Inactivation of Continuously Released Airborne Virus by Upper-Room UVC LED Irradiation Under Realistic Testing Conditions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript deals with the use of UV to virus inactivation in indoor environments.

The topic is sufficiently interesting to deserve publication.

The methods are sound and explained with enough precision.

 

There are only some minor topics that should be addressed.

1) In Fig.4 is reported a plot of a fitted function, but no details could be find in the text.

Which equation was used to fit ? How many free parameters ?

What about the chi-square and the error associated to the estimated Ceq and Ktotal ?

2) In Figure 5 and table 4 several parameters describing the kinetics are reported without any estimate about their uncertainties. How the curves shown in figure were obtained.

If a fit was performed, as stated on pag.12, please give some details about the results.

3) It would be nice to discuss the energy costs of such a sanitization technology in the Conclusions section.  

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,
The study addresses an important and timely topic.

It requires attention to ensure maximum clarity, scientific rigor, and impact. We offer the following points for your consideration, aiming to help you further strengthen your valuable work.

Making the k_AC Formulas Easier to Understand (Primarily Eq. 7-9): The mathematical model used to calculate k_AC is a central part of your study. While we appreciate the technical work involved, the derivation and the assumptions behind Equations 7, 8, and 9 could be explained in a way that is more accessible to readers who may not be specialists in aerosol modeling or advanced air disinfection mathematics. For instance, the specific way the virus concentrations are related in Equation 7 might not be immediately intuitive to everyone. We kindly ask if you could provide a clearer, perhaps more simplified, step-by-step explanation of how you arrived at these formulas. Highlighting the main assumptions of your "Incremental Evaluation Model" in plain language and showing how they lead to these equations would be very helpful. The goal is to ensure that a wider range of readers can confidently follow the logic and understand the basis for your k_AC calculations, as these are key to the paper's conclusions.

Explaining the Natural Virus Loss (k_Nat): The rate at which the virus disappears naturally (k_Nat) in your tests seems higher than in some other studies.: Could you discuss in simpler terms why this might be? For example, did the room conditions (like air movement from the manikins, or the specific temperature/humidity) play a big role? It's also a bit puzzling why k_Nat was different in your two main test types (continuous release vs. decay). A clear explanation would be helpful, as k_Nat affects your k_AC results.

Showing How Much Results Might Have Varied (Experimental Variability): When you present your results, it's not clear if you did each experiment multiple times. If you did, showing how much the numbers varied between tests would help readers understand how consistent your findings are.
 Please clarify if you repeated your experiments. If so, it would be good to include information like averages and how much they varied (e.g., standard deviation). If not, please mention this as a point to consider for future work.

Clarifying Results for Position 1 (Near the Source): The results for k_AC at Position 1 seem a bit inconsistent across different parts of the paper, and you mention the model wasn't a good fit there for one test. Please double-check all numbers for Position 1. If the model doesn't fit well, please explain clearly what that k_AC value means or if it should be viewed with caution. The "short-circuit" idea is interesting, but more direct evidence or a clearer explanation of airflow would make it stronger.

Explaining Why k_AC Changes at Position 2: The k_AC value at Position 2 changed a lot when the starting virus concentration was different.
Could you discuss in simpler terms why this might be? For example, does the UV system work differently if there's less virus, or is it something about how the math works when numbers are very small?

Room Conditions: Could you briefly explain why you chose the specific room height and humidity for your tests, and if these choices might have affected the results?
Model Details [Ref. 75]: If the full details of your "Incremental Evaluation Model" are in another paper (reference 75) that isn't easy for everyone to get, a very brief summary of its main ideas in this paper would be helpful.

Clarity on Funding and System Origin:: The company that paid the publication fee also holds a patent on the UV system's optical design. Please state this relationship very clearly in the "Conflicts of Interest" section so all readers are aware of it, even if it didn't influence your work.

Making the Conclusions Clear and Actionable: It would be great if the Conclusions started by directly listing the most important practical takeaways from your study.
Next Steps: Could you suggest a few clear next steps for research in this area?

Minor Points for Presentation:
Title & Keywords: Could making the title and keywords a bit more specific (e.g., mentioning "LED-UV" or "Phi6") help people find your important work more easily?
Introduction Focus: The introduction is good, but perhaps some very technical UV details could be simplified or moved to the discussion if used for comparison.
Materials and Methods Readability:
Some parts are very technical. Would a simple table listing the main equipment help?
Terminology: Using "concentration of active/infectious viruses" instead of "virulence" (line 215) would be clearer.
Discussion - Broader Context: It would be helpful to compare your results a bit more with other similar studies to show where your work fits in.
Discussion - "Realistic" Conditions: Since you tested in "realistic" conditions, a brief mention of how small real-world disturbances might affect the results could be useful.
Final Polish: A final read-through for any typos or unclear phrasing would be great.
Consistent Terms: Make sure terms like "k_AC" and "k_Nat" are used the same way everywhere.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Reducing human exposure to pathogenic particles is one of the essential elements of controlling the indoor environment. Preventing the spread of particles is particularly important in hospital rooms, where medical personnel work every day and patients stay, who usually have reduced resistance to infections due to illness. The most effective way to reduce the concentration of pathogenic particles is to capture them as close as possible to the source of emission. In the literature, you can find descriptions of several interesting solutions.
I consider the topic of the article to be very important, timely and specific. The article is very well written, well organized,  it contains all the required parts. The introduction is extensive and interesting at the same time, covering many aspects of eliminating airborne pathogens.
My general observation is that the mannequins used in the experiment did not breathe, except for one. In my opinion, this fact significantly influenced the shape of the convective flow in the room, and specifically the structure of the turbulent component and, as a result, the range of the convective flow.
A less important general comment concerns the problem of ozone formation in air exposed to UV-C radiation. Human exposure to ozone should at least be discussed.
I also have also a few additional particular comments to the content of the presented article. The order of the comments does not reflect their significance. It results only from the order of appearance in the text of the article.
My remarks and comments:
1.    Line 167, “heat load generated by a person” - What value of the CLO coefficient was adopted during the tests; Thermal resistance coefficient, CLO value describes the degree of insulation provided by an article of clothing.
2.    Line 173, “The flow velocities were determined at a height of 1.25 m” - Information about the location relative to the mannequin and how far the sensor was located from the heated surface of the mannequin would be very valuable.
3.    Line 201, “a ventilation rate of 9 strokes/min” - It is important to specify the duration of the individual respiratory phases, how long the inhalation lasted and how long the exhalation lasted. Was a pause between breaths simulated? The durations of the respiratory cycle phases have a significant impact on the formation of flows in the breathing zone, the interaction of the convective flow and cyclic exhalations.
4.    Line 201, “0.7L” - I suggest adding a literature source confirming the validity of the assumed breathing parameter.
5.    Lines 248-249, “on LED technology and utilized UVC radiation” - it would be good to know the spectral characteristics of the radiation source.
6.    Line 270, “natural convection” - This is very important information, it explains how pathogenic particles were transported into the radiation zone. Hence the question about the location of air supply and exhaust becomes important. In the abstract the authors declare "(air exchange rate) was 47 h-1 in the first scenario and 30 h-1 in the second" then in line 183 they wrote "Air exchange rate 0 h-1 (static, no air exchange applied in the test facility". I suggest adding some explanations so that the reader knows what kind of air flows we are dealing with.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

thank you for the effort in improving the paper.

I think that some work is still needed:

1. Justify the Mathematical Model Choice.
   • The authors need to add a dedicated paragraph in the Materials and Methodssection (before section 2.5) justifying their choice of the "Incremental Evaluation Model" [77]. They should briefly explain why standard, simpler models (like a basic well-mixed assumption) are insufficient for their "realistic testing conditions" and how their chosen model specifically addresses the challenges of spatio-temporal gradients and short-circuit airflow.
2. Provide a Conceptual, Step-by-Step Explanation.
   • To address the need for clarity, they should provide a simplified, conceptual explanation of their gradient model. This doesn't mean simplifying the math, but explaining the logicbehind the equations in plain language. For example: "To account for the non-uniform virus concentration, our model divides the room and the measurement time into smaller segments. It calculates the concentration change in each segment considering both the virus decay and the influx from the previous segment, thus building a dynamic, location-specific profile..."
3. Strengthen the Discussion on Model Discrepancy.
   • In the Discussionsection, they must expand on the significant difference between the k_AC values obtained from the two models (32–73 h⁻¹ vs. 161–566 h⁻¹). They should explicitly state which model's results they consider more reliable for their setup and why. They need to guide the reader to the correct interpretation, rather than just presenting two conflicting numbers. They should state that the k_AC from the well-mixed model is likely an overestimation and should be interpreted with extreme caution.
4. Improve Text Structure for Readability.
   • The "Materials and Methods" section, particularly section 2.5, is extremely dense. It would benefit greatly from being broken down with more specific sub-headings (e.g., "2.5.1 Modeling for Non-Uniform Conditions (Gradient Model)", "2.5.2 Modeling for Idealized Conditions (Well-Mixed Model)"). This would make the structure clearer and easier to follow.

Regards

Author Response

Please see the attachment.

Author Response File: Author Response.pdf