Development of a New Evaporation Exposure Model: Chemical Product Evaporation Model (CPEM)
Round 1
Reviewer 1 Report
The authors describe a new chemical exposure model to simulate the volatilization of chemicals from indoor/household products while considering the change in volume/weight of the product itself, which is a valuable addition to existing models that assume constant product volume. While the mathematical backbone of the model and model results are nicely explained and described, the necessity and application of the model is not sufficiently demonstrated throughout the manuscript, as the authors did not measure concentrations in air and product themselves. I have some major comments that should be addressed before publication.
Major comments:
(1) The introduction is heavily focused on models but does not describe if there have been studies that identified or discussed that variability in product volume can be an issue.
(2) Please cite experimental studies that focused on indoor exposure and application of previous models. 16 references are not appropriate for a scientific article and there are many studies that focus on indoor exposure from consumer products.
(3) More sophisticated literature research would increase the significance of the work; if it is significant? Since the authors did not measure chemical concentrations themselves, the necessity for the increased complexity of the new model is unclear. Please demonstrate that the new model is needed and that previous models are not applicable in certain cases (which cases? Which consumer products other than the one’s tested find application?).
(4) The Figures in the manuscript are really big and still not sufficiently stand-alone, there is a lot of space left to add information. Please consider changes / additions in the Figure to make them clearer. Possible changes include arrows to describe the differences between the model calculations. Overlying graphs can be removed and just described in the text as not significantly different from each other (saturated vs unsaturated). The key information is the difference between the models with and without product volume change consideration.
(5) Similar to the introduction, the discussion lacks context. 3 References cited in the discussion clearly shows that the authors need to demonstrate the significance and utility of their model, if possible on the basis of experimental studies / data.
Minor comments:
Abstract: Should be filled with more information and data as well as an introductory sentence describing the context of the study.
Line 42: Replace “for instance” with “however”
Table 2: Can be moved to Supporting Information
Line 221: Remove “Also”
Figure 4c is not a significant addition in information compared to Figure 4b. Suggest removing one of the two.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript describes a model intended for use in describing the evaporation (and hence potential air concentration exposure) of a target volatile substance in consumer products - the family of products in mind are things like air fresheners which are intentionally designed to evaporate into the room air. The model proposed extends prior models to incorporate the effect that other substances (ie solvent) may also evaporate during product use. Depending on relative evaporation rates, this changes the effective target substance concentration in the product and can (under some frankly poor product design circumstances) lead to an increase in target product air concentration than is expected in prior models.
The idea behind the development of this model is sound, and I think the model seems "correct." However, the application of the model is not discussed well - ie what are the overall ramifications of such model to exposure, and are there any actual products of those samples that the model would predict a different outcome than prior models. The latter point speaks to a kind of "who cares?" idea that I think is not really addressed, but could be. The parameter space chosen for the model feels arbitrary, with many assumed parameters that are not supported in text through even brief discussion. Most importantly, the only actual measured data in this paper is done haphazardly with lack of proper controls. The authors really claim to be measuring a volume decrease but actually are measuring mass decreases and use a bad approximation to covert between mass and volume. This is important because these volume decreases are used as basis for perhaps the most important parameter in their new model. Interestingly, somewhere 2/3 or so in the paper, the authors seem to abandon this strained notion of volume changes and discussion mentions more of mass changes. This confusion between volume and mass change must be rectified (see further comments below). Also use of "P" and "p" as variables is seen as problematic (see comments below).
The use of the word "product" is many times confusing as it has overlapping context between "consumer goods" and "outcome of reaction/process". Because of this especially a thorough working of the English should be done by a native speaker to help capture context. Slightly awkward phrasing and grammar pervade the paper.
Overall, I cannot suggest this paper be published. While the high-level idea behind the model development is sound, the key finding - model dependency on "p" - is based on flawed experimental design and data, which calls into question the validity of all data in the paper which is based on these results. I recommend Rejection with encouragement that the authors should recast the paper with proper mass not volume treatment. I choose Rejection because I believe the full overhaul of the paper is needed beyond even "Major Revisions". I believe the paper should be re-worked and submitted anew.
More specific comments relative to sections of the paper are contained below.
I do not like the name "chemical product evaporation model" The meaning of this is not clear out of context - it seems as though it would apply to raw chemicals - ie a can of solvent. Or that it refers to a reaction product model. Unfortunately I do not have a better suggestion, but please re-think the name.
Check manuscript throughout to ensure variables are italicized and units as are not italicized - this is done inconsistently
ln. 77 "????? is the concentration in a product" this refers to a particular substance of interest - so maybe write concentration of active ingredient ? or target substance? etc.
ln 78-79 " ???? is the saturated vapor pressure of the substance," in reference also with last comment, should this be "target substance " and make it target substance in Ln. 77 as well for consistency.
The use of the term "other materials" is too ambiguous. I think you mean the idea of "other volatile materials that are not the target substance " rather than the idea of "all other substances in the product" ... right? If so I think perhaps "other volatile materials" or just "other volatiles" is better than "other materials"
I think at the beginning of the section ( ~ln. 63) you should make a 1-2 sentence introduction saying something like "Here we build the model... and we are concerned with treating a target substance of interest and the other volatile compounds in the product..." The idea is to give the reader the notion that there are two kinds/classes of evaporating species you are treating, and to name those classes (i.e. "target substance" and "other volatiles" if you choose those terms) and use those names when definitely ing variables and in ensuring discussion. Try to always use those names and not synonyms. This will avoid confusion as you define and establish the model.
Eq. 7 This is not acceptable to use variable "P" here. choose a different variable or at a minimum assign a subscript. "P" is always a pressure, and in other use in this same construction of model it is a pressure, but here you are using the (naked) variable as a rate. This must be changed.
ln. 102 This is a new model introduced to the paper so I want to confirm that V, Q, and S are the same as defined earlier - room volume, vent rate, product surface area... correct?
ln. 111 "p=0" is meaningless here, this variable is currently undefined. re-write to use words in the section heading to convey the idea of "p=0"
ln. 123 an equation and its description are missing here entirely!
Related to my objection to "P" variable I don't like "p" as your rate coefficient variable. consider other variables. Greek letter nu and rho perhaps?
ln. 137 what do you mean by "pattern"? this is not something you measure.
ln 138 "The weight was converted to a volume assuming a unit density." I feel this is frankly a horrible assumption and undermines the whole concept behind this study. Unless all of the products are majority water-based, using unit density (ie density of water 1 g/ml) will always be at least slightly wrong. Your whole model aims to treat "volume reduction" but you're ACTUALLY just looking as mass changes.
The crux of your whole study is that you are concerned that concentrations may change due to essentially solvent evaporation while the solute target substance is also evaporating. So thinking about this volume of solvent is not a bad idea - I'm with you here. The concentration idea here you propose and use in equations and throughout the text is mass/volume, or essentially a MOLARITY (moles of solute/volume of solution) type concept. BUT you're not measuring volume. You're measuring MASS. So you would be much better using a MOLALITY concept of concentration (moles of solute/mass of solvent). No assumptions needed, no information of density needed. All you need is the initial mass of target substance and "everything else". Rectifying this point is key to making this model and paper valid.
Ln. 169 "Because it was difficult to measure the release area of each product" Why was this difficult? Liquids and gels should have been "re-packaged" into identical vessels/test tubes/beakers/whatever that all had same air/surface contact area (i.e. circular area of the testube.) I can imagine that different diffusers have very different areas based on their design (Reed diffuser for instance depends on number and size of "sticks") but this can be controlled for by using the diffuser liquid in the same vessel also and comparing between "testtube" and diffuser. This while idea can give controls for the volume reduction and then SOME idea must be able to be gotten for the area of the products themselves. As it stands, I do not trust that most of the results (ie. fig. 1 and table 2 for instance) represent meaningful findings. Rectifying this point is also key to making this model and paper valid.
"e0X" "e-XX" is unprofessional formating. Please change to "x 10-XX" (superscripts) notation - in other words, proper scientific notation.
ln. 194 "By measuring the weight change every hour, it was confirmed that the volume of all products decreased at a constant rate" This is based on a (potentially bad) assumption, and does not confirm such a rate. I see clear curvature for instance in Fig. 1a. Are these for particular products? (i.e. as an example) which ones? The values do not match that in Table 2. Are they instead averages for a class of products?(if so, that is not a valid thing to do).
Also where is the data for diffusers? it is not seen in Table 2 - or is not labelled as such.
Ln. 196-197 "The range of p was 3.3e-6 to 1.05e-3 ℎ−1, the median was 3.78e-4 ℎ−1, and the mean was 3.16e-4 ℎ−1" These statistics are meaningless as the "p" values are compared over a range of different types of products, with apparently no control for surface area, and no reason that I see that they should be expected to behave at all similarly.
Fig. 2 and 3 are not very aesthetically pleasing and are somewhat hard to see the data, but in the end they are not all that important to the overall story and show expected behavior. The captions should make clear that the K and S minimum and maximum are from the range quoted in the table.
ln. 244 "The more active the ventilation, the more frequently the substance was removed from the air, and the average concentration gradually decreased." I do not understand what the latter part of this sentence refers to, because the plot shows continual increase. Do you mean that ventilation would only serve to slightly lower the overall exposure to the target substance?
ln. 305 you write "...it would be impossible to estimate the total amount..." but of course it is possible. you suggest on Ln. 309 to measure the leftover substance in spent product and on Ln. 311 suggest CPEM model. So I think you might mean "difficult" perhaps, but not "impossible"
Section 4.3 / Fig 5 seems more important than the authors indicate (ie by not discussing it much) It is not fully clear to me the full ramifications of the indirect estimation and why it is so much different in Fig. 5b. I think the authors should explain more.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
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Comments for author File: Comments.docx
Author Response
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Author Response File: Author Response.docx
