Individual external dose monitoring of all citizens of Date City by passive dosimeter 5 to 51 months after the Fukushima NPP accident (series): II. Prediction of lifetime additional effective dose and evaluating the effect of decontamination on individual dose

In the first paper of this series, we showed that the ratio c of individual dose to ambient dose did not change with time in Date City, Fukushima Prefecture, after the Fukushima Daiichi Nuclear Power Plant accident. The purpose of the present paper, the second in a series, is to estimate the lifetime doses of the Date City residents, based on continuous glass badge monitoring data, extrapolated by means of the ambient-dose-rate reduction function obtained from the airborne monitoring data. As a result, we found that the external exposure contribution to the mean additional lifetime dose of residents living in Date City is not expected to exceed 18 mSv. In addition, effects of decontamination on the reduction of individual doses were not evident. This method of combining individual doses and the ambient doses, as developed in this study, has made it possible to predict with reasonable certainty the lifetime doses of residents who continue to live in this radiologically contaminated area.

Following the Expression of Concern issued on this article on 11 January 2019, IOP Publishing is now retracting this article. On 4 June 2020, IOP Publishing received confirmation from the authors of 2017 J. Radiol. Prot. 37 623 (the second in a series of two research articles) that ethically inappropriate data were used in the study reported in this article. This confirmation follows an investigation into the matter by Date City Citizen's Exposure Data Provision Investigation Committee, which finds that some subjects within the study did not consent to their data being used for research, and it is unclear whether the unconsented data was provided to the author. IOP Publishing believes that the authors were unaware of the ethical problems with this data, which was supplied by a third party. The results of this investigation are available (in Japanese) at https://www.city.fukushima-date.lg.jp/soshiki/3/41833.html (IOP Publishing and the Society for Radiological Protection take no responsibility for the content at this link).
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Individual external dose monitoring of all citizens of Date City by passive dosimeter 5 to 51 months after the Fukushima NPP accident (series): II. Prediction of lifetime additional effective dose and evaluating the effect of decontamination on individual dose 1

. Introduction
In the previous paper in this series [1], we analysed the results obtained from the 'glass badge' individual dosimeters (radio-photoluminescence glass dosimeters: Glass Badge ® ) used by residents of Date City, Fukushima Prefecture, from 5 to 51 months after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, and concluded as follows: (1) The individual doses obtained by individual dose monitoring with glass badges and the ambient doses at the residences of the glass-badge participants ('grid dose' as defined in [1]) estimated from the airborne monitoring survey carried out in the same period [2] were proportional with a coefficient of 0.15. (2) The obtained coefficient of 0.15 did not change with time, across the six airborne surveys conducted between November 2011 (4th survey) and November 2014 (9th survey). (3) As a result, the conversion factor from the ambient dose rate to the personal dose, 0.6, adopted by the Ministry of the Environment [3], was found to be about four times larger than the coefficient 0.15 obtained by these actual measurements.
In particular, conclusion (2) implies that the individual doses and the ambient doses decreased at the same rate. This further suggests that the lifetime doses of the residents who continue to live in the contaminated zones can be inferred from ambient doses obtained from the airborne monitoring surveys. In this paper, the second in a series of three papers based on the Date City glass badge data, we first obtain a function f (t) for the temporal change of the ambient dose rate based on the airborne monitoring data. We then compare the time-integrated ambient-dose-rate curve F t f d t ò t t = ( ) ( ) to the additional cumulative doses measured from 5 to 51 months after the FDNPP accident. We further provide the estimates for the lifetime cumulative doses of the residents, by making use of F(t).
In addition, we examine the effect of decontamination on the individual doses of the residents, who continued to use glass badges and lived in the designated decontamination area throughout the study period. We then discuss the implications of our findings regarding the effectiveness of decontamination in the reduction of long-term cumulative doses.
This paper discusses the results of these two studies and aims to establish a method to reliably predict the long-term cumulative external doses of residents living in the contaminated areas.

Modelling the temporal change of the ambient dose rate
The Japanese government regularly monitors the area radiologically affected by the Fukushima Daiichi NPP accident using aircraft flying at an altitude of about 300 m [2] and converts the measured values to the ambient dose rate H 10 * at 1m above the ground. The averaged data within the 250 m grids have been disseminated as maps and numerical data [4]. The aerial survey was first conducted in April 2011 and the most recent was the tenth, conducted in November 2015. In our previous paper [1], we used data from the fourth through ninth airborne monitoring surveys, which overlapped in timing with the glass-badge measurements from Sept. 2011 to 2014Q3. Note that when obtaining the additional exposure dose rate from the ambient dose rate obtained by airborne monitoring, 0.04 μSv h −1 , the value corresponding to the natural-radiation dose rate adopted by the Ministry of the Environment [3], is subtracted.
In order to extract the temporal change of the ambient dose rate in Date City, we first tabulated, for each grid point in Date City and for the fifth to tenth surveys, the backgroundsubtracted ambient-dose-rate ratio derived from each monitoring survey subsequent to (but not including) the fourth. The results are shown in figure 1. As shown, there is a good linearity between the ambient dose rates of nth and 4th surveys. We therefore fitted a straight line (also shown in figure 1) for each survey from the 5th to the 10th. The slope of each straight line represents the average reduction of the ambient dose rate relative to the 4th survey, which we plot with filled circles in figure 2, versus years after the accident.
Next, the time dependence of the ambient dose rates was modelled by the following function, known from previous studies [5,6], which contains the physical half-lives of Cs-134 and Cs-137 (T 134 = 2.06y and T 137 = 30.17y), fast-and slow-ecological half-lives (T fast and T slow ) and the fraction of the fast-ecological decay component (a fast ), where H 0.65 10 * ( ) denotes that this function is normalised to the ambient dose rates of the 4th airborne survey conducted at t = 0.65y. Here, the coefficient k was derived as follows: Radioactive contamination in Date City is due primarily to radioactive materials released from the FDNPP No. 2 reactor on March 15, 2011 [7]. Analysis of radioactivity in each nuclear reactor of FDNPP released from JAEA in 2012 [8] at the shut down point of the  [9], the coefficient of k = 2.95 was derived.

Estimated additional lifetime doses
Date City started measuring individual doses with glass badges in August 2011, when measurements were done for pregnant women and children aged 15 years and younger for one month. From September 2011, the measurement periods were extended to 3 months (1 Quarter) and measurements are still being continued at the time of writing (table 1 of [1]). The dosimeter supplier, Chiyoda Technol Corporation, has been using the value of 0.54 mSv y −1 as the contribution of the natural background radiation throughout Fukushima Prefecture since the FDNPP accident. This was measured in Oarai in Ibaraki Prefecture, and is subtracted from the glass-badge readings when they report the additional external doses to the relevant municipalities [10], including Date City.
In the present paper, we selected the subjects who held the glass badges continuously from 2011Q3 to 2015Q1 (from September 2011 to June 2015), as this makes it possible to directly obtain their cumulative individual doses for the said period. The numbers of subjects thus selected were n = 476 for zone A, n = 693 for zone B and n = 3280 for zone C (see section 2.3 for the definition of the three zones). The additional cumulative external dose for the first 4 months after the accident, before the glass-badge measurement period, was assumed to be 1.4 mSv, which is the average value of the external dose estimated for the residents living in the northern Fukushima Prefecture region, as was published in the basic survey included in the Fukushima Health Management Survey [11].
The cumulative individual doses H P (t) were then compared to the following function, is the ambient dose reduction function obtained in equation (1), i is for zone A, B and C. The mean ambient dose rates at the time of the 4th airborne monitoring were

Difference in the decontamination method for each zone in Date City
Date City pioneered the decontamination efforts, such as top soil removal, as described in the decontamination implementation plan (1st edition) published in October 2011 [12]. In the 2nd edition, published in August 2012 [13], the city was divided into three zones, zone A (>3.5 μSv h −1 ), zone B (between 1 μSv h −1 and 3.5 μSv h −1 ), and zone C (<1 μSv h −1 ). Zoning was set mainly for the purpose of determining the methods and priority of decontamination, and to carry out decontamination properly and promptly. According to the implementation plan, the residential premises and the surrounding forest up to 20 m from residential boundaries were to be decontaminated in zone A, residential premises were to be decontaminated in zone B, and 'hot spots' were to be cleaned in zone C. The method for zone A is equivalent to the method described in the Decontamination Guidelines, published by the Ministry of the Environment [14]. The decontamination of residential premises based on the implementation plan in Date City was completed in March 2014.

Evaluation of the influence of decontamination on individual doses
Since the decontamination methods adopted by Date City differed for each zone, in order to correctly evaluate the effect of decontamination on individual doses it is necessary to select a single zone (i.e., common decontamination method), and to select the subjects who continuously held the glass badges while their houses were decontaminated. We focused on 1700 houses where decontamination was done in zone A, among which all family members continuously held glass badges from 2011Q3 to 2014Q1 (September 2011 to June 2014), and decontamination was carried out during 2012Q3 (from October to December 2012). This narrowed the number of subjects to 425, living in 132 houses. The glass badge data and the GIS information of the subjects were anonymized by Date City, and was provided to the authors. Figures 3 and 4 illustrate the approximate positions of the 132 houses targeted and the time periods when decontamination was performed respectively. The distribution of the measured values of the glass badges of the selected subjects over time was evaluated, as was the influence of decontamination on the glass badge measurement values.

Temporal change of the ambient dose rate
By fitting the model equation (1) to the ambient dose rate ratios of the 5th to 10th airborne surveys relative to the 4th survey (filled circles in figure 2), the parameters in equation (1) have been determined as follows: a fast = 0.68 ± 0.01, T fast = 0.32 ± 0.01 y, T slow > 700 y (therefore, the slow ecological component can be ignored for our purposes). The line in figure 2 is the best-fit curve.

Discussion
The current study has a number of unique characteristics. First, no other cases of continuous population-scale mass monitoring of the individual external doses of residents living in longterm contaminated areas have been recorded, meaning that no comparable individual dose data is available. After the Chernobyl accident, for example, external doses were generally estimated based on the ambient dose rates [15], rather than on individual dose monitoring. Therefore, the lifetime cumulative doses of residents, important information for those living in contaminated areas, were not derived from measured individual doses.
The scale of the decontamination carried out in Fukushima Prefecture after the FDNPP accident is also unprecedented, meaning that comparable quantitative data regarding the effectiveness of decontamination in reducing individual doses on a population scale is unavailable. In Japan after the FDNPP accident, the dose reduction due to decontamination has until now generally been evaluated by comparing the ambient dose rates before and after  figure 5-1) is for zone A, 5-2 is for zone B and 5-3 is for zone C, who continuously held glass badges during the study period. The boxes cover 25-percentile to 75-percentile of the distribution, and the whiskers cover the 1-percentile to 99percentile of the distribution. The dots represent outliers. The dark solid curve is the estimated median H p (t) (equation (2)), while upper and lower light curves correspond to the 1-and 99-percentile estimates, respectively. The dotted line along the right vertical axis of the graph represents the estimated median lifetime doses (up to 70 years) from external exposure pathways, 18 mSv for zone A, 15 mSv for zone B, and 11 mSv for zone C. decontamination, measured using survey meters at various outdoor points on the premises. As such, evaluation of the effect of decontamination on the individual doses of the residents has until now not been implemented either retrospectively or prospectively.
Fortunately, however, except for the initial 4 months after the FDNPP accident, individual doses have been continuously recorded in many municipalities in Fukushima  Prefecture. In most cities, towns and villages, these individual measurements targeted limited groups of subjects and time periods. Date City, which began measuring personal doses for schoolchildren and residents living in relatively high-dose areas from August 2011 and has continued to measure tens of thousands of citizens uninterruptedly, is an important exception. The availability of appropriate population-scale individual dose data, the scale of the decontamination that was performed in the areas where the subjects reside, and the availability of consistent aerial survey data from before and after the decontamination period, make the current study possible.
We reported in the first instalment of this series [1] that the correlation between the individual doses measured with glass badges in Date City and the ambient dose rates derived from the airborne monitoring of residences did not change over time. Based on that conclusion, in this report, we estimated the cumulative doses over a lifetime by extrapolating the individual doses of the Date City residents who continuously held a glass badge, by making use of equation (2). As a result, the median of the additional lifetime doses from external exposure pathways for residents continuing to live in zone A, the most contaminated area in Date City (and who received the highest doses in Fukushima Prefecture after the FDNPP accident), was estimated to be 18 mSv. The ICRP adopts an allowable dose band of 1-20 mSv per year for individuals in existing exposure situations [16]. The present study indicates that the median additional lifetime (i.e., 70 y) dose by the FNDPP accident of Date City residents will not exceed 20 mSv.
In the Chernobyl accident, it was estimated that the residents in Russia, Belarus and Ukraine received about a quarter of their total lifetime exposure (up to 70 years after the accident) during the first year following the accident [15]. Similarly, the 1st-year external cumulative dose of Date City residents was estimated to be ∼1/6 of their total lifetime exposure.
In August 2011, the Cabinet Office of Japan established a basic policy of decontamination, aiming at reducing the external dose rate received by the general public by 50% (including physical decay and weathering effects) in the two years from the end of August 2011 to the end of August 2013. The report published in March 2015 by the Ministry of the Environment (MoE: responsible for decontamination) [17], showed that the additional external dose rate as estimated from survey meter measurements decreased by approximately 62% in those two years in the area decontaminated according to the guidelines, to which the decontamination was said to contribute about 22%, and physical decay etc contributed 40%. In Date City, the reduction of the grid dose (airborne monitoring survey) in the city between November 2011 and October 2013 was 60%, and the reduction of the individual dose (glass badge) in zone A during the same period was 62%, similar to the result shown in the MoE report.
It is possible, however, to explain the 60% reduction of the ambient dose rate in Date City by decay and weathering alone. In addition, comparison between the individual-and ambient-dose rates in Date City in the present study failed to confirm a 22% reduction of the individual doses due to decontamination, for the residents in zone A, where the decontamination was carried out according to the government guidelines. The authors believe that reasons why no decontamination effect could be seen in Date City, unlike the results presented in the MoE report, include: 1. The ratio of the areas actually decontaminated to the area covered by the airborne monitoring is small. Thus, decontamination has little effect in reducing the 'grid' dose. 2. The residents do not necessarily spend all their time only in the decontaminated residential areas, and 3. the measurements included in the MoE report were done at outdoor points on the premises, not reflecting the effect of decontamination on indoor dose rates.
The information available for the present study, however, does not make it possible to evaluate the behaviour patterns of individual residents or the retention status of each dosimeter, so it is difficult to further discuss the reasons why the effect of decontamination is not reflected in the individual doses. No comparable data is yet available to help determine whether or not this would be the case in other areas. But because residents of zone A in Date City received some of the highest external doses to the public, and decontamination was performed relatively early there, meaning its effects would likely be most evident, the authors believe that their findings may possibly prove to be representative.

Conclusions
In the present paper, we have shown that the ambient-dose-rate reduction function obtained from the airborne monitoring data can be applied uniformly to all zones in Date City.
Regardless of the magnitude of the ambient dose rate, the difference in decontamination method, or whether or not decontamination was carried out, the reduction function was the same throughout Date City. Based on these results, the lifetime additional doses can be realistically estimated from the measured individual doses, extrapolated by means of the ambient-dose-rate reduction function. For establishing this method, which combines the individual dose and the ambient dose, the existence of large-scale and long-term glass-badge data of Date City was essential. Although the proportionality coefficient c = 0.15 may not be universal, the authors believe that the present method will make it possible in the future to reliably estimate the lifetime doses of residents living in contaminated areas based on regular airborne monitoring surveys, supplemented by smaller-scale individual dose monitoring surveys for the purpose of determining the ambient-dose-to-individual-dose coefficient c.