The more detailed descriptions on the calibration procedure and evaluation had been added according to the comments. Also the acknowledged limitations from the calibration had been added in the following paragraphs.

Comments 4: The application of reference levels for CO2 and PM2.5 is at times misaligned, and the 2000 ppm CO2 "intervention level" often exceeds international IAQ guidelines; therefore, clearer justification for these comparisons and thresholds, or alignment with more stringent limits, is necessary.

Response 4: For this point, the chapters involving the reference levels have been modified accordingly, especially in chapter 2.6, introducing this point. The reference levels for CO2 and PM2.5 are taken from our previous review work, as cited in the paper, which summarised the reference levels from many international IAQ guidelines. And in this previous work, it was also found that the majority of guidelines took 1000 ppm for CO2; meanwhile, in the documents from the UK and Germany, they had the second level at 2000 ppm, which is recommended to have intervention. So in this and previous works, we also included this point as a reference. The explanation of this is added in this revision.

Comments 5: Acknowledged battery limitations impacting monitoring intervals and duration suggest that "many subtle variations" in IAQ might have been missed, potentially affecting dynamic insights; hence, the discussion must explicitly address how these technical constraints affect findings and highlight the necessity for longer, higher-frequency monitoring in future work.

Response 5: This limitation has been mentioned in the limitations session at the end, and more explanations have been added to this limitation to highlight the importance of longer periods and higher frequency in monitoring. In case of misunderstanding, in both the methodology session and chapter 3.3, there are more paragraphs have been added for better understanding when displaying the results in the diagrams.

Comments 6: Reliance on the CO2 I/O ratio as a proxy for ventilation or window operations lacks direct validation, limiting conclusive links between specific occupant behaviours and IAQ changes; this limitation must be acknowledged, suggesting future research incorporate direct monitoring methods for window opening or actual air exchange rates.

Response 6: We agree with this point and were aware of the limitation as we didn’t have the proper tool for recording the window’s motions during the fieldwork periods, and it was a compromised solution in data display. This limitation is now discussed in the session when discussing the CO2 I/O ratio as a proxy. Also, in the limitation session, this point is mentioned again to highlight it.

Comments 7: It is also recommended to discuss the key findings in a standalone Discussion section.

Response 7: For this point, we separate the previous “result and discussion” chapter into 2 parts. One of the 2 parts only displays the general results from the monitoring devices and questionnaires, now titled “3. Results from monitoring devices and questionnaires” and another subchapter includes the subdivided discussions with data analysis, titled “4. Discussion”.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

The authors have satisfactorily responded to the comments. Limitations around the survey response rate, calibration uncertainty, and use of proxies (e.g. CO₂ I/O ratio) are clearly acknowledged. The manuscript  includes a separate Results and Discussion section, improving structure and clarity. Hypotheses are appropriately distinguished from causal claims, particularly regarding occupant health. In order to improve the quality of the paper further, the authors are advised to consider the following amendments:

• The manuscript would benefit from professional English editing to address frequent grammar issues.
• Simplify awkward phrasing and avoid vague terms like slightly lower or a few more occasions.
• Ensure consistent and clear use of technical terms (e.g. use “air infiltration” instead of “inward infiltrations”).
• Where possible, quantify vague descriptors.
• Rephrase strong claims to reflect data limitations. Suggestions (e.g. ventilation advice) should be framed as recommendations, not conclusions.
• Acknowledge that future work should include direct monitoring (e.g. motion/window sensors) and explore statistical or modelling tools to enhance short-term IAQ analyses.

Comments on the Quality of English Language

English language can be improved.

Author Response

Thank you very much for your time and comments on this manuscript! Based on your comments, we did a thorough modification of the English expressions, among all chapters, trying to correct all the grammar mistakes and unclear expressions in the paragraphs.

Comments 1: The manuscript would benefit from professional English editing to address frequent grammar issues.

Response 1: The entire menuscript has been modified again for the English expressions. The unclear expressions and mistakes in grammar were modified through this process. For further remaining mistakes, we may ask for help from the journal editor.

Comments 2: Simplify awkward phrasing and avoid vague terms like slightly lower or a few more occasions.

Response 2: For this point, the sentence from results to conclusions has been checked and modified one by one to double-check the expression to be specific and more accurate in meaning.

Comments 3: Ensure consistent and clear use of technical terms (e.g. use “air infiltration” instead of “inward infiltrations”).

Response 3: These unclear expressions in the result, discussions, and conclusion chapters have been rephrased and modified to avoid misunderstandings, especially the technical terms.

Comments 4: Where possible, quantify vague descriptors.

Response 4: Same as the previous response, the unclear descriptions have been modified. The long sentence with unclear indications has also been split into short sentences with clearer indications on the subject of each sentence, wherever found unclear in the manuscript.

Comments 5: Rephrase strong claims to reflect data limitations. Suggestions (e.g. ventilation advice) should be framed as recommendations, not conclusions.

Response 5: In the chapter on results and discussions, those strong claims have been modified in the way of recommendations, and the expression of findings is now in hypothetical expressions.

Comments 6: Acknowledge that future work should include direct monitoring (e.g. motion/window sensors) and explore statistical or modelling tools to enhance short-term IAQ analyses.

Response 6: This point is added and highlighted in the limitation and future development section at the end of the paper.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Response to the authors and editor,

investigates the impact of a Saharan dust intrusion event on indoor air quality (IAQ) in a student accommodation facility in Milan, Italy. The research aimed to understand how user behavior, particularly window operations, influences IAQ during periods of elevated outdoor particulate matter (PM) levels.

The findings revealed the date outdoor PM2.5 and PM10 levels peaked coinciding with a sandstorm event, and subsequently decreased after rainfall. Indoor PM levels were generally lower than outdoor levels and affected by window operations. The study also found a negative trend between outdoor PM levels and temperature.

It is a meaningful approach, especially for investigating the issues in real life. However, there are some flaws to improve. Would recommend major revision.

 

Comments,

Introduction

Line 43. don’t think it is necessary to say its from a PhD project. Please focus more on the reason to conduct this research and the logical connection and thinking process to develop the manuscript from research background to specific research objectives. 

Need subtitle for each section, like method, and results.

line 106, not clear about the meaning.

In figure 2 or 3, they are clear for what they are now, adding more information about the measurements would be better. like, what parameters were measured.

For the method, need subtitle to show each part clearly. location selection, data obtainment, data analysis and so on.

For the result, the current method applied was insufficient to reveal the differences for the measured parameters between groups. Would suggest more statistical analysis.

Reviewer 2 Report

Comments and Suggestions for Authors

This article has significant flaws in research design and lacks innovative insights, thus currently not meeting publication standards. The authors are advised to refine the scientific focus (choosing either the sandstorm or user behavior as the core subject) and redesign experiments and data analysis accordingly to potentially meet publication criteria.

Specific issues:

1. The study introduces both "sandstorm" and "user behavior" as variables but fails to define the core research objective. If focusing on sandstorms, prioritize analyzing their unique impacts(e.g., PM component variations, vertical transport mechanisms). If studying user behavior, emphasize its specialized role under sandstorm conditions (e.g., behavioral pattern differences during extreme weather).
2. Excessive data accumulation dilutes the core value of sandstorm research, weakening scientific contributions. For example, CO2 is primarily influenced by indoor human activities and lacks physical or chemical connections to sandstorms. Temperature and humidity parameters are superficially compared without integrating them into sandstorm dynamics (e.g., humidity’s enhancement effect on PM deposition).
3. Inadequate exploration of potential innovative points, such as unresolved gaps in indoor PM research under sandstorms (e.g., long-distance transported particles' penetration rates and residence times).
4. Sections 4.1, 4.2, 4.4, and 4.5 of the Discussion are better suited for the Results section. Section 4.3 (temperature-humidity correlations) lacks substantive discussion value.
5. Most findings do not go beyond professional common sense. Examples include: indoor PM trends aligning with outdoor levels during sandstorms; user behavior (e.g., window opening, HVAC system usage) significantly affecting IAQ and health; indoor CO2 fluctuations linked to occupancy; and reduced window openings during rainfall leading to CO2 accumulation. These conclusions are predictable based on existing domain knowledge.

Reviewer 3 Report

Comments and Suggestions for Authors

This study monitored indoor air quality (IAQ) in a Milan student residence during a Saharan sandstorm in June 2024, focusing on PM2.5 and PM10 using 30 sensors and questionnaires. Interesting results were identified, indicating a link between outdoor and indoor PM levels when windows were open, with window closure reducing infiltration. The research also examined daily PM variations and preliminary vertical distribution, highlighting the impact of occupant behaviour on IAQ. A notable strength of this study lies in its timely and real-world observation of a Saharan dust event's impact on IAQ within student accommodation. The analysis of PM2.5 and PM10 levels shows the relationship between atmospheric conditions and occupant activity on pollutant concentrations. Furthermore, the analysis demonstrates the potential for residents to directly affect their indoor PM exposure through the management of window openings. The initial exploration into the vertical distribution of PM levels within the building, whilst preliminary, raises questions related to building design and occupant exposure to indoor air pollutants (IAPs). Despite an interesting finding, the authors need to revise the manuscript in line with the comments below to improve the quality of the paper.

• The authors are recommended to include a Literature Review section to show studies carried out on this topic, highlighting the gaps in knowledge that this study aims to address. Including some background information can help focus more on the knowledge gap this study addresses.
• It is recommended to add an introductory paragraph in the Methods section, summarising the methods and key aims.
• The characteristics of the case studies could be introduced in a separate subsection as an overview of the case study building.
• The description of the student survey can benefit from more detail regarding the response rate, recruitment strategy, and limitations.
• The authors need to ensure there are no new findings in the Discussion section. They need to move such data to the Findings section and ensure to argue the key findings in the Discussion section. Expand on how the findings relate to existing literature. How do the observed temperature and IAQ trends compare with similar studies?
• To strengthen the findings, it is recommended to include a discussion of the statistical significance of the observed differences in PM levels, particularly when comparing indoor and outdoor concentrations and variations across different building heights.
• Whilst the focus on the sandstorm event is significant, a brief discussion of other potential factors that can affect IAQ in residential buildings (e.g., indoor sources of PM, ventilation rates, other pollutants) can provide a comprehensive context for the findings.
• The role of HVAC systems is mentioned in the introduction, but details regarding their presence, type, and operation within the monitored student house are lacking. It is recommended to clarify this, arguing its potential interaction with natural ventilation strategies that could be valuable.
• While reference levels for CO₂ and PM are mentioned, briefly justifying their relevance and the specific context (e.g., regulatory guidelines, health recommendations) of this study can add value to this paper.
• The low response rate for the occupant questionnaires limits the generalisability of the subjective feedback. The authors are recommended to acknowledge this as a significant limitation when drawing conclusions based on this data.
• The quality of the figures needs to be improved, possibly by increasing the font size. The authors should also ensure that each figure’s caption is placed close to the corresponding figure.