Improving Indoor Air Quality in a Higher-Education Institution Through Biophilic Solutions^†

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The article lacks information on the organization of air exchange. How is outdoor air supplied to the rooms and how is it removed?

2. The article lacks information on the course of variability of outdoor air parameters (EAQ). In the case of natural ventilation, EAQ has a fundamental impact on IAQ.

3. Was a room tightness test performed and the air exchange rate determined? Were they at the same or similar level?

4. The article lacks a building diagram indicating the location of the rooms in question in relation to the other rooms in the building.

5. Line 118-119 - Error! 118 Reference source not found - requires explanation

6. Line 164-166 - These windows, in both classrooms, were 164 equipped with adjustable curtains for shading, which were installed three months prior to 165 the IAQ monitoring". Did the authors verify what the VOC emissions from these curtains were?

7. Line 172 - ". It should be noted that, as the classrooms lack air conditioning systems ....". Did the rooms have any heating system? If not, how is the thermal comfort of the room users ensured during periods of low temperatures?

 

8. Line 233, 236, 242, etc. – table no. 3 is missing in the text of the article

 

9 . Figure 4a - how can we justify the increase in temperature after closing the door to the classroom at 2:23 pm and its decrease in the same conditions after 3:21 am?

 

10. Line 204-207 - the level of the respiratory tract is not equal to the height of the table surface. A seated adult has their respiratory tract approximately 40-50 cm above its surface

11.  Line 230-231 - "Regarding thermal conditions, air movement inside the classrooms was 230 not measured." - On what basis was it determined that the ventilation conditions were identical? Air exchange in rooms with natural ventilation is the result of many components, such as the technical condition of windows and doors. The most appropriate solution would be to perform a room tightness test and determine the air exchange rate for each of the tested rooms.

12. Line 233, 236, 242, etc. - Table 3 is missing from the article text

13. Figure 4a - how can we justify the increase in temperature after closing the door to the classroom at 2:23 pm and its decrease in the same conditions after 3:21 am?

14. Figure 4b - Why does the RH level drop before the doors were opened at 2:52 pm and rise after they were opened after 3:21 pm? Why does the RH level practically not change for the first half hour of use of the rooms?

15. Figure 4b - How do the authors explain the further increase in RH in the neutral room (NR) after the doors were opened at about 4:30 pm

 

16. Figure 6a - Why does the CO2 concentration remain at the same level (500 ppm) for the first half hour of room use and then increase steadily even after opening the door to the room. What was the CO2 concentration in the outside air at that time?

17. Figure 6a - why did the CO2 concentration not decrease after the declared opening of the door in room NR (2:52 PM) as in the other cases? Opening the door increases the level of room ventilation and reduces the concentration of pollutants inside the room.

18. Figure 6a. In the first phase of room use, the CO2 concentration was almost identical in both rooms. How can the difference in CO2 concentration in Room GR and Room NR be justified in the second part of the day (after 2:52 PM) if the way the room was used and the number of users were the same?

19. If the rooms were used in the same way (the same number of people), how can we explain the different rate of increase in VOCT for Room NR between 1:55 pm and 2:52 pm and 3:21 pm and 4:47 pm?

20. Figure 6b - How can we explain such a large increase in VOC in Room NR?

21. Line 333-334 - Between 1:55 pm and 3:21 pm in Room NR, CO2 and VOCT values ​​do not confirm the door opening as indicated in Figure 6. A similar situation occurs in Room GR between 1:55 pm and 2:52 pm. How can we explain this?

22. Line 339 - 340 - In the first and second measurement periods in Figure 6a, the differences are small and not significant as the authors claim.

23. Line 410-413 – "Despite the absence of reference values ​​for temperature in Portuguese legislation concerning indoor air quality, Decree-Law no. 243/86 of August 20 [61] recommends that temperatures in workplaces oscillate between 18°C ​​and 22°C, with the possibility of reaching 25°C in certain climatic conditions." – why were the tests carried out at an internal air temperature below 18°C?

24. Line 427-429 - "In accordance with the recommended values ​​set forth in Portuguese legislation [61], the GR consistently meets the temperature requirement, with values ​​consistently above 428 18°C ​​or near, most of the time, reaching 20.5ºC..." - this is not confirmed by the values ​​in Figure 4a and the data from Table A1

25. Line 471-473 – "This monitoring also underscores the significance of sufficient natural ventilation in enhancing indoor air quality (IAQ). It has been demonstrated that there is a decrease in pollutant values ​​whenever the door is opened" – this is not confirmed by the CO2 and VOC values ​​in Figure 4

Author Response

We sincerely appreciate the reviewers' thoughtful and constructive feedback, which has greatly contributed to improving the quality of our manuscript. We have carefully considered all comments and have implemented the suggested revisions accordingly. A detailed point-by-point response follows the attached folder. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Introduction: lengthy, large paragraph on the health risks associated with air pollution, this study is not about exploring the health effects of air pollution. It is not until line 97 that the topic of this study begins.

Experimental design: lack of comparison of the monitoring of the two classrooms before the experiment was conducted to prove that the two classrooms had the same environmental background, and the opening and closing of the door activities had a large impact on the monitoring indicators to prove that the difference in the monitoring values between the two classrooms was only caused by the placement of green plants.

Data accuracy issues: the results in Figure 8 and Table A1 contradict with Figure 9, there is no median in the box line plot, and the median and quartile overlap is only shown if the distribution is uniform and absolutely normal. Whether to consider using the Mann-Whitney U test to enhance the validity of the test for the median. Are the anomalies in PM2.5 and PM10 due to the low precision of the monitoring equipment, despite the statistical results stated in Table A2?

Discussion: lengthy and lack of relevant studies to prove the reliability of the viewpoints.

Conclusion: lack of valid indirect and clear conclusions, and the conclusions conflict with the experimental design and the purpose of the study. The study hypothesized that the two groups of experiments were independent and related respectively, but in the end it was inconclusive whether the two groups were independent or related.

 

Line 150: Please Abbreviations 2.1. building characterization. Whether they have the same layout, finishes and display materials, and climatic environment such as temperature and humidity. Also the concentration of outdoor air pollution during the monitoring period, as outdoor pollution can enter the house through cracks in windows and doors.

Line 202: Please add the frequency of data collection.

Line 246: How were the methods mentioned used in the study? Why were these methods used? Please add citations.

Line 268: In the results section, the authors have written each paragraph according to the different monitoring objectives. It is suggested to re-diagramme the results to make it easier for the reader to read, e.g. the line graph for temperature and the box plot together. Because the text does not find a statement about the correlation between temperature and humidity, putting them together could lead to misunderstandings.

Line 278: The horizontal coordinates of Figure 4 are not equal time intervals. Please write down the exact time of opening and closing the door. Figures 6 and 8 also need to be revised.

Line 280: The fourth time period of figure (b) is missing p-value, please add

Line 282: The four phases are not of equal length, what is the basis for their division?

Lines 285-323: Please be brief, not a restatement of the figure but an analysis of the figure is needed, e.g. what might be the cause of the state of the door or the difference in temperature and humidity at different times of the day.

Line 290/297/298: Avoid vague and subjective expressions such as similar, rightward slant, light tail, more symmetrical and regular distribution. Use specific data instead. There are many other similar expressions in the results section, please correct them.

Line 317: in Table A2 respectively

Line 350: Add the relationship between CO₂ concentration and people density (e.g. ‘25 people contribute about 500L of CO₂ per hour based on each person exhaling about 20L of CO₂ per hour’). What is the specific effect of plant photosynthesis on CO₂ uptake (e.g., ‘rate of CO₂ uptake per hour by green plants’)?

Line 383-399: This paragraph lacks an explanation of the mechanism.

Line 397: Long-term, low-dose exposures can also pose a health risk (e.g., long-term exposure of children to PM₂. ₅ may affect lung development).

Line 411: Are there any new regulations that can be cited, the 1986 standards may not be applicable to today's health needs, and the act is aimed at indoor workplaces, classrooms are educational institutions may not be subject to the regulations.

Line 415: "Conversely, relative humidity levels exceeding 70% are conducive to the uncontrolled growth of fungi and bacteria " Add citation.

Line 420: Same problem as above, Liu et al. study shows optimal learning performance at 40% humidity, contradicting the 1986 legislation.

Line 440: This conclusion cannot be justified in this study.

Line 443-460: The harms of pollutants should be presented in the introduction.

Line 464-467: Lack of sufficient evidence to support the conclusion.

Line 540: Since natural ventilation plays a key role, vegetation is not the only influencing factor in this study, and therefore does not support the conclusion that vegetation improves the environment in the study.

Line 549-559: A brief summary of the main idea of the paper is sufficient.

Line 583: Wrong title.

Author Response

We sincerely appreciate the reviewers' thoughtful and constructive feedback, which has greatly contributed to improving the quality of our manuscript. We have carefully considered all comments and have implemented the suggested revisions accordingly. A detailed point-by-point response follows the attached folder. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

There is already a relatively large body of literature on the impact of plants on the indoor environment, and a generally recognized consensus has already developed. While the beneficial impact of plants on people's well-being is beyond dispute, the possibility of using plants for air biofiltration in public buildings raises serious doubts.

Unfortunately, the authors did not ensure the proper design of the study, which means that the obtained results have no significant scientific value.

No one has obtained similar results in real facilities since NASA's studies on air purification from volatile organic compounds. It should be noted that in NASA's studies, the degree of filling the research chambers with green mass was high. In a chamber with a volume of 0.4 m³, 2 plants were placed, filling approximately 5 plants/m³. Moreover, the lighting intensity was 3500 lux, and the chambers were perfectly airtight.

In the described study, 20 plants were placed in a room with a volume of 241 m³, which corresponds only to 0.08 plants/m³. Based on fig. 3a, the lighting intensity can be estimated at 300 lux, and the ventilation intensity based on Figure 6a at <1.5 l/s per person.

It should be remembered that both the VOC bioremediation and photosynthesis processes occur only when the limit value of photosynthetic radiation intensity is exceeded. In semi-darkness, plants exhibit the phenomenon of photorespiration (plants absorb oxygen and release carbon dioxide). A significant phenomenon, which the authors of the text do not analyze, is transpiration and evaporation of water from a moist substrate. Due to these processes, over 90% of the water used during watering the plants evaporates into the air in the room. The heat for this process is taken from the environment, which means that the plants, by humidifying the room, additionally cool it slightly.

At the same time, 15-16 pots with soil substrate (with an estimated volume of about 1,3 m³) were introduced into the room. Studies have often shown that in low-light conditions, the effect of VOC adsorption on the porous structure of the substrate is more important than the absorption of VOCs by the green part of the plant.

It is worth carefully studying Woolverton's classic publication [40]. Table 1 shows that placing only pots without plants caused a 33.3% decrease in formaldehyde concentration in the chamber, while a reduction of 66.6% was observed when plants in pots were placed. This means that sorption processes were of similar importance to bioremediation itself.

Another issue is the measuring equipment used. The authors provided relative measurement errors in the text. However, the data from https://www.kaiterra.com/sensedge-mini-indoor-air-quality-monitor is probably more telling.

Particulate Matter Sensor accuracy ±3 μg/m³

TVOC Sensor accuracy ±15%

CO2 Sensor accuracy ±40 ppm ±3%

Temperature Sensor accuracy ±1 °C

Relative Humidity Sensor accuracy ±5 % RH

Many differences, including temperature or relative humidity values, are smaller than the device's measurement accuracy. With sensors of such low accuracy, it is worth calibrating them against each other before use by comparing the readings of 2 sensors placed next to each other.

Similarly, conducting control measurements in both rooms without plants could show that they can be treated identically. An even better idea would be to swap groups of students between rooms halfway through such a control study.

The authors did not demonstrate measurably, but only declared that both rooms are identical. The room was filled with insufficient plants to present the impact of plants on the indoor environment. The measurement differences attributed to plants are puzzling and partially contradictory to existing knowledge, which has not been discussed. One of the reasons for this is the low accuracy of the measuring devices. Data analysis does not take into account the low accuracy of measurements. The second is improper planning of the study.

Detailed comments:

Line 118 "Error. Reference source not found."

Incorrectly described window orientation in Figure 2a. Replace Northwest with Northeast.

In Figure 2a, there are no markings for all types of plants (only types 1-3 are marked).

Author Response

We sincerely appreciate the reviewers' thoughtful and constructive feedback, which has greatly contributed to improving the quality of our manuscript. We have carefully considered all comments and have implemented the suggested revisions accordingly. A detailed point-by-point response follows the attached folder. 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The article  is recommended to accept after minor revisions. The following improvements are proposed to enhance the paper's quality and clarity of presentation.

1. Some references are outdated (e.g., Wolverton 1984). It is recommended to supplement recent studies from the past five years (e.g., meta-analyses on plant purification efficiency published after 2020).
2. The paper investigates the effects of NGS on IAQ. Including measurable outcomes (e.g., 'NGS demonstrates a CO₂ reduction of ≥20%') would strengthen the study's empirical validity.
3. The current 3-hour winter monitoring period is insufficient for comprehensive analysis. We recommend extending both the observation duration and including multiple seasons to account for seasonal variations in plants' purification performance.
4. As natural ventilation influences the diffusion of airborne contaminants, it is essential to incorporate air velocity measurements in the collected data for proper analysis.
5. We recommend including updated research (2020-present) on phytoremediation efficiency for both PM2.5/PM10 and common VOCs (e.g., formaldehyde, benzene) to strengthen the literature review.
6. The experimental design effectively controls variables between the two adjacent classrooms. However, adding a third control space featuring conventional air purification technology would enable more rigorous isolation and quantification of the vegetation's specific contributions to air quality improvement.
7. This research has valuable practical implications for school IAQ management. The impact would be strengthened by adding specific implementation advice, particularly regarding the most effective plant species for classroom deployment.
8. The inclusion of multiple plant species is commendable. A more detailed discussion comparing their respective absorption capacities for different pollutant types would provide greater scientific and practical value.

Author Response

We sincerely appreciate the reviewers' thoughtful and constructive feedback, which has greatly contributed to improving the quality of our manuscript. We have carefully considered all comments and have implemented the suggested revisions accordingly. A detailed point-by-point response follows the attached folder. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Unfortunately, the article still lacks a graph of the variation of outdoor air parameters during the tests. This does not allow for a clear determination of the effect of outdoor air parameters on its indoor parameters. The information provided on page 8 is insufficient to clearly exclude the effect of outdoor air pollution on IAQ.

Author Response

Dear reviewer,

Thank you once again for your valuable feedback and thorough review.

Unfortunately, we do not have the additional data you requested beyond what was already provided in the first review. We fully acknowledge that such data could indeed enhance the study, and we will certainly take this into consideration in future monitoring efforts.

Prior to conducting the indoor monitoring, we assessed the outdoor environment and confirmed that the indoor and outdoor conditions were identical (with windows and doors open for ventilation). Given this setup, we did not expect to see significant variations in the outdoor environment over the 3-hours monitoring period. Nevertheless, as mentioned, this aspect will be considered and addressed in subsequent monitoring.

We greatly appreciate your insightful suggestions, which will undoubtedly contribute to the improvement of our future work.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper has been improved. Suggest accept.

Author Response

Dear reviewer,

We sincerely appreciate your time and valuable feedback on our manuscript. We are grateful for your agreement with the article’s findings and conclusions.

Thank you once again for your thoughtful review and positive assessment.

 

Reviewer 3 Report

Comments and Suggestions for Authors

It is challenging to analyze the revised manuscript because only new text fragments are visible, and it is not possible to identify deleted or replaced parts.

A previous review stated:

“There is already a relatively large body of literature on the impact of plants on the indoor environment, and a generally recognized consensus has already developed. While the beneficial impact of plants on people's well-being is beyond dispute, the possibility of using plants for air biofiltration in public buildings raises serious doubts. Unfortunately, the authors did not ensure the proper design of the study, which means that the obtained results have no significant scientific value.”

Since the authors do not fully agree with this statement in response to the comments, the reasons for this statement are described in more detail below. Assuming identical rooms, with identical ventilation, used identically, differing only in the fact of introducing plants into the "green room" (GR), the following results should be expected in the GR compared to the Neutral room (NR):

• Slightly lower air temperature in GR than NR (the opposite effect was obtained)
• Slightly higher relative air humidity in GR than in NR (in fact, slightly higher moisture content expressed in g/kg of dry air) (the opposite effect was obtained)
• Depending on the lighting intensity and the share of plants using C3 and C4 type photosynthesis, a slight decrease or even a small concentration of carbon dioxide would be possible. However, a significant reduction in CO2 concentration was observed. The obtained difference in CO₂ concentration, ~500 ppm, with an increase above the external background of ~3000 ppm, would correspond to a reduction in the number of people in the room by ~4 people. This would require ~100 m² of leaves in good lighting conditions.
• To VOC, taking into account the reservations concerning the photosynthesis process, as well as sorption, a relatively minor decrease (a significant reduction in VOC concentration was observed).

 

In the further part of the first review, clues were also indicated to facilitate the authors' search for the causes of these surprising results. It was suggested that differences in the construction and equipment of the rooms, heat gains from neighboring rooms, variations in the intensity of natural ventilation (resulting, for example, from different wind pressure conversion coefficients on the building façade), inaccuracies in the measuring equipment, and the lack of consideration for the sorption effect, among other factors, could be contributing factors.

In response, the authors emphasized that, in their opinion, rooms should be treated as identical. The reason for the surprising results was not the quality of the measuring equipment.

 

In that case, I kindly ask the authors to:

1. Explain what processes cause the air temperature to increase as a result of introducing plants into the room
2. Explain what processes cause the air humidity to decrease and not increase as a result of introducing plants into the room.
3. Estimate the area of plant leaves in the room, determining the lighting conditions (PAR), and checking whether individual species, depending on their type of photosynthesis (C3 or C4), were in photosynthesis or photorespiration mode. Could you explain that such assumptions make a difference in the increase in CO2 in GR by as much as 500 ppm?
4. Considering that one plant in good lighting conditions corresponds to a VOC purifier with a CADR of 0.01 - 0.04 m³/h, explain what processes are responsible for the decrease in VOC concentration by 50%.

Author Response

Dear reviewer,

Thank you very much for your time and insightful evaluation of our manuscript. We sincerely appreciate your constructive feedback. Your insightful comments have been carefully considered, and we have incorporated them to enhance the clarity and rigor of the manuscript. We truly appreciate your expertise and support throughout the review process.

The answers to the questions are detailed below:

1. Explain what processes cause the air temperature to increase as a result of introducing plants into the room

We would like to clarify that the increase in temperature in the classroom with plants is indeed possible and can be explained by two main factors:

• Plants near windows or light sources may retain heat more effectively, contributing to a localized increase in temperature;
• The presence of plants can influence air circulation and heat distribution within an enclosed space, potentially leading to slightly higher temperatures.

The text between lines 511 and 514 has been added.

2. Explain what processes cause the air humidity to decrease and not increase as a result of introducing plants into the room.

Regarding the decrease in relative humidity in the classroom with plants, we consider this outcome plausible due to two main factors:

• Certain plants can absorb moisture directly from the air through their leaves or soil, reducing ambient humidity;
• As the temperature in the room with plants increased, the air's capacity to hold moisture also increased, which can lead to a decrease in relative humidity despite the presence of plants.

The text between lines 522 and 526 has been added.

3. Estimate the area of plant leaves in the room, determining the lighting conditions (PAR), and checking whether individual species, depending on their type of photosynthesis (C3 or C4), were in photosynthesis or photorespiration mode. Could you explain that such assumptions make a difference in the increase in CO2 in GR by as much as 500 ppm?

Regarding the reduction of CO₂, several factors should be considered:

• The plants may have enhanced air mixing within the classroom, even without natural ventilation;
• The localized effect of the plants, positioned near the walls and ceiling, may have contributed to the reduction of CO₂;
• Regarding the observed reduction in CO₂ levels, it is also important to consider the photosynthetic pathways through which plants capture and fix CO₂, namely the C3 mechanisms;
• Most indoor plants used in our study utilize the C3 photosynthetic pathway, where CO₂ is directly fixed by the enzyme Rubisco into a three-carbon compound during the Calvin cycle. This process is efficient under moderate temperatures and higher CO₂ concentrations, such as those found in an indoor environment with occupants. Thus, plants using the C3 mechanism can actively reduce CO₂ levels by increased photosynthetic activity;
• In our study, the likely predominance of C3 plants, combined with the elevated indoor CO₂ concentrations due to occupant presence, may have stimulated greater photosynthetic uptake of CO₂, contributing significantly to the measured decrease.

The text between lines: 207 to 216 and 569 to 581 has been added, and new references were included.

4. Considering that one plant in good lighting conditions corresponds to a VOC purifier with a CADR of 0.01 - 0.04m/h, explain what processes are responsible for the decrease in VOC concentration by 50%.

Regarding VOCs decrease it is important to highlight that plants can reduce VOC concentrations not only through direct absorption by their leaves but also through the action of their root microbiomes. The soil and associated microorganisms can break down VOCs, contributing to a cleaner indoor air environment. The combined effect of plant foliage, soil, and microbial degradation is a well-documented mechanism for VOC reduction.

The text between lines 603 and 606 has been added, and new references were included.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Unfortunately, the authors' explanations do not convince me. I maintain almost all of my critical comments on the presented manuscript. Before starting the study, the authors did not sufficiently explore the subject of plant physiology, especially the intensity of these processes. They did not estimate what results should be expected in the case of their study. The fundamental mistake was the use of rooms with natural ventilation. It isn't impossible to talk about controlling the experiment, then. Seemingly adjacent rooms, very similar in terms of architecture, can differ significantly in natural ventilation intensity (even by several dozen %). In studies conducted in such rooms, differences in the concentrations of metabolically generated carbon dioxide help explain discrepancies in ventilation intensity (CO₂ is treated as a tracer gas). In the described study, it was assumed a priori that the ventilation intensity is identical, and the differences in concentrations (approx. 500 ppm) are the result of unrealistic plant activity. In the previous review, I noted that such an effect could only be expected if the room was filled with many more plants and exposed to long-term, lighting with the intensity of photosynthetic radiation PAR (approx. 100 μmol/(m² s)). The description of the study does not fit this statement. Similarly, the results of bioremediation presented in the paper are unconvincing. The literature on the subject does not confirm such a significant impact in real-world situations (please analyze my previous comment on equivalent CADR).

I am a strong supporter of biophilic solutions, including the placement of ornamental plants in rooms. I allowed myself to formulate such negative comments for the authors because I spent a considerable amount of time studying the physiological processes of plants in both laboratory conditions and real-world situations, as well as their interaction with the indoor environment. In these studies, a significant effect of plant transpiration and evaporation of moisture from the substrate, leading to air humidification and its slight cooling, was consistently revealed, consistent with the literature data. Taking seriously the studies that deny this phenomenon would require a much better-controlled experiment.