ASSOCIATION OF 210Po CONTENT IN URINE WITH CONCENTRATION OF 222Rn IN DWELLINGS

ASSOCIATION OF 210Po CONTENT IN URINE WITH CONCENTRATION OF 222Rn IN DWELLINGS Abstract Effect of indoor radon concentrations higher than 500 Bq m−3 in dwellings on excretion of 210Po was studied in 28 volunteers. The study was further augmented by following eight individuals from the group for 5 months at 1-month intervals. Association between 222Rn concentration in homes and 210Po excretion was found when the dataset containing values of the studied 27 subjects was combined with data on 40 Prague inhabitants from the previous study. Sources of uncertainties involved in the analysis are discussed. INTRODUCTION The aim of the study is to find the effect of high 222Rn activity concentration in dwellings on 210Po excretion in urine. Based on the studies on uranium miners (1–3) and on glass surfaces in “radon rich” dwellings (4), it can be assumed that 210Pb and 210Po body burden is proportional to the indoor 222Rn concentration. This burden should result in a higher content of 210Po in urine. Since the 1950s, numerous attempts were made to associate the concentration of 210Po in urine of uranium miners with radon concentration (RC) in mines and consequently estimate the exposure of their lungs to radon and its daughters. Whereas Sultzer and Hursh (1) found this approach promising, Hölgye (2) proved that 44–58% of miners’ body burden of 210Po comes from inhalation of mine dust. Analyses of bones and tissues obtained from autopsy or surgery of former uranium miners were made by Blanchard and Moore (3). Seventy-eight per cent of 210Po and 79% of 210Pb body burden were found in skeleton, and these data correlated well with exposure to radon and its daughters. Correlation was found by Novak and Panov (5) between the concentration of 210Po in urine and blood and exposure duration in former miners from an abandoned mine. On the other hand, no correlation was proved between the actual concentration of 222Rn in air and 210Po concentration in urine of miners at a working mine. Most of the studies cited above take smoking into account. It is generally known that smokers have higher intakes of 210Po than non-smokers, which results in higher 210Po excretion than in non-smokers (3,6). Higher intake is also connected with consumption of food rich in 210Po, such as sea-food, fish, reindeer meat, liver or kidneys (7,8). This must be considered when studying the effect of radon in dwellings and workplaces on polonium excretion. Excretion of 210Po and 210Pb in urine was previously studied on 40 non-smokers living in Prague (9) and abstaining from eating fish, sea-food, liver and kidneys for at least 1 week prior to and during the collections of urine. The further study was concentrated on daily variations of this excretion (10). Day to day variations expressed as coefficients of variation (CV) of the arithmetic means are in the range 12–37%. In the present work, association between concentrations of 222Rn in homes of 27 (data for one volunteer were omitted) volunteers living in different parts of the Czech Republic and their urinary excretion rate of 210Po was investigated. The data were collected for persons where higher concentrations of indoor 222Rn were expected. The data from the study on 40 Prague inhabitants (9) were also used in this evaluation in order to cover broader interval of concentrations of 222Rn in homes (include low 222Rn indoor concentrations), increase sample size and thus increase statistical power. EXPERIMENTAL From 2015 till 2017, healthy adults living in dwellings with indoor activity concentration of 222Rn exceeding 500 Bq m−3 provided 24-h urine samples—one sample from each volunteer. These 28 volunteers were non-smokers and abstained from eating fish, sea-food, liver and kidneys for at least 1 week prior to and during the collections of urine. Some volunteers shared the same household; therefore, there are some data for more persons corresponding to the same radon activity concentrations. In 2018, eight volunteers from the above group participated in further research aimed at time stability of the excretion of 210Po. Urine samples were taken by these persons on a monthly basis from February till June 2018. In the urine samples, activities of 210Po were determined. The method used in the previous study (10) was used in 210Po determination. An acquired sample of urine was acidified first with concentrated nitric acid, then the exact activity of 209Po tracer was added and the urine was gently heated. After the solution was evaporated to 250 ml, it was covered by a watch glass and carefully wet-ashed. The sample was taken to near dryness, cooled and 5 mol hydrochloric acid was added to the residue and evaporated to remove nitrates. This step was repeated one more time. The residue was dissolved in 75 ml of 0.8 mol HCl and filtered to a Teflon beaker. The volume was adjusted to 150 ml by adding distilled water and 0.2 g of ascorbic acid was dissolved in the solution to reduce iron present in urine from Fe (III) to Fe (II). Polonium was auto-deposited on a silver disc (one side was painted to avoid deposition of polonium) for 4 h at 80–90°C under constant stirring. Afterwards, discs were rinsed with demineralized water and ethanol and dried out. The alpha activities of 210Po and 209Po were counted using alpha spectrometer Alpha Analyst with 1200 mm2 Passivated implanted planar silicon (PIPS) detectors for 588 000 s. Radon activity concentrations in homes were measured by integral radon dosemeters localized in rooms of a dwelling for at least 2 months during heating season. The average concentrations used in the evaluation were calculated averaging all values obtained for inhabitable rooms measured in each flat. For two persons working in a building with indoor RC higher than 1000 Bq m−3, RCs used in the evaluation were adjusted according to the time spent at their respective workplaces and RCs there. Data for 40 Prague inhabitants taken from the previous study (9) were included into the evaluation in order to achieve adequate statistical power and broaden the scale of RCs toward lower values. Statistical analyses were done in R software (11) with additional packages (12,13). RESULTS Data on 210Po excretion in 27 volunteers and 40 Prague inhabitants from the previous study(9) vs. average radon activity concentrations in homes are presented in Figure 1. Simultaneous confidence band for regression line on level 0.95 is also plotted. This band covers the regression line simultaneously for all RC values with confidence level 95% (do not confuse it with point-wise confidence interval for mean value constructed for each RC value separately). Figure 1 Open in new tabDownload slide Twenty-four-hour urinary excretion of 210Po vs. average RC in homes. Estimated linear relationship is plotted with dashed line. Gray region denotes the simultaneous confidence band for regression line on level 0.95 Figure 1 Open in new tabDownload slide Twenty-four-hour urinary excretion of 210Po vs. average RC in homes. Estimated linear relationship is plotted with dashed line. Gray region denotes the simultaneous confidence band for regression line on level 0.95 For indoor RCs in the range of 505–3440 Bq m−3, the values of urinary excretion of 210Po were found to be in the range of 1.2–21.7 mBq d−1. Data for one subject with radon activity about 5000 Bq m−3 were removed from the plot as explained in the discussion. In Figure 1, data on 40 Prague inhabitants are plotted vs. indoor RC of 98.9 Bq m−3 as the average for Prague. Based on the above data combined with the data on 40 Prague inhabitants (9) the dependence of daily excretion of 210Po (EPo) in mBq d−1 on RC in dwelling in Bq m−3 was estimated (weighted linear regression model) as $$$${E}_{\mathrm{Po}}=4.1+2.57\bullet{10}^{-3}\mathrm{RC}$$$$ (1) This fit is also shown in Figure 1 as a dashed line. Both the intercept (p < 10−5) and the slope (p < 10−5) differ statistically significantly from zero (F-test of submodel). Ninety-five per cent confidence interval for the intercept is (2.4, 5.0) and ninety-five per cent confidence interval for the slope is (1.8 × 10−3, 3.5 × 10−3). See also the gray region in Figure 1 that reflects the uncertainty of the estimates. Daily excretion of 210Po in urine of eight occupants of homes with RC higher than 500 Bq m−3 providing samples at monthly intervals is presented in Table 1 together with arithmetic means and their standard deviations. Table 1 Daily excretion of 210Po in urine sampled at monthly intervals (mBq d−1). No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 Excretion values are presented with standard uncertainties of determination and arithmetic means with their CV. Open in new tab Table 1 Daily excretion of 210Po in urine sampled at monthly intervals (mBq d−1). No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 Excretion values are presented with standard uncertainties of determination and arithmetic means with their CV. Open in new tab No seasonal trend in the excretion common to all participants was observed (p = 0.42, Analysis of Variance Table with Satterthwaite’s method for mixed effects models (14)). The heterogeneity of values for monthly intervals is higher than determination uncertainty (p < 10−4, Cochran’s test of heterogeneity (12,14)). It was estimated (12,14) that only about 10% of total month-to-month variations are caused by determination error. Month-to-month variations expressed as CV are in the range 12.5–47%, the largest coefficient of variation corresponding to the lowest excretion. DISCUSSION The dataset used in estimating the association between daily excretions of 210Po and concentrations of 222Rn and its daughters in homes contains spot values of the daily excretion from the years 2015–2017 for all persons, repeated measurements from 2018 for seven persons and data for 40 Prague inhabitants taken from the previous study(9). One person (No. 5) with repeated measurements living in high RC (5000 Bq m−3) compared to the rest was omitted before the statistical analyses. Removing this observation is rather conservative, but its x value is not fully compatible with the rest of the data. It is a so-called leverage point (do not confuse it with outlying point). This leverage point has a high impact on the results, and hence this observation was removed to assure that our results are not mainly determined by the data for Prague inhabitants and this one leverage point and that remaining (and valuable) data have low impact on the results. However, note that the data point for this person was not in significant disagreement with the results of the statistical analysis (while considering the statistical uncertainty of the results). Without omitting this data, the slope is slightly higher (2.8 × 10−3) and the intercept is lower (3.6). The dataset also contains repeated measurements for a subset of individuals. Because of repeated measurements for some subjects, the statistical mixed effects model (13) was used for obtaining some results. For the purpose of analyses for model (1) 210Po excretions for each individual with repeated measurements were averaged and the weights in the regression model were properly adjusted according to this fact. The dataset is subject to both, uncertainties of indoor RC and uncertainties of measured daily excretion further influenced by day-to-day (month-to-month) variations. Uncertainty in x-variable usually leads to underestimating the slope and overestimating the intercept. Therefore, the true slope in model (1) can be actually higher and the true intercept can be actually lower than the estimated values. Besides the measurement errors, there are other factors contributing to the uncertainty of RC. The first factor is taking the mean of RC from different rooms as representative for a dwelling and the second is using the mean value of indoor RC in Prague as representative for 40 volunteers from Prague(9). Without additional data from 40 Prague inhabitants, the dataset would not have statistical power to obtain significant results for model (1). These data have relatively high influence on the result. Data on repeated sampling by eight persons are characterized by month-to-month variations. When expressed as CV they are in the range 12.5–47%, which corresponds to day-to-day variations in the range 12–37% observed in seven Prague inhabitants(10). Even when taking into account uncertainties in RCs, differences in daily excretion of 210Po can hardly be connected solely with current indoor RC (p < 10−10, ANOVA-like table for random-effects (13)). The polonium in urine is generally believed to come from food and primarily from 210Pb stored in the bones (1), what corresponds also to the findings of Novak and Panov (5) studying former uranium miners. The duration of living and/or working in an environment with high indoor concentration of radon may be decisive for 210Pb content in bones. Individual physiology, dietary habits and other factors also affect body burden with 210Pb and 210Po and concentration of 210Po in urine. Among the people providing urine samples at monthly intervals there were two couples, namely subject 1 living with subject 2 and subject 3 living with subject 4. As can be seen in Table 1, there are big discrepancies in the polonium excretion rate between the spouses. Besides individual physiology and dietary habits, this phenomenon can be explained by further occupational burden of one of the couple e.g. from dust at the workplace. Occupational burden is also apparent for person No. 6 and person No. 7 who work in a building with RC higher than 1000 Bq m−3. Therefore, RCs used in the evaluation were adjusted according to the time spent at home and at their respective workplaces as stated in the experimental section. CONCLUSION Association between 222Rn concentration in homes and 210Po excretion was found in the dataset containing values for the studied 28 subjects and 40 Prague inhabitants from the previous study(9). Month-to-month variations in eight more thoroughly studied subjects agree with earlier found day-to-day variations(10). FUNDING This work was supported by the Ministry of the Interior of the Czech Republic, identification code MV-12331-5/OBVV-2018. ACKNOWLEDGMENTS Special thanks are due to Jana Petrášková for invaluable technical assistance in alpha spectrometry. The authors are also grateful to Dr. Petr Rulík for his useful suggestions and continuous interest in this study. Appreciation is extended to Věra Bečková for her assistance and constructive criticism during the preparation of the manuscript. REFERENCES 1. Sultzer , M. and Hursch , J. B. Polonium in urine of miners exposed to radon . Arch. Ind. Hyg. Occup. Med. 9 ( 2 ), 89 – 100 ( 1954 ). WorldCat 2. Hölgye , Z. Urinary excretion of polonium-210 by miners of Czechoslovak uranium mines . Čas. Lék. Čes. 108 ( 48 ), 1450 – 1455 ( 1969 ) (article in Czech) . WorldCat 3. Blanchard , R. L. and Moore , J. B. Body burden, distribution and internal dose of 210Pb and 210Po in a uranium miner population . Health Phys. 21 ( 4 ), 499 – 518 ( 1971 ). Google Scholar Crossref Search ADS PubMed WorldCat 4. Mártin Sánchez , A. , Pérez de la Torre , J. and Ruano Sánchez , A. B. Experimental studies about the ratio between 210Po deposited on surfaces and retrospective indoor 222Rn concentrations . Radiat. Prot. Dosim. 160 ( 1–3 ), 206 – 209 ( 2014 ). Google Scholar Crossref Search ADS WorldCat 5. Novak , L. J. and Panov , D. Polonium-210 in blood and urine of uranium mine workers in Yugoslavia . Am. Ind. Hyg. Assoc. J. 33 ( 3 ), 192 – 196 ( 1972 ). Google Scholar Crossref Search ADS PubMed WorldCat 6. Okabayashi , H. , Suzuki-Yasumoto , M. , Hongo , S. and Watanabe , S. On the evaluation of Po-210 bioassay for uranium mine workers in Japan for the personal exposure index to radon daughters . J. Radiat. Res. 16 ( 2 ), 142 – 151 ( 1975 ). Google Scholar Crossref Search ADS PubMed WorldCat 7. Okabayashi , H. A study on the excretion of Pb-210 and Po-210 . J. Radiat. Res. 23 , 242 – 252 ( 1982 ). Google Scholar Crossref Search ADS PubMed WorldCat 8. Pietrzak-Flis , Z. , Chrzanowski , E. and Dembinska , S. Intake of 226Ra, 210Pb and 210Po with food in Poland . Sci. Total Environ. 203 , 157 – 165 ( 1997 ). Google Scholar Crossref Search ADS PubMed WorldCat 9. Hölgye , Z. Excretion rates of 210Po and 210Pb in Prague inhabitants . Radiat. Prot. Dosim. 156 ( 1 ), 1 – 6 ( 2013 ). Google Scholar Crossref Search ADS WorldCat 10. Hölgye , Z. , Hýža , M. , Mihalík , J. , Rulík , P. and Škrkal , J. Variation of 210Po daily urinary excretion for male subjects at environmental level . Radiat. Environ. Biophys. 54 ( 2 ), 251 – 255 ( 2015 ). Google Scholar Crossref Search ADS PubMed WorldCat 11. R Core Team . R: A Language and Environment for Statistical Computing . ( Vienna, Austria : R Foundation for Statistical Computing ) ( 2018 ) . https://www.R-project.org/ . Google Preview WorldCat COPAC 12. Viechtbauer , W. Conducting meta-analyses in R with the metafor package . J. Stat. Softw. 36 ( 3 ), 1 – 48 ( 2010 ). Google Scholar Crossref Search ADS WorldCat 13. Kuznetsova , A. , Brockhoff , P. B. , Rune , H. B. and Christensen , R. H. B. lmer test package: tests in linear mixed effects models . J. Stat. Softw. 82 ( 13 ), 1 – 26 ( 2017 ). Google Scholar Crossref Search ADS WorldCat 14. Higgins , J. P. T. and Thompson , S. G. Quantifying heterogeneity in a meta-analysis . Stat. Med. 21 ( 11 ), 1539 – 1558 ( 2002 ). Google Scholar Crossref Search ADS PubMed WorldCat © The Author(s) 2019. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

ASSOCIATION OF 210Po CONTENT IN URINE WITH CONCENTRATION OF 222Rn IN DWELLINGS

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Oxford University Press
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0144-8420
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1742-3406
DOI
10.1093/rpd/ncz244
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Abstract

Abstract Effect of indoor radon concentrations higher than 500 Bq m−3 in dwellings on excretion of 210Po was studied in 28 volunteers. The study was further augmented by following eight individuals from the group for 5 months at 1-month intervals. Association between 222Rn concentration in homes and 210Po excretion was found when the dataset containing values of the studied 27 subjects was combined with data on 40 Prague inhabitants from the previous study. Sources of uncertainties involved in the analysis are discussed. INTRODUCTION The aim of the study is to find the effect of high 222Rn activity concentration in dwellings on 210Po excretion in urine. Based on the studies on uranium miners (1–3) and on glass surfaces in “radon rich” dwellings (4), it can be assumed that 210Pb and 210Po body burden is proportional to the indoor 222Rn concentration. This burden should result in a higher content of 210Po in urine. Since the 1950s, numerous attempts were made to associate the concentration of 210Po in urine of uranium miners with radon concentration (RC) in mines and consequently estimate the exposure of their lungs to radon and its daughters. Whereas Sultzer and Hursh (1) found this approach promising, Hölgye (2) proved that 44–58% of miners’ body burden of 210Po comes from inhalation of mine dust. Analyses of bones and tissues obtained from autopsy or surgery of former uranium miners were made by Blanchard and Moore (3). Seventy-eight per cent of 210Po and 79% of 210Pb body burden were found in skeleton, and these data correlated well with exposure to radon and its daughters. Correlation was found by Novak and Panov (5) between the concentration of 210Po in urine and blood and exposure duration in former miners from an abandoned mine. On the other hand, no correlation was proved between the actual concentration of 222Rn in air and 210Po concentration in urine of miners at a working mine. Most of the studies cited above take smoking into account. It is generally known that smokers have higher intakes of 210Po than non-smokers, which results in higher 210Po excretion than in non-smokers (3,6). Higher intake is also connected with consumption of food rich in 210Po, such as sea-food, fish, reindeer meat, liver or kidneys (7,8). This must be considered when studying the effect of radon in dwellings and workplaces on polonium excretion. Excretion of 210Po and 210Pb in urine was previously studied on 40 non-smokers living in Prague (9) and abstaining from eating fish, sea-food, liver and kidneys for at least 1 week prior to and during the collections of urine. The further study was concentrated on daily variations of this excretion (10). Day to day variations expressed as coefficients of variation (CV) of the arithmetic means are in the range 12–37%. In the present work, association between concentrations of 222Rn in homes of 27 (data for one volunteer were omitted) volunteers living in different parts of the Czech Republic and their urinary excretion rate of 210Po was investigated. The data were collected for persons where higher concentrations of indoor 222Rn were expected. The data from the study on 40 Prague inhabitants (9) were also used in this evaluation in order to cover broader interval of concentrations of 222Rn in homes (include low 222Rn indoor concentrations), increase sample size and thus increase statistical power. EXPERIMENTAL From 2015 till 2017, healthy adults living in dwellings with indoor activity concentration of 222Rn exceeding 500 Bq m−3 provided 24-h urine samples—one sample from each volunteer. These 28 volunteers were non-smokers and abstained from eating fish, sea-food, liver and kidneys for at least 1 week prior to and during the collections of urine. Some volunteers shared the same household; therefore, there are some data for more persons corresponding to the same radon activity concentrations. In 2018, eight volunteers from the above group participated in further research aimed at time stability of the excretion of 210Po. Urine samples were taken by these persons on a monthly basis from February till June 2018. In the urine samples, activities of 210Po were determined. The method used in the previous study (10) was used in 210Po determination. An acquired sample of urine was acidified first with concentrated nitric acid, then the exact activity of 209Po tracer was added and the urine was gently heated. After the solution was evaporated to 250 ml, it was covered by a watch glass and carefully wet-ashed. The sample was taken to near dryness, cooled and 5 mol hydrochloric acid was added to the residue and evaporated to remove nitrates. This step was repeated one more time. The residue was dissolved in 75 ml of 0.8 mol HCl and filtered to a Teflon beaker. The volume was adjusted to 150 ml by adding distilled water and 0.2 g of ascorbic acid was dissolved in the solution to reduce iron present in urine from Fe (III) to Fe (II). Polonium was auto-deposited on a silver disc (one side was painted to avoid deposition of polonium) for 4 h at 80–90°C under constant stirring. Afterwards, discs were rinsed with demineralized water and ethanol and dried out. The alpha activities of 210Po and 209Po were counted using alpha spectrometer Alpha Analyst with 1200 mm2 Passivated implanted planar silicon (PIPS) detectors for 588 000 s. Radon activity concentrations in homes were measured by integral radon dosemeters localized in rooms of a dwelling for at least 2 months during heating season. The average concentrations used in the evaluation were calculated averaging all values obtained for inhabitable rooms measured in each flat. For two persons working in a building with indoor RC higher than 1000 Bq m−3, RCs used in the evaluation were adjusted according to the time spent at their respective workplaces and RCs there. Data for 40 Prague inhabitants taken from the previous study (9) were included into the evaluation in order to achieve adequate statistical power and broaden the scale of RCs toward lower values. Statistical analyses were done in R software (11) with additional packages (12,13). RESULTS Data on 210Po excretion in 27 volunteers and 40 Prague inhabitants from the previous study(9) vs. average radon activity concentrations in homes are presented in Figure 1. Simultaneous confidence band for regression line on level 0.95 is also plotted. This band covers the regression line simultaneously for all RC values with confidence level 95% (do not confuse it with point-wise confidence interval for mean value constructed for each RC value separately). Figure 1 Open in new tabDownload slide Twenty-four-hour urinary excretion of 210Po vs. average RC in homes. Estimated linear relationship is plotted with dashed line. Gray region denotes the simultaneous confidence band for regression line on level 0.95 Figure 1 Open in new tabDownload slide Twenty-four-hour urinary excretion of 210Po vs. average RC in homes. Estimated linear relationship is plotted with dashed line. Gray region denotes the simultaneous confidence band for regression line on level 0.95 For indoor RCs in the range of 505–3440 Bq m−3, the values of urinary excretion of 210Po were found to be in the range of 1.2–21.7 mBq d−1. Data for one subject with radon activity about 5000 Bq m−3 were removed from the plot as explained in the discussion. In Figure 1, data on 40 Prague inhabitants are plotted vs. indoor RC of 98.9 Bq m−3 as the average for Prague. Based on the above data combined with the data on 40 Prague inhabitants (9) the dependence of daily excretion of 210Po (EPo) in mBq d−1 on RC in dwelling in Bq m−3 was estimated (weighted linear regression model) as $$$${E}_{\mathrm{Po}}=4.1+2.57\bullet{10}^{-3}\mathrm{RC}$$$$ (1) This fit is also shown in Figure 1 as a dashed line. Both the intercept (p < 10−5) and the slope (p < 10−5) differ statistically significantly from zero (F-test of submodel). Ninety-five per cent confidence interval for the intercept is (2.4, 5.0) and ninety-five per cent confidence interval for the slope is (1.8 × 10−3, 3.5 × 10−3). See also the gray region in Figure 1 that reflects the uncertainty of the estimates. Daily excretion of 210Po in urine of eight occupants of homes with RC higher than 500 Bq m−3 providing samples at monthly intervals is presented in Table 1 together with arithmetic means and their standard deviations. Table 1 Daily excretion of 210Po in urine sampled at monthly intervals (mBq d−1). No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 Excretion values are presented with standard uncertainties of determination and arithmetic means with their CV. Open in new tab Table 1 Daily excretion of 210Po in urine sampled at monthly intervals (mBq d−1). No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 No Month II III IV V VI Mean CV (%) 1 15.9 ± 1.0 14.9 ± 1.1 17.9 ± 1.2 21.7 ± 1.4 13.8 ± 0.9 16.8 18.5 2 8.6 ± 0.7 6.8 ± 0.6 7.4 ± 0.6 6.5 ± 0.5 5.2 ± 0.5 6.9 18.0 3 9.9 ± 0.7 12.0 ± 1.0 11.7 ± 0.9 14.1 ± 1.0 11.8 ± 0.9 11.9 12.5 4 2.0 ± 0.2 2.7 ± 0.3 1.2 ± 0.2 3.1 ± 0.3 4.6 ± 0.4 2.7 46.9 5 12.8 ± 0.9 7.5 ± 0.6 21.3 ± 1.5 20.3 ± 1.6 15.5 42.2 6 9.6 ± 0.7 9.4 ± 0.7 6.7 ± 0.5 11.6 ± 0.8 9.3 21.5 7 20.4 ± 1.4 16.1 ± 1.1 11.4 ± 0.9 21.1 ± 1.4 17.3 25.9 8 6.5 ± 0.5 6.7 ± 0.5 8.2 ± 0.6 9.1 ± 0.7 7.6 16.3 Excretion values are presented with standard uncertainties of determination and arithmetic means with their CV. Open in new tab No seasonal trend in the excretion common to all participants was observed (p = 0.42, Analysis of Variance Table with Satterthwaite’s method for mixed effects models (14)). The heterogeneity of values for monthly intervals is higher than determination uncertainty (p < 10−4, Cochran’s test of heterogeneity (12,14)). It was estimated (12,14) that only about 10% of total month-to-month variations are caused by determination error. Month-to-month variations expressed as CV are in the range 12.5–47%, the largest coefficient of variation corresponding to the lowest excretion. DISCUSSION The dataset used in estimating the association between daily excretions of 210Po and concentrations of 222Rn and its daughters in homes contains spot values of the daily excretion from the years 2015–2017 for all persons, repeated measurements from 2018 for seven persons and data for 40 Prague inhabitants taken from the previous study(9). One person (No. 5) with repeated measurements living in high RC (5000 Bq m−3) compared to the rest was omitted before the statistical analyses. Removing this observation is rather conservative, but its x value is not fully compatible with the rest of the data. It is a so-called leverage point (do not confuse it with outlying point). This leverage point has a high impact on the results, and hence this observation was removed to assure that our results are not mainly determined by the data for Prague inhabitants and this one leverage point and that remaining (and valuable) data have low impact on the results. However, note that the data point for this person was not in significant disagreement with the results of the statistical analysis (while considering the statistical uncertainty of the results). Without omitting this data, the slope is slightly higher (2.8 × 10−3) and the intercept is lower (3.6). The dataset also contains repeated measurements for a subset of individuals. Because of repeated measurements for some subjects, the statistical mixed effects model (13) was used for obtaining some results. For the purpose of analyses for model (1) 210Po excretions for each individual with repeated measurements were averaged and the weights in the regression model were properly adjusted according to this fact. The dataset is subject to both, uncertainties of indoor RC and uncertainties of measured daily excretion further influenced by day-to-day (month-to-month) variations. Uncertainty in x-variable usually leads to underestimating the slope and overestimating the intercept. Therefore, the true slope in model (1) can be actually higher and the true intercept can be actually lower than the estimated values. Besides the measurement errors, there are other factors contributing to the uncertainty of RC. The first factor is taking the mean of RC from different rooms as representative for a dwelling and the second is using the mean value of indoor RC in Prague as representative for 40 volunteers from Prague(9). Without additional data from 40 Prague inhabitants, the dataset would not have statistical power to obtain significant results for model (1). These data have relatively high influence on the result. Data on repeated sampling by eight persons are characterized by month-to-month variations. When expressed as CV they are in the range 12.5–47%, which corresponds to day-to-day variations in the range 12–37% observed in seven Prague inhabitants(10). Even when taking into account uncertainties in RCs, differences in daily excretion of 210Po can hardly be connected solely with current indoor RC (p < 10−10, ANOVA-like table for random-effects (13)). The polonium in urine is generally believed to come from food and primarily from 210Pb stored in the bones (1), what corresponds also to the findings of Novak and Panov (5) studying former uranium miners. The duration of living and/or working in an environment with high indoor concentration of radon may be decisive for 210Pb content in bones. Individual physiology, dietary habits and other factors also affect body burden with 210Pb and 210Po and concentration of 210Po in urine. Among the people providing urine samples at monthly intervals there were two couples, namely subject 1 living with subject 2 and subject 3 living with subject 4. As can be seen in Table 1, there are big discrepancies in the polonium excretion rate between the spouses. Besides individual physiology and dietary habits, this phenomenon can be explained by further occupational burden of one of the couple e.g. from dust at the workplace. Occupational burden is also apparent for person No. 6 and person No. 7 who work in a building with RC higher than 1000 Bq m−3. Therefore, RCs used in the evaluation were adjusted according to the time spent at home and at their respective workplaces as stated in the experimental section. CONCLUSION Association between 222Rn concentration in homes and 210Po excretion was found in the dataset containing values for the studied 28 subjects and 40 Prague inhabitants from the previous study(9). Month-to-month variations in eight more thoroughly studied subjects agree with earlier found day-to-day variations(10). FUNDING This work was supported by the Ministry of the Interior of the Czech Republic, identification code MV-12331-5/OBVV-2018. ACKNOWLEDGMENTS Special thanks are due to Jana Petrášková for invaluable technical assistance in alpha spectrometry. The authors are also grateful to Dr. Petr Rulík for his useful suggestions and continuous interest in this study. Appreciation is extended to Věra Bečková for her assistance and constructive criticism during the preparation of the manuscript. REFERENCES 1. Sultzer , M. and Hursch , J. B. Polonium in urine of miners exposed to radon . Arch. Ind. Hyg. Occup. Med. 9 ( 2 ), 89 – 100 ( 1954 ). WorldCat 2. Hölgye , Z. Urinary excretion of polonium-210 by miners of Czechoslovak uranium mines . Čas. Lék. Čes. 108 ( 48 ), 1450 – 1455 ( 1969 ) (article in Czech) . WorldCat 3. Blanchard , R. L. and Moore , J. B. Body burden, distribution and internal dose of 210Pb and 210Po in a uranium miner population . Health Phys. 21 ( 4 ), 499 – 518 ( 1971 ). Google Scholar Crossref Search ADS PubMed WorldCat 4. Mártin Sánchez , A. , Pérez de la Torre , J. and Ruano Sánchez , A. B. Experimental studies about the ratio between 210Po deposited on surfaces and retrospective indoor 222Rn concentrations . Radiat. Prot. Dosim. 160 ( 1–3 ), 206 – 209 ( 2014 ). Google Scholar Crossref Search ADS WorldCat 5. Novak , L. J. and Panov , D. Polonium-210 in blood and urine of uranium mine workers in Yugoslavia . Am. Ind. Hyg. Assoc. J. 33 ( 3 ), 192 – 196 ( 1972 ). Google Scholar Crossref Search ADS PubMed WorldCat 6. Okabayashi , H. , Suzuki-Yasumoto , M. , Hongo , S. and Watanabe , S. On the evaluation of Po-210 bioassay for uranium mine workers in Japan for the personal exposure index to radon daughters . J. Radiat. Res. 16 ( 2 ), 142 – 151 ( 1975 ). Google Scholar Crossref Search ADS PubMed WorldCat 7. Okabayashi , H. A study on the excretion of Pb-210 and Po-210 . J. Radiat. Res. 23 , 242 – 252 ( 1982 ). Google Scholar Crossref Search ADS PubMed WorldCat 8. Pietrzak-Flis , Z. , Chrzanowski , E. and Dembinska , S. Intake of 226Ra, 210Pb and 210Po with food in Poland . Sci. Total Environ. 203 , 157 – 165 ( 1997 ). Google Scholar Crossref Search ADS PubMed WorldCat 9. Hölgye , Z. Excretion rates of 210Po and 210Pb in Prague inhabitants . Radiat. Prot. Dosim. 156 ( 1 ), 1 – 6 ( 2013 ). Google Scholar Crossref Search ADS WorldCat 10. Hölgye , Z. , Hýža , M. , Mihalík , J. , Rulík , P. and Škrkal , J. Variation of 210Po daily urinary excretion for male subjects at environmental level . Radiat. Environ. Biophys. 54 ( 2 ), 251 – 255 ( 2015 ). Google Scholar Crossref Search ADS PubMed WorldCat 11. R Core Team . R: A Language and Environment for Statistical Computing . ( Vienna, Austria : R Foundation for Statistical Computing ) ( 2018 ) . https://www.R-project.org/ . Google Preview WorldCat COPAC 12. Viechtbauer , W. Conducting meta-analyses in R with the metafor package . J. Stat. Softw. 36 ( 3 ), 1 – 48 ( 2010 ). Google Scholar Crossref Search ADS WorldCat 13. Kuznetsova , A. , Brockhoff , P. B. , Rune , H. B. and Christensen , R. H. B. lmer test package: tests in linear mixed effects models . J. Stat. Softw. 82 ( 13 ), 1 – 26 ( 2017 ). Google Scholar Crossref Search ADS WorldCat 14. Higgins , J. P. T. and Thompson , S. G. Quantifying heterogeneity in a meta-analysis . Stat. Med. 21 ( 11 ), 1539 – 1558 ( 2002 ). Google Scholar Crossref Search ADS PubMed WorldCat © The Author(s) 2019. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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