Unconventional oil and gas development and ambient particle radioactivity

Unconventional oil and natural gas development (UOGD) expanded extensively in the United States from the early 2000s. However, the influence of UOGD on the radioactivity of ambient particulate is not well understood. We collected the ambient particle radioactivity (PR) measurements of RadNet, a nationwide environmental radiation monitoring network. We obtained the information of over 1.5 million wells from the Enverus database. We investigated the association between the upwind UOGD well count and the downwind gross-beta radiation with adjustment for environmental factors governing the natural emission and transport of radioactivity. Our statistical analysis found that an additional 100 upwind UOGD wells within 20 km is associated with an increase of 0.024 mBq/m3 (95% confidence interval [CI], 0.020, 0.028 mBq/m3) in the gross-beta particle radiation downwind. Based on the published health analysis of PR, the widespread UOGD could induce adverse health effects to residents living close to UOGD by elevating PR.

11. Line 295: Were different circular sector degrees tested? For example, 45 degrees? You would anticipate a stronger response. 12. To clarify, a well would be included as upwind if, on day t, it was completed (this date is indicated in drilling info? Does complete mean it has been fracked?) and the wind blew the necessary direction? Providing an explicit definition would be helpful. Also defining "completed" and how this was determined for each well. 13. Thank you for providing the leave-one-out analysis, it's helpful. 14. Figure 1: could the formations in the sub-analysis be added here in grey? You may want to consider using another color scheme throughout because many readers are red/green colorblind. 15. As a negative exposure control you could test the effect of wells located downwind of the monitors, these should contribute much less or no exposure.
Reviewer #2 (Remarks to the Author): Dear Authors, Thank you for the opportunity to review your manuscript. This is a study that works to answer the question of the scale of impact of particulate radiation exposure through the air pathway sources from oil and gas development. This manuscript is novel in that no other study has attempted to do this to date and opens up another potential pathway of exposure to chemical constituents from O&G development that has not previously been quantified and thoroughly explored.
In general the manuscript is is good condition and the approach and findings are clear and well argued. As you will see if my annotation of your manuscript, I have asked a number of questions to clarify language around framing, definitions and results.
One contextualizing piece of information that could be discussed in this manuscript is whether the atmospheric enhancements of PR observed exceed known human health thresholds and appear to pose health risks to populations within the buffer distances investigated.
I am also curious to know if you might be able to use the distance decay that you observed across geographic space to be able to make educated predictions as to what the concentrations of PR would be at shorter distances from O&G operations. Your statement that they are likely higher seems reasonable, but as a reader, I am wondering if you can take this in a more quantitative direction.
Reviewer #4 (Remarks to the Author): I have tried to read the article completely, but there is a great disorder in the presentation of the data and results. Its understanding is impossible, with many conceptual errors, being the information not well organized. Tha conclusions of the MS have not been demonstrated. Therefore, my decision has been to reject this MS in the current form.

Reviewer #5 (Remarks to the Author):
This is a well-written manuscript that attempts to address an interesting question, namely the extent to which unconventional oil and gas exploration contributes to the radioactivity of airborne particulate material. In fact, the manuscript goes a step further than that, linking findings of increased radioactivity to health impacts. Although the manuscript is interesting, there are some fundamental issues with the findings as presented and the extrapolations being made.
The following two statements from the abstract provide a useful basis for illustrating these issues. Statement 1. "By analyzing airborne particle radioactivity records from the U.S EPA Radiation monitoring Network, we find that upwind unconventional oil and gas wells could elevate downwind ambient particle radioactivity by up to 3.78×10-3 pCi/m3, or 44.8% over background level." This may be true, but is it of concern? The USEPA reports the average outdoor radon activity concentration in air to be 0.4 pCi/l (400 pCi/m3), with the average indoor radon activity concentration being 1.3pCi/l (1300 Bq/m3). The suggested potential maximum elevation in radioactivity of up to 0.00378 pCi/l (3.78 pCi/m3) due to unconventional oil and gas production is negligible when set in this context.
Whilst the Curie (Ci) is still used as a unit of measurement in North America, the Becquerel (Bq) is the SI unit most widely adopted for quantifying radioactivity and allows for direct comparison with other international datasets. Converting the maximum elevation in air activity concentration reported in the abstract to SI units gives 1.40x10-4 Bq/m3 (0.00014 Bq/m3). The World Health Organisation reports the global average outdoor radon to be in the range of 5 -15 Bq/m3. Even recognising that the manuscript under review only reports gross beta activity (i.e. alpha and gamma activity are not considered), whether comparing with USEPA data or global data, the maximum elevation in activity reported in the present paper is negligible. Statement 2. "These results confirm a link between unconventional oil and gas development and a new harmful environmental exposure." Here is perhaps the greatest area of concern within the paper. The authors are claiming that, what we now recognise to be a negligible potential elevation in the radioactivity of airborne particulates is leading to a new harmful exposure of the public. This is a bold claim and the data presented do not support this.
What matters when relating radioactivity to a biological endpoint is dose (radiation energy absorbed within biological tissue). The authors may argue that the difference is whether or not the activity is particle bound, but the reported negligible potential elevation in radioactivity will make little difference to the total dose to an individual. Even if we were to (i) approximate the slight elevations in alpha and beta emissions that may also have been identified if the authors had considered these, and (ii) apply radiation weighting factors to account for the relative biological effectiveness of these different emitters to enable the potential increase in equivalent dose to be calculated, the change in dose would still be negligible.
Why then are the authors proposing significant harm from a negligible elevation in dose? Looking at the references used in the paper to support the statements around the health impacts of particulate bound radioactivity, the three studies cited were all conducted by the authors of the paper under review here. It is good that the authors have opted to publish all three studies as Open Access, facilitating the evaluation of the scientific basis on which the claimed health impacts are based. However, when looking at these papers, it appears that two out of the three are focussing on two different potential effects within the same cohort of individuals and the papers discuss 'associations' rather than robust causal links. To lend weight to any claims that the authors may wish to make about the potential health impacts of elevated radioactivity of airborne particulate material, it would be helpful if the authors cited studies from other researchers that demonstrate causal links (if such studies exist).
Overall, as presented, I do not think that this paper is suitable for publication.

Response to Reviewers
Dear Reviewers, Thank you for your insights and comments regarding our manuscript entitled "Unconventional Oil and Gas Development and Ambient Particle Radioactivity." These comments, both major or minor, have helped us improve the manuscript significantly. We appreciated the opportunity to revise our manuscript according to your considered and careful reviews and are glad to present our revision.
We structure the response letter in two parts. We first reply to the comments offered by more than one reviewer. We then reply to the remaining comments of each reviewer individually.

Common comments
1. More than one reviewer suggested us to convert the unit of study from pCi/m 3 to mBq/m 3 for better interpretability.
Reviewer #1 wrote: For interpretability, I recommend reporting results in piC/L (multiple results by 1/1000) rather than piC/m3 because EPA recommendations are made in piC/L. Alternatively, 37 Bq/m3. Reviewer #5 wrote: While the Curie (Ci) is still used as a unit of measurement in North America, the Becquerel (Bq) is the SI unit most widely adopted for quantifying radioactivity and allows for direct comparison with other international datasets.
Reply: We thought this comment very helpful and converted the unit of all of our results.
After conversion, the national average level of PR during our study period is 0.35 mBq/m 3 . Meanwhile, our primary result is that an additional 100 upwind UOGD wells within 20 km associates with an increment of 0.024 mBq/m 3 in the gross-beta radiation downwind from the wells. For areas with 580 upwind UOGD wells within 20 km (the 95% percentile), UOGD could elevate the radiation level by 0.14 mBq/m 3 .
2. More than one reviewer pointed out that the UOGD-related enhancements in PR, even though they are measurable, may be too trivial to cause any health or environmental effects.
Reviewer #2 wrote: One contextualizing piece of information that could be discussed in this manuscript is whether the atmospheric enhancements of PR observed exceed known human health thresholds and appear to pose health risks to populations within the buffer distances investigated.
Reviewer #5 wrote: This may be true, but is it of concern? The USEPA reports the average outdoor radon activity concentration in air to be 0.4 pCi/l (400 pCi/m3), with the average indoor radon activity concentration being 1.3pCi/l (1300 Bq/m3). The suggested potential maximum elevation in radioactivity of up to 0.00378 pCi/l (3.78 pCi/m3) due to unconventional oil and gas production is negligible when set in this context.
The World Health Organisation reports the global average outdoor radon to be in the range of 5 -15 Bq/m3. Even recognising that the manuscript under review only reports gross beta activity (i.e. alpha and gamma activity are not considered), whether comparing with USEPA data or global data, the maximum elevation in activity reported in the present paper is negligible.

Reply:
We thought these comments very helpful and revised our manuscript accordingly.
First, we added a paragraph in the discussion section (Lines 288-300) associating our results with the previously published health effect analysis of particle radioactivity. The two publications found that an increase of 0.07 mBq/m 3 increase in 28-day average associated with adverse health outcomes. Our study finds that UOGD could enhance the gross-beta level by up to 0.14 mBq/m 3 . The magnitude of the increase related to UOGD is above the threshold identified in these papers. As a result, we do not think this influence is negligible.
Second, in response to this comment, we re-wrote the first half of the paragraph (Lines 46-52) to clarify the relationship and difference between the activity concentration of radon and gross-beta radiation. The activity concentration of Pb-210 (t 1/2 =22.3 years) is much lower than the concentration of Rn-222 ((t 1/2 =3.8 days) due to its much longer half-life than Rn-222. As a result, the gross-beta radiation is also much lower than the activity concentration of radon. In the atmospheric environment, the activity concentration of radon (15 Bq/m 3 ) is over forty thousand times higher than the gross-beta level (3.5 ×10 -4 Bq/m 3 ). A measurable increase in gross-beta suggests a measurable increase in radon level. Actually, enhanced levels of radon have been reported in Reference 24 and Reference 25.
Finally, we included a paper entitled "The Role of Ambient Particle Radioactivity in Inflammation and Endothelial Function in an Elderly Cohort" (Reference 37) which was written by a co-author of this paper and is recently accepted for publication by Epidemiology, a well-known journal in environmental epidemiology. This paper reports an elevated level of an inflammatory biomarker associated with a 0.14 mBq/m 3 increase in the 7-day average gross-beta radiation. The results of this paper supports observable health effects of UOGD at environmentally relevant levels consistent with the present study.

For Reviewer #1
Dear Reviewer #1, We are grateful for your considered and constructive comments about our manuscript. They have been incredibly helpful.
We are more confident about our results after conducting the extra sensitivity analysis you suggested. Our visualizations are more informative and reader-friendly after being revised according to your comments. We have extended our discussion to address the issues you indicated in our discussion sections.
We hope our revisions address your concerns.

Regards,
Longxiang Li Comment 1: The introduction regarding radioactivity could be streamlined to more accurately reflect the pathway between UOG suggested by the authors. Uranium decays to radium, lead, and polonium, but that's not entirely clear as written.

Reply:
We appreciate the reviewer's suggestion to clarify the potential mechanism by which UOGD could elevate PR. We re-wrote the paragraph accordingly (Lines 46-53).

Comment 2:
For interpretability, I recommend reporting results in piC/L (multiple results by 1/1000) rather than piC/m3 because EPA recommendations are made in piC/L. Alternatively, 37 bq/m3.

Reply:
We addressed this comment in our reply to the shared comments, above. Comment 3: It seems possible that PM2.5 is on the causal pathway between wells and particle gross b activity; wells can produce PM2.5. What happens if this variable is removed from the model?
Reply: In response to this comment, we repeated the analysis with slightly modification of the regression formula, removing the PM 2.5 term in the equation. According to the simplified model, an additional 100 upwind UOGD wells are associated with a 0.025 mBq/m3 increase in PR (95% CI: 0.021, 0.03 mBq/m3). This is slightly higher than the estimated effect in the original analysis (0.024 mBq/m3, 95% CI: 0.020, 0.028 mBq/m3). We agree with you that PM 2.5 is on the causal pathway. However, we determined not to add it to the main text because it does not influence our results remarkably.

Comment 4:
Timing. It seems that timing of well exposures might matter quite a bit. For example, well pad construction and drilling would like result in more upturn of soil and possibility of dispersion of radioactive materials than other phases, although the authors do provide hypotheses on other ways these materials could disperse (e.g., improperly stored drill cuttings or wastewater). The authors currently include any "completed" wells as the exposure of interest. Would it be possible to look at timing of completed spudding? In the intro, the authors mention having access to construction records.
Reply: Unfortunately, we cannot investigate the construction-dependent association because of the lack of detailed construction records. In response to your comment, we added this as a limitation in the discussion section (Lines 281-285).

Comment 5:
Related to sub-regional analyses: Line 349: how do levels of uranium differ by the three regions? Any explanation for the larger coefficients at the 30 and 35 km radii in the Bakken?
While the authors conclude that there's not much going on in the Marcellus, the point estimate is large (though not statistically significant) for the 20km radius. A bit more discussion would be helpful as this paper will certainly garner media attention. If distance to RadNet sites explains the  Table S1). It is reasonable to have a large estimated coefficient when the range of a predictor is small while the range of the dependent variable is kept unchanged. This heterogeneity highlights the importance of conducting the leave-one-out sensitivity analysis.

Comment 6:
The models contain a large number of covariates, is it possible to include a conceptual framework showing how the authors believe all these variables fit together? This could be added to the appendix.

Reply:
We have revised the related paragraphs in response to this suggestion. But we also added this into the main text because it significantly improves the clarity of our manuscript. We grouped all predictors in the model into three groups: O&G well count, radiation emissionrelated environmental factors (Lines 105-116), radiation transport-related factors (Lines 117-125), and spatial-temporal trends.
In addition, we added some comments on the formula of the regression model (Lines 132-141) and summarized these variables in Table 1 (Line 461). We presented the estimated associations between PR and these environmental factors in Table 2 (Line 468) and compared these coefficients with previous studies (Lines 278-280).

Comment 7:
The paper requires editing for clarity, there are several typographical and sentence structure errors.

Reply:
We have double-checked the revised manuscript and sent it to native English speaking colleagues for additional proof-reading.

Comment 8:
The last sentence of the abstract is strongly worded and implies causality. I am wavering over whether it's too strong.
Reply: We agree. It is too strong. We re-wrote the abstract paragraph complying with the word count limit of Nature Communications (Lines 13-24). Most importantly, we replace that strongword sentence with the conclusion sentence based on our findings and previously published results (Lines 22-24).
Comments 9: Lines 29-32: this statement is misleading and only refers to oil and gas produced from tight formations, not all oil and gas as stated. Please update.

Reply:
We agree that this sentence overestimated the contribution of UOGD. We deleted this sentence in the revised version because our study focuses on the existence-dependent influence, instead of the production-dependent influence. (Lines 26-32). Comment 14: I see that relative humidity is included but wonder about rainfall. While perhaps highly correlated rainfall should modulate.

Reply:
We revisited the analysis in response to this comment. We understand that precipitation governs the wet deposition process and determines the residence time of aerosol. These effects may influence the particle-bound radioactivity level. However, the estimated coefficient for precipitation is not statistically significant, with adjustment for relative humidity, soil moisture, planetary boundary layer height, temperature, PM 2.5 concentration, and the origin of air mass.
Likely, these environmental factors jointly explain the variation of precipitation.   Figure S5). Reply: In response to this comment, we added a sensitivity analysis to the statistical analysis section (Lines 153-155). Then the results are now mentioned in the discussion section (Lines 202-203) and detailed in the supplementary information Section 2.2, Figure S4. Our results are not sensitive to a change in the angle of the circular sector. In addition, the influence of UOGD is stronger when we shrink the angle from 90° to 60°, as expected. Figure S1. The increment in PR associated with an increase of 100 upwind UOGD wells in circular sectional buffers with different central angles.
Comment 18: To clarify, a well would be included as upwind if, on day t, it was completed (this date is indicated in drilling info? Does complete mean it has been fracked?) and the wind blew the necessary direction? Providing an explicit definition would be helpful. Also defining "completed" and how this was determined for each well.
Reply: To clarify the definition of "completed" well, we add Lines 93-95. A well is completed when all construction jobs are finished (including fracking) and is ready to be transferred to the operator for production. We also included Figure 2 to demonstrate how we calculate the upwind completed UOGD well count.
Comment 19: Figure 1: could the formations in the sub-analysis be added here in grey? You may want to consider using another color scheme throughout because many readers are red/green colorblind.

Reply:
We have revised the figure according to your comment. The revised Figure 1 is attached here for a quick review.
We also adjust all the figures to make them more color-blind friendly. Thank you for pointing this out.

For Reviewer #2
Dear Reviewer #2, Thank you so much for your constructive comments.
We feel that the manuscript is improved significantly by addressing your comments. We tentatively extrapolate our results to a smaller spatial scale. We also clarify the health implications section by comparing our results with the previously published health effect analysis.
We sincerely hope our revisions address your concerns regarding our previous manuscript.

Longxiang Li
Comment 1: One contextualizing piece of information that could be discussed in this manuscript is whether the atmospheric enhancements of PR observed exceed known human health thresholds and appear to pose health risks to populations within the buffer distances investigated.

Reply:
We have substantially revised our manuscript to address these concerns. After converting the unit of our results, an additional 100 upwind UOGD wells within 20 km associates with a 0.024 mBq/m 3 increase in the level of PR (95% CI: 0.020, 0.028 mBq/m 3 ). In areas with 580 upwind UOGD wells within 20km, the increase is approximately 0.14 mBq/m 3 . This enhancement is above the levels that are found associated with significant adverse health effects in previous studies (0.07 mBq/m3 in 28-day average and 0.14 mBq/m 3 in 7-day average). We detailed these comparisons in Lines 290-302.  Figure S6. We attach the figure here for a quick review. We used the power function of downwind distance with a negative exponent to model the distance-decay. The exponent with the best fit is -2.5. Based on this decay gradient, an additional 100 UOGD wells within 10 km is associated with an increase of 0.14 mBq/m 3 . Reply: We agree. It is too strong. We re-wrote the abstract paragraph complying with the word count limit of Nature Communications (Lines 13-24). Most importantly, we replace that strongword sentence with the conclusion sentence based on our findings and previously published results (Lines 22-24).

Comment 4:
Lines 28 needs to be more specific.

Reply:
We agree it needs clarification. We have revised it accordingly in Lines 13-14. accounted for 96% and 97% of the domestic crude oil and natural gas production, respectively." Your comments: The term unconventional oil and gas development has two main definitions: The Engineering Definition: The application of directional drilling and hydraulic fracturing The Geological Definition: The development of hydrocarbons from low permeability reservoirs and primarily from source rock. What you are referring to is the Geological definition and EIA states that ~96% of O&G from TIGHT OIL AND SHALE GAS PLAYS is attributable to the application of horizontally drilled wells. This is different from the calculation of the proportion of hydrocarbon production attributable to horizontal drilling and hydraulic fracturiong which is much lower (i.e., there is a lot of oil and gas that is still produced with the application of other techniques such as enhanced oil recovery, etc.

Reply:
We agree that this sentence overestimated the contribution of UOGD. We deleted this sentence in the revised version because our study focuses on existence-dependent influence, instead of the production-dependent influence. (Lines 26-32).

Reply:
We agree and additional references are included in Lines 30-32 of the revised manuscript.
Comment 7:Lines 39-41. We wrote, "If inhaled, these radioisotopes, such as Lead-210 (210Pb) and Polonium-210 Po, a known strong carcinogen), on the particulate contribute to internal radiation when they decay and emit α-, β-, and γradiations." Your comment: This sentence does not make sense.

Reply:
We have revised this for clarity in Lines 50-56.
Comment 8: Lines 42-43. W wrote, "Several recent cohort studies have found associations between PR and short-term health outcomes, including a decrease in forced vital capacity8, an increase in systolic and diastolic blood pressure …" Your comment: Perhaps put this into more plain language such as "lung function".

Reply:
We have revised accordingly in Lines 56-58.
Comment 9: Line 52. Additional reference is recommended.

Reply:
We agree; additional references are now included in Lines 229.

Reply:
We have revised to clarify the difference between UOGD and COGD in Lines 234-245.
We cited reference regarding the higher-than-background level of NORM in sedimentary rock rich in organic matter, such as black shale. We did not find a publication supporting the depthdependent distribution of NORMs. However, it is probably true because deeper formations are older than the shallower formations. Please also refer to our reply to Comment 11, because these two comments are closely related.
Comment 11: Line 59. We wrote "UOG wells produces larger volumes of drill cuttings and flowback water compared to COG wells, because of the combination of lateral drilling and hydraulic fracturing." Line 63. We wrote "Increasing lateral drilling length and " Your comment: These volumetric differences are much more tied to the length of the wells than to the direction of drilling a hydraulic fracturing

Reply:
We agree that the volume of drill cutting is governed by the drilling length. In response to the comment, in the revised manuscript we write that lateral drilling creates a large volume of cuttings sourced from the highly radioactive formation, while the vertical drilling mostly goes through formations of lower radioactivity (Lines 241-243).
Comment 12: Line 63. We wrote "Increasing lateral drilling length and water usage could exacerbate the condition in the future" Your comment: Why just LATERAL drilling length? Why not just say increased length of wellbores? Why would increased water usage alone increase risk of PR production?

Reply:
We have revised the statement to clarify the mechanism in Lines 234-245. Briefly, water usage alone does not matter a lot. The large volume of produced water may be problematic.
Comment 13: Line 108. We wrote "However, these results are not directly comparable, as remarkably changing number of wells at different buffers, or differences in the number of COG and UOG wells, may influence the magnitude of the results" Line 115. We wrote "This value is selected to represent the number of upwind O&G wells in regions with intensive oil and gas development and is also closely associated with the area of buffer," Your comment: This discussion is confusing

Reply:
We have revised to clarify the discussion regarding the distance-decay influence (Lines 259-262).

Comment 14:
Lines 132-138. We discussed the potential leakage mechanism of PR from UOGD.
Your comment: I would not consider all of these "off-site" as many of these operations occur on the well pad. Perhaps refer to this class of activities as "surface activities, transmission and waste handling"?

Reply:
We sincerely appreciate your suggestions to improve this discussion. We revised this paragraph accordingly (Lines 225-233).
Comment 15: Lines 201. We cited a report published on EIA's website with a wrong access date as "Accessed: 10th December 2019" Your comment: This does not seem possible. Please note the actual date accessed.

Reply:
We accessed the report on 12 th Oct 2019. However, this reference was removed in our revised manuscript in response to Comment 5, above.

For Reviewer #5
Dear Reviewer #5, Thank you for sharing with us your helpful and constructive critical comments. We agree that some wording in the previous manuscript may have been overly strong and have revised accordingly.
If we understand correctly, your primary concern is whether UOGD could elevate PR to a "harmful" level. The short answer should be yes, considering the results of three peer-reviewed and published studies. We agree that studies conducted by outside researchers may provide additional insight. However, this is a relatively new field with limited existing knowledge. The first paper was published less than three years ago. A lack of previous studies may reflect more on the novelty of this area than on its value or importance.
I hope our detailed reply to your comments could at least in part address your concerns. We look forward to your feedback.
Best regards,

Longxiang Li
Comment 1: While the Curie (Ci) is still used as a unit of measurement in North America, the Becquerel (Bq) is the SI unit most widely adopted for quantifying radioactivity and allows for direct comparison with other international datasets.
Reply: In response to this comment, we have converted all the results into the SI units. After conversion, the national average level of PR during our study period is 0.35 mBq/m 3 . Meanwhile, our primary result is that an additional 100 upwind UOGD wells within 20 km is associated with an increase of 0.024 mBq/m 3 in the gross-beta radiation downwind from the wells. For areas with 580 upwind UOGD wells within 20 km (the 95% percentile), UOGD could elevate the radiation level by 0.14 mBq/m 3 , in agreement with the calculation by the reviewer.

Comment 2:
This may be true, but is it of concern? The USEPA reports the average outdoor radon activity concentration in air to be 0.4 pCi/l (400 pCi/m3), with the average indoor radon activity concentration being 1.3pCi/l (1300 Bq/m3). The suggested potential maximum elevation in radioactivity of up to 0.00378 pCi/l (3.78 pCi/m3) due to unconventional oil and gas production is negligible when set in this context.
The World Health Organisation reports the global average outdoor radon to be in the range of 5 -15 Bq/m3. Even recognising that the manuscript under review only reports gross beta activity (i.e. alpha and gamma activity are not considered), whether comparing with USEPA data or global data, the maximum elevation in activity reported in the present paper is negligible.
What matters when relating radioactivity to a biological endpoint is dose (radiation energy absorbed within biological tissue). The authors may argue that the difference is whether or not the activity is particle bound, but the reported negligible potential elevation in radioactivity will make little difference to the total dose to an individual. Even if we were to (i) approximate the slight elevations in alpha and beta emissions that may also have been identified if the authors had considered these, and (ii) apply radiation weighting factors to account for the relative biological effectiveness of these different emitters to enable the potential increase in equivalent dose to be calculated, the change in dose would still be negligible.

Reply:
We appreciate this critical argument. .
We have revised the first half of the paragraph (Lines 46-52) to clarify the relationship and difference between the activity concentration of radon and the particle-bound gross-beta radioactivity. The activity concentration of Pb-210 (t 1/2 =22.3 years) is much lower than the concentration of Rn-222 (t 1/2 =3.8 days) due to its much longer half-life than Rn-222. As a result, the gross-beta radiation is also much lower than the activity concentration of radon. In the atmospheric environment, the activity concentration of radon (3.7 Bq/m 3 ) is over ten thousand times higher than the gross-beta level (3.5 e -4 Bq/m 3 ). Due to the noble nature of radon, its carcinogenic health effects are primarily caused by its radioactive progeny instead of radon itself.
Gross-beta radiation, mostly contributed by Pb-210, the first long-lived progeny of the decay products of radon and is theoretically closer to the health outcome than the activity concentration of radon. Even though the absolute increase in beta radiation associated with UOGD is small, it represents approximately a 45% increase over the background level, which may not be neglectable.
Particle-bound radioactivity is the radioactive character of the ambient.The behavior of PM 2.5 in our body plays an essential role in calculating the effective dose. However, our knowledge in this field is still limited. That is why we used population-based studies to evaluate the health effects of PR instead of a physical model. According to our previous published papers, a 0.14 mBq/m 3 increase in 7-day average PR and a 0.07 mBq/m 3 increase in 28-day average PR are both significantly associated with increased blood pressure, lower lung function and increase levels of inflammatory biomarker. The UOGD-related enhancement in PR is up to 0.14 mBq/m3, above these thresholds.

Comment 3:
Why then are the authors proposing significant harm from a negligible elevation in dose? Looking at the references used in the paper to support the statements around the health impacts of particulate bound radioactivity, the three studies cited were all conducted by the authors of the paper under review here. It is good that the authors have opted to publish all three studies as Open Access, facilitating the evaluation of the scientific basis on which the claimed health impacts are based. However, when looking at these papers, it appears that two out of the three are focussing on two different potential effects within the same cohort of individuals and the papers discuss 'associations' rather than robust causal links. To lend weight to any claims that the authors may wish to make about the potential health impacts of elevated radioactivity of airborne particulate material, it would be helpful if the authors cited studies from other researchers that demonstrate causal links (if such studies exist).