From the journal Environmental Science: Atmospheres Peer review history

29-Jan-2024

Dear Dr Salthammer:

Manuscript ID: EA-CRV-01-2024-000006
TITLE: Carbon monoxide as an indicator of indoor air quality

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

I have carefully evaluated your manuscript and the reviewers’ reports, and the reports indicate that major revisions are necessary.

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Prof. Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

************

Reviewer 1

The manuscript is very up-to-date and offers a comprehensive critical literature review on carbon monoxide as an indicator of indoor air quality. Although the manuscript is not systematic according to the PRISMA criteria, it still offers a very significant and representative literature review on the topic. The chemical background of the manuscript regarding carbon monoxide is of a very high quality. The measurement techniques are carefully described, and they well illustrate the methods and means for measuring and determining carbon monoxide. In addition, Figures 1-3 further well illustrate the operating principles of measuring devices.
The figures are very well planned and add value to the manuscript. Especially Figure 6 at the end is very valuable. However, I would move the Figure 6 from section 9 (Conclusion) to section 8 (Discussion).
Table 2 gives an interesting view of carbon monoxide concentrations in indoor air, outdoor air and test chambers. If there is a need to shorten the manuscript, I would recommend moving Table 1 to the supplementary materials.

Reviewer 2

The paper titled “Carbon monoxide as an indicator of indoor air quality” presents a review of the sources, effects, and mitigation strategies of carbon monoxide (CO) in indoor environments. The paper addresses a timely and important issue of indoor air quality, which affects the health and well-being of millions of people worldwide. However, the paper in the current form is not good enough to be published although it has potential if the authors work more in it.

Below some comments:
- It would be desirable to include a review of other studies that have used CO as an indicator of indoor air quality in the introduction.
- The reference for the data you provided is unclear. Could you please specify the source?
"Carbon monoxide is a colorless and odorless gas with a molar mass of 28.01 g/mol. At T = 298K and p = 101325 Pa, 1 ppm=ˆ 1.14mg/m3. The density of 1.25 kg/m3 is almost identical to the density of air. The solubility in water is low and the Henry constant is corespondingly small.?"
- It is unclear whether Table 1 is necessary for the aim of the paper. It is included in section 3 (Physical properties of carbon monoxide), but the purpose of this section is not clear. Could you please clarify?
- The sources described in the paper are mostly general sources on outdoor chemistry. A more complete description of indoor sources, including plenty of references, should be included.
- A table with different measurement techniques, including advantages, disadvantages, range of concentration, limit of detection, and references where they have been used in indoor settings, should be added.
- If the paper is focused on indoor CO, why is there a section titled “CO concentrations in indoor and outdoor air”?
-It is unclear whether Figure 4 is from another paper or if it is new data. Could you please provide a reference?
- Table 2 should be more complete, including the equipment used to measure CO, the type of building, the room where it was measured, etc. In comments, only things that cannot be generalized should be included.
“Indoor guide values” should be changed to “Indoor air quality guidelines”.
-The format of the tables needs to be improved. For example, units should be in headings instead of being placed next to every number.
-Could you please clarify what you mean by “The most common averaging time is 8 h and the modal value for the 8 h average is in accordance with the WHO guideline. In addition, the modal value of the 15min also corresponds to the WHO recommendation, while small deviations can be observed for 24 h with a modal value of 8mg/m3?” It seems that statistical data is being used. Have they performed any statistical analysis, and what are the sources of the data?
- The discussion section needs to be revised to make it clearer. It should specify exactly which kind of sensors are recommended, the number of sensors to be used, and their location to be useful. Additionally, other sensors for other indoor pollutants that need to be measured at the same time as CO should be mentioned, along with reasons for their inclusion.

[This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.]

1
Manuscript: EA-CRV-01-2024-000006
RESPONSE TO REVIEWERS
Comments are in “italic”, responses in “blue”
Please note that I use LaTex. Therefore, corrections and changes in the manuscript look
different than those in Office Word.
REVIEWER REPORT(S):
Referee: 1
Comments to the Author
The manuscript is very up-to-date and offers a comprehensive critical literature review on
carbon monoxide as an indicator of indoor air quality. Although the manuscript is not systematic
according to the PRISMA criteria, it still offers a very significant and representative literature
review on the topic. The chemical background of the manuscript regarding carbon monoxide
is of a very high quality. The measurement techniques are carefully described, and they well
illustrate the methods and means for measuring and determining carbon monoxide. In addition,
Figures 1-3 further well illustrate the operating principles of measuring devices.
I am very grateful for the positive evaluation of the manuscript. Of course, I am familiar with
PRISMA and have already written a few reviews using this standard. But I quickly realized that
PRISMA makes little sense in this case. It was not my intention to conduct a complete literature
review of carbon monoxide or to write about the technical details of sensors. Rather, my
intention was to highlight the potential importance of carbon monoxide as an indoor air quality
indicator and to explain the benefits for future use. However, a review based on PRISMA
criteria is not really suitable for this. Nevertheless, I believe that I have searched the literature
adequately.
The figures are very well planned and add value to the manuscript. Especially Figure 6 at the
end is very valuable. However, I would move the Figure 6 from section 9 (Conclusion) to
section 8 (Discussion).
I agree and have now embedded Figure 6 in the Discussion section.
2
Table 2 gives an interesting view of carbon monoxide concentrations in indoor air, outdoor air
and test chambers. If there is a need to shorten the manuscript, I would recommend moving
Table 1 to the supplementary materials.
I agree with the reviewer that Table 1 could be moved to a supporting information. The only
problem here is that there is no supporting information yet. I do not think it is really useful to
create a separate document just for one table. Therefore, I would like to leave Table 1 in the
main text for the sake of a coherent presentation.
However, if the editor requests this in order to shorten the manuscript, I am of course willing to
move Table 1.
Referee: 2
Comments to the Author
The paper titled “Carbon monoxide as an indicator of indoor air quality” presents a review of
the sources, effects, and mitigation strategies of carbon monoxide (CO) in indoor
environments. The paper addresses a timely and important issue of indoor air quality, which
affects the health and well-being of millions of people worldwide. However, the paper in the
current form is not good enough to be published although it has potential if the authors work
more in it.
I appreciate that the reviewer basically assesses the work as positive. I take the criticisms very
seriously and where possible I followed the suggestions. If that was not possible, I explained
my reasons in detail.
Below some comments:
- It would be desirable to include a review of other studies that have used CO as an indicator
of indoor air quality in the introduction.
I agree and have included a short section with key references. Please note that these
references will be discussed in detail later.
- The reference for the data you provided is unclear. Could you please specify the source?
"Carbon monoxide is a colorless and odorless gas with a molar mass of 28.01 g/mol. At T =
298K and p = 101325 Pa, 1 ppm=ˆ 1.14mg/m3. The density of 1.25 kg/m3 is almost identical
3
to the density of air. The solubility in water is low and the Henry constant is correspondingly
small.?"
Yes - I now cite the references in the text and refer to Table 1.
- It is unclear whether Table 1 is necessary for the aim of the paper. It is included in section 3
(Physical properties of carbon monoxide), but the purpose of this section is not clear. Could
you please clarify?
I am convinced that the Table 1 is essential for understanding the analytical properties of
carbon monoxide and its dynamics in the indoor environment. I think the fact that the density
of CO corresponds to that of air is an important information. The ionization and dissociation
energy are needed for discussing photoionization, the proton affinity for PTR-MS, and the
electrochemical potential for the sensors working according to the redox principle. The kOH
constant is necessary for discussing the chemical reactions of CO. Some other parameters
were included for the sake of completeness and I see no reason to delete them.
- The sources described in the paper are mostly general sources on outdoor chemistry. A more
complete description of indoor sources, including plenty of references, should be included.
I respectfully disagree. In Table 2, all concentrations and sources are indoor-related, which is
also clear from the literature sources cited. However, the reviewer is right that the table was
not complete. I have therefore added the emission data from cookstoves.
I would also like to point out once again that the purpose of this article is not to cite as many
literature sources as possible or to provide a full literature overview. Rather, the references
were specifically chosen to discuss typical sources, emissions and concentrations.
- A table with different measurement techniques, including advantages, disadvantages, range
of concentration, limit of detection, and references where they have been used in indoor
settings, should be added.
I am sorry, but that is not the focus of this work and such a table would be endless. I have cited
the original papers and reviews accordingly in which the various techniques are discussed.
There are dozens of companies that offer CO sensors and devices that work with NDIR or
PAS. There are also various special techniques. In addition, the detection limit is not crucial for
CO sensors. What is more important is whether a signal that reaches the sensor is interpreted
correctly.
4
However, I have to partly agree with the reviewer here too. Therefore, I have produced a small
table with some basic information and references. I hope that is acceptable.
- If the paper is focused on indoor CO, why is there a section titled “CO concentrations in indoor
and outdoor air”?
Outdoor air quality can have a noticeable impact on indoor air quality. This is particularly true
for a combustion gas such as CO. Many studies therefore measure inside and outside. If this
was the case in the studies cited, I have included the value or range.
-It is unclear whether Figure 4 is from another paper or if it is new data. Could you please
provide a reference?
As already stated I the figure caption, the data were provided b my colleague Dr. Erik Uhde
from WKI. However, I agree with the reviewer that their origin is unclear. I now provide
additional information in the figure caption and in the text.
- Table 2 should be more complete, including the equipment used to measure CO, the type of
building, the room where it was measured, etc. In comments, only things that cannot be
generalized should be included.
I am not sure what the reviewer exactly means. I am certainly unable to specify the conditions
for all measurements. Take as an example the work of Pigg et al. (2018): how should I specify
the room for measurements in 514 homes? That information is not even provided in the paper
itself. Besides, I do not see the point. The requested information is not relevant for this work.
It would just result a huge and confusing table. Anyone interested in further details can access
the original work at any time. To my regret I cannot follow the reviewer's request.
“Indoor guide values” should be changed to “Indoor air quality guidelines”.
This is not so easy. Some of the WHO values were derived for ambient air and later adapted
for the indoor environment. If possible, I made the requested change.
-The format of the tables needs to be improved. For example, units should be in headings
instead of being placed next to every number.
5
I was able to comply with the reviewer's request in Table 3, but not in Table 2. Different units
are used there. I therefore have to write the unit directly after the respective number.
-Could you please clarify what you mean by “The most common averaging time is 8 h and the
modal value for the 8 h average is in accordance with the WHO guideline. In addition, the
modal value of the 15 min also corresponds to the WHO recommendation, while small
deviations can be observed for 24 h with a modal value of 8mg/m3?” It seems that statistical
data is being used. Have they performed any statistical analysis, and what are the sources of
the data?
Yes, this part was unclear. Thank you. I refer to the paper by Dimitroulopoulou et al. and have
now explained their statistical analysis more detailed in the text.
- The discussion section needs to be revised to make it clearer. It should specify exactly which
kind of sensors are recommended, the number of sensors to be used, and their location to be
useful. Additionally, other sensors for other indoor pollutants that need to be measured at the
same time as CO should be mentioned, along with reasons for their inclusion.
It has already been explained in detail in the text that carbon monoxide is a priority pollutant
according to WHO criteria and should be measured for precautionary reasons. It will now also
be explained which parameters should also be measured. Here I am referring to one of our
current publications (Morawska et al., 2024). I think it is impossible to say which sensor should
be located where in a building. This depends on the individual building type, which is also
mentioned in the text.
It is also outside the scope of this paper to recommend specific sensor types. For this purpose,
reference is made to various current publications. Furthermore, as a public service employee,
I am not allowed to recommend specific products. However, I admit that the word “perfor-
mance” in the headline was misleading and I have changed that.
In summary, I tried to take the reviewer's requests into account wherever possible. However, I
hope it is clear that this work cannot provide a detailed comparison of measurement methods
and sensor types. It is also not possible to give general recommendations for a sensor network.
Of course, a CO sensor should not be located too far from a potential CO source, but that is
certainly a trivial matter.

07-Feb-2024

Dear Dr Salthammer:

Manuscript ID: EA-CRV-01-2024-000006.R1
TITLE: Carbon monoxide as an indicator of indoor air quality

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Prof. Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 1

I think the corrections are sufficient and I support publishing the article.