福島県立医科大学 学術機関リポジトリ

Title

Difference in the Cesium Body Contents of Affected Area
Residents Depending on the Evacuation Timepoint Following
the 2011 Fukushima Nuclear Disaster

Author(s)Igarashi, Yu; Kim, Eunjoo; Hashimoto, Shozo; Tani, Kotaro;
Yajima, Kazuaki; Iimoto, Takeshi; Ishikawa, Tetsuo; Akashi,
Makoto; Kurihara, Osamu

Citation

Health physics. 119(6): 733-745

Issue Date

2020-12URL

http://ir.fmu.ac.jp/dspace/handle/123456789/1592

Rights© 2020 Health Physics Society. This is a non-final version ofan article published in final form in "Health Phys. 2020
Dec;119(6):733-745".

DOI

10.1097/HP.0000000000001249

Text Version author

This document is downloaded at: 2024-10-16T12:11:14Z

Fukushima Medical University

Final Manuscript (Abstract, Text, References, Footnotes, List of
Figure Captions)

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DIFFERENCE IN THE CS BODY CONTENTS OF AFFECTED AREA RESIDENTS
DEPENDING ON THE EVACUATION TIMEPOINT FOLLOWING THE 2011 FUKUSHIMANUCLEAR DISASTER

Yu Igarashi,*,† Eunjoo Kim,* Shozo Hashimoto,* Kotaro Tani,* Kazuaki Yajima,* Takeshi Iimoto,*,†Tetsuo Ishikawa, ‡ Makoto Akashi § and Osamu Kurihara**

National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku,
Chiba-city, Chiba, Japan†The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-city, Chiba, Japan‡

Fukushima Medical University, 1-Hikarigaoka, Fukushima-city, Fukushima, Japan§

Rurgasaki Public Health Center, 2983-1, Ryugasaki-shi, Ibaraki, Japan

Corresponding author: Dr. Eunjoo Kim, Department of Radiation Measurement and Dose Assessment,
Center for Advanced Radiation Emergency Medicine, Quantum Medical Science Directorate, National
Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, Chibacity, Chiba, 263-8555, Japan.
Tel: +81-43-206-4734, Fax: +81-43-284-1769. Email: kim.eunjoo@qst.go.jp

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ABSTRACT
Estimating the internal thyroid dose received by residents involved in the 2011 Fukushima Daiichi
Nuclear Power Plant (FDNPP) accident has been a challenging task because of the shortage of direct
human measurements related to the largest contributing radioisotope to the dose, 131I. In a previous
dose estimation, we used the results of whole-body counter (WBC) measurements targeting 134Cs and137

Cs, based on the assumption that these radioisotopes were incorporated at the same time as 131I in

the early phase of the accident. The main purpose of this study was to clarify whether the trace of theearly intake remained in the WBC measurements that were started several months after the accident.
In the present work, WBC data of 1,639 persons from Namie town, one of the heavily contaminatedmunicipalities, were analyzed together with their evacuation behavior data. The results demonstrated
that the Cs detection rate in the WBC results was several times higher in the late evacuees (whoevacuated outside the 20-km radius of the FDNPP at 3:00 p.m. [Japanese Local Time] on 12 March orlater) compared to the prompt evacuees (who evacuated before 3:00 p.m. on 12 March). Among theadults, the Cs detection rates (and the 90th percentile values of the 137Cs intake) of the prompt and lateevacuees were about 20% (5.4  103 Bq) and 60% (1.6  104 Bq), respectively. Approximately 20% of
the individuals analyzed were categorized as late evacuees. These differences in Cs would be causedby exposure to the radioactive plume in the afternoon on 12 March, which was likely to influence thelate evacuees. On the other hand, the intake on 15 March when the largest release event occurred was
expected to be relatively small for Namie town's residents. In conclusion, the trace of the early intake
remained in the WBC measurements, although this would not necessarily be true for all subjects. The
results obtained from this study would provide useful information for the reconstruction of the earlyinternal thyroid doses from radioiodine in the future.

Key words: Fukushima, nuclear accident, internal dose, cesium, whole-body counter, evacuation

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INTRODUCTION
The Fukushima Daiichi Nuclear Power Plant (FDNPP) run by Tokyo Electric Power Company
suffered from a massive earthquake following the destructive tsunami that occurred on 11 March 2011.
Consequently, the FDNPP reactors in operation (Units 1, 2, and 3) lost all cooling functions for thecores' heating after a normal shutdown, resulting in the release of huge amounts of radionuclides into
the surrounding environment (National Diet of Japan 2012). The estimated release amounts of 131I and137

Cs were 151 PBq and 14.5 PBq, respectively, which were roughly one-tenth and one-fifth of thosein the Chernobyl accident (Katata et al. 2015). The FDNPP accident has been tentatively ranked as thehighest level (Level 7) on the International Nuclear Event Scale (INES) (Nuclear Emergency Response
Headquarters Government of Japan 2011).
The Japanese government repeatedly issued evacuation orders while expanding the on-alert area
(Fukushima Prefecture 2018a). Approximately 78,000 residents who lived within a 20-km radius of
the FDNPP (designated as the restricted area as of April 22, 2011) were requested to evacuate outside
by an order at 6:25 p.m. (Japanese Local Time, hereinafter the same) on 12 March shortly after thehydrogen explosion event at Unit 1 (at 3:36 p.m.). Regarding Namie town focused in the present study,
one of the heavily contaminated municipalities with the radionuclides (mainly 134Cs, 137Cs) due to this
accident, the municipal government independently instructed about 19,600 residents living within the
20-km radius to evacuate in the morning on the same day (National Diet of Japan 2012).The easternpart of Namie town is located within the 20-km radius (Fig. 1).
Many studies have been performed to determine how much radiation dose the residents ofFukushima Prefecture received due to the accident (Ishikawa 2017; Kurihara 2018b). One common
view of the various publications by Japanese experts was that following the FDNPP accident, both theexternal and internal doses of the residents were generally low, and the future radiation-induced healtheffects would be undetectable. However, there have been many concerns, particularly regarding the
thyroid exposure to young children mainly from intake of 131I. This would have been related to the factthat many malignant cases have been unexpectedly identified in thyroid ultrasound examinations that
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have been conducted as a part of the Fukushima Health Management Survey (FHMS) (Yasumura et al.
2012).The target subjects of these thyroid ultrasound examinations are all prefectural residents aged
0–18 years of March 11, 2011 (~370,000), and about 200 malignant cases (including suspected
malignancies) were identified as of December 2017 (Fukushima Prefecture 2018b). The Exploratory
Committee of the FHMS stated in their interim report that these malignant cases were unlikely to becaused by radiation exposure because of (1) the much lower doses compared to those in the Chernobyl
NPP accident, (2) the earlier detection than the latent period of thyroid cancer, (3) no malignant casesin children aged ≤5 years, and (4) the absence of a significant regional difference in the detection ratecompared to the companion studies conducted at other prefectures in Japan, although further studies
were required (Fukushima Prefecture 2018c).
The internal thyroid doses of subjects have been estimated by Japanese scientists (Tokonami et al.
2012; Matsuda et al. 2013a; Kim et al. 2016a). The results of these studies indicated that the majority
of individual thyroid doses were <20–30 mSv, although there might be a very limited number ofsubjects who received higher doses (Kamada et al. 2012). The major obstacle for the thyroid dose
estimation is the shortage of direct human measurements of 131I with a physical half-life of 8.02 d.
These measurements were available only for about one month after the FDNPP accident because of
decay out. The number of these measurements totaled only about 1,300 subjects of the public: direct
thyroid measurements of 62 subjects by Tokonami et al. (2012), whole-body counter (WBC)measurements of 173 subjects by Matsuda et al. (2013a), and a screening survey of 1,080 subjects forinternal thyroid exposure by the Nuclear Emergency Response Local Headquarters (NERLH) (Kim etal. 2016a). The NERLH screening survey covered relatively a large number of subjects, but it was
conducted at only three municipalities located mostly outside of the 30-km radius of the FDNPP:
Kawamata town, Iwaki city, and Iitate village. Residents who evacuated from the restricted area were
not targeted in that survey.
To overcome the difficulty in the thyroid dose estimation, Hosoda et al. (2013) and Kim et al.
(2016c) used the results of late WBC measurements by the National Institute of Radiological Sciences
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(NIRS) and the Japan Atomic Energy Agency (JAEA), and they applied the derived intake ratios of
131

I to 134Cs (or 137Cs). The numbers of subjects measured by the NIRS and the JAEA were 174 and

9,927 (by the end of January in 2012), respectively. These measurements were started several months
after the accident (27 June at the NIRS and 11 July 2011 at the JAEA). As a result, only 134Cs and/or
137

Cs could be detected. The thyroid dose estimation in the above studies were performed based on theassumption that the Cs body contents detected in the subjects came from the subjects' intake via
inhalation at the same time in the early phase of the accident as well as 131I.
On the other hand, Matsuda et al. (2013b) analyzed their WBC measurement results of 372 subjects
who were dispatched to Fukushima Prefecture at any timepoint from April 2011 to March 2012, andthose authors proposed that the major route of intake of Cs would be inhalation until May in 2011, andthen ingestion in June and later months. Nomura et al. (2016) assessed the relationship between selfprotection measures at the initial stage (i.e., evacuation, indoor sheltering) and the Cs body contents
for 525 subjects who were examined with WBCs at Minami-soma Municipal General Hospital. These
subjects were enrolled from among the residents who lived in Minami-soma city at the time of theaccident; the southern part of the city was included in the restricted area. Based on their analyses ofthe WBC measurements, Nomura et al. deduced that the effect of the self-protection measures was
insignificant. Although our study cannot be directly compared with the Matsuda and Nomura studies
because of the differences of subjects and measurements, the conclusions from these two studies seemto pose a critical problem regarding the thyroid dose estimations by Hosoda et al. and Kim et al.;namely, the trace of the early intake might not have remained in the WBC measurements by the NIRS
and the JAEA because of the possible ingestion of Cs from the daily diet and the relatively shortbiological half-life of Cs, i.e., ~100 days for adults and ~30 days for children (5 yrs) (ICRP 1990). We
therefore conducted the present study to clarify this issue, based on analyses of the relationshipbetween the Cs content and the evacuation times of the Namie residents. The thyroid dose estimation
was not fully addressed in this paper; however, the present study expects to provide useful information
for the reconstruction of the early internal thyroid doses by using Cs as a tracer of radioiodine.
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MATERIALS AND METHODS
Subjects and their behavior data
The individuals analyzed in this study were all residents of the Namie town, Japan who underwent
WBC measurements at the JAEA by the end of January 2012 (Momose et al. 2012) and responded toself-administered questionnaires about their behaviors for the first 4 months after the March 2011accident as part of the Basic Survey for the external dose estimation (Ishikawa et al. 2015), one of themain components in the FHMS. The total of 1,639 subjects and their composition by age and gender
are shown in Fig. 2. Personal behavior data were generated from answer sheets of the above
questionnaires and contained the whereabouts (the place name and its latitude and longitude), the timespent indoors/outdoors or moving, and the type of the building where the person stayed (e.g., a wooden
house, a concrete building), although not all of the items were analyzed in this study. These data were
provided hourly until 25 March and daily from 26 March to 11 July (only for representative places tostay and commute). We found that the data for April and later months were missing for most of the
1,639 subjects.
The distance from the subject at each time to the FDNPP (latitude 37.421071 N, longitude
141.032755 E), one of the important indexes in the analyses, was calculated by using a program
package of the Python language (Geod) (Python 2019). The locations of the subjects were visualizedon maps using our own program written in the Python language as necessary. For the two group
analyses described below in the results, the locations of individuals when they moved residences were
determined assuming a uniform linear motion between the places of departure and destination. The useof the personal behavior data in this study was approved by the Research Ethics Committee of Japan's
National Institutes for Quantum and Radiological Science and Technology (QST) – National Instituteof Radiological Science (NIRS) QST-NIRS (13-011).

Whole-body counter measurements
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Details of the WBC measurements of Fukushima Prefecture residents by the JAEA (for the firstyear after the accident) are described elsewhere (Kurihara et al. 2018a). Briefly, the JAEA started themeasurements on 11 July 2011 (2 months after the accident). The WBC units that were mainly usedwere two standing-type units and one chair-type unit at two JAEA sites in Ibaraki Prefecture because
no WBCs were available for measurements of the residents in Fukushima Prefecture at that time.
Surface contamination check was performed using a closed-end Geiger-Muller (GM) survey meter
before the WBC measurements, which would have minimized false-positive detection due to theremained contaminant on clothes. The nominal minimum detectable activity (MDA) values for 134Cs
and 137Cs were 300–400 Bq for a counting time of 2 min. These values were evaluated taking into
account the variation of background counts (including the component from 40K in the human body)and were increased by about 100 Bq compared to those before the accident. The statistical uncertainties
in the net peak area corresponding to 300Bq were about 20-45%. Subjects were selected by the
Fukushima prefectural government; residents living in the municipalities near the FDNPP wereprioritized. Residents of Namie town were measured at relatively early times (mostly during the period
from July to September in 2011). Children aged under 3 years (at that measurement timepoint) were
excluded from the subject populations because of difficulties in the measurements with the WBC units.

Calculations of intake and committed effective dose
The calculations of intake and the committed effective dose (CED) based on the results of the WBC
measurements were performed using the following equations:𝐼𝑖,134 =𝑀134
𝑅𝑖,134 (𝑡)(1)𝐼𝑖,137 =𝑀137
𝑅𝑖,137 (𝑡)

(2)

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𝐶𝐸𝐷𝑖 = 𝐼𝑖,134 ∙ 𝑒𝑖,134 + 𝐼𝑖,137 ∙ 𝑒𝑖,137(3)

where, the subscript i is the age group defined in the ICRP publications: 5 yr, 10 yr, 15 yr, and Adult(ICRP 1995); note that 0 yr and 1 yr age groups are not included in subjects of the WBC measurements.
The subscripts 134 and 137 indicate 134Cs and 137Cs, respectively; M is the Cs body content determinedby the WBC measurements; R is the whole-body retention rate as a function of the elapsed time after
intake (until measurement); e is the effective dose per unit intake (DPUI) in the case of Type F
compounds with an activity median aerodynamic diameter (AMAD) of 1 m.
The retention rate and DPUI values were taken from the database of the MONDAL systemdeveloped by Ishigure et al. (2004) and the ICRP database of dose coefficients (ICRP 1998). The intake
scenario was assumed to be acute intake via inhalation on 12 March 2011, as in previous studies (Kimet al. 2016c; Kurihara et al. 2018a). This intake scenario was set to avoid underestimation of individual
doses rather than to obtain realistic dose estimates on the assumption that dietary intake would havebe minimized due to prompt regulations for contaminated food and drink. Persons who occasionallyconsumed highly contaminated foodstuffs such as wild animals or home-grown vegetables were foundto be very few in number (Hayano et al 2013; Tsubokura et al. 2013).
In the present analyses, the intake was treated as zero for the subjects with negative detection. Here,the negative detection is the cases where the Cs body content was neither detected nor exceeded thenominal MDA values, whereas the positive detection (described later) is the cases where the Cs content
was larger than the nominal MDA values. Several percentile values of intake and the CED wereobtained using the Excel PERCENTILE function. For explanation, the Cs intake is obtained using theCs whole-body retention rate, as shown in the above equations. It should be noted that the Cs wholebody retention rates are considerably different between the age groups and rapidly decreases for the
younger age groups, which means that even a small amount Cs content can be converted into a verylarge Cs intake (or a very large CED) for children with increasing the elapsed time. This needs to becarefully considered as described in Discussion.
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RESULTS
Cs detection rate in the WBC measurements
Table 1 provides the numbers of subjects with positive detection of 134Cs or 137Cs for each age andgender group along with the Cs detection rate. As shown, the Cs detection rate was higher the group isolder, and the gender difference in the Cs detection rate was significant in the Adult group. The highest
Cs detection rates were for the Adult male group (n=91): 51.6% for 134Cs and 56.0% for 137Cs.

The relationship between the movement of resident after the accident and the Cs detection rate
Fig. 3 illustrates the whereabouts of the 1,639 residents of Namie town on the first few days after
the March 2011 accident; note that the locations of some individuals are overlapped at the same places.
Note that the locations of residents when they were moving were determined on this figure assuming
a uniform linear motion between the whereabouts of departure and destination. As demonstrated in the
figure, the majority of the Namie residents originally stayed near the coastal area of Namie town within
the 20-km radius and then started to evacuate rapidly on the morning of 12 March. The residents
remaining near the FDNPP were sparse as of 12:00 a.m. on 15 March. It was found that the major
evacuation route from Namie town was twofold: a Tsushima route (toward the northwest from the
FDNPP) and a Minami soma route (toward the north from the FDNPP). Tsushima is the west districtin Namie, and we defined the Tsushima area as the areas outside the 20-km radius of Namie town inthis study (Fig. 1).
Fig. 4 shows the trends of the numbers of individuals (the Adult group) staying within the 20-km
radius (<20-km), those outside the 20-km radius (20-km), and those who moved during the period
between 12:00 a.m. on 12 March and 12:00 a.m. on 16 March. The number of individuals (<20-km)
started decreasing in the morning of 12 March, suggesting that the evacuation order by the municipal
government functioned well. The number of people moving fluctuated on 13 March and later,
indicating that evacuations were repeatedly performed during the daytime. It was also clear that some
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individuals returned to inside the 20-km radius after they had evacuated outside the radius. The same
figures for the other age groups and all of the 1,639 individuals are provided in Appendix 1.
Fig. 5 illustrates the trends of the 317 individuals in the Adult group, separating those who stayed
within the 20-km radius and those who moved outside the 20-km radius. Here the distances from thewhereabouts of individuals moving to the FDNPP at each timepoint were determined in a manner
similar to that shown in Fig. 3. In the figure, 116 individuals with positive detection and 201 individuals
with negative detection regarding 137Cs (see Table 1) are also provided separately, along with the trendsof the 137Cs detection rate for these two groups. This analysis was performed in reference to Nomura
et al. (2016). As demonstrated, the individuals with positive detection moved to remote places atrelatively early times; however, the time of evacuation for these individuals appeared to be somewhat
late compared to those with negative detection.
One remarkable finding was that the detection rate for the subjects who remained within the 20-km
radius started increasing at around noon on 12 March and reached the maximum (77%, 27 of 35
individuals) at 8:00 p.m. on the same day, and then gradually decreased afterwards. A similar tendency
was seen in the other age groups and all of the 1,639 individuals (Appendix 2). The detection rate forthe individuals who stayed outside the 20-km radius did not significantly change with time; note thatthe 137Cs detection rate in the Adult group is 36.6% (Table 1), which is indicated in the figure's lower
panel.

Difference in the internal dose due to the timepoint of evacuation
Fig. 6 shows the results of the comparison of the 137Cs body content between two subject groups inbox-and-whisker plots for each age group; one group is the Namie residents who evacuated outside
the 20-km radius before 3:00 p.m. on 12 March (G1: the prompt evacuation group), and the other groupis those who evacuated outside the 20-km radius on 3:00 p.m. on 12 March or later (G2: the lateevacuation group). The individuals with negative detection are marked as nearly zero in the figure.
As shown in Fig. 6, the distribution of the 137Cs body content values was clearly different between
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the two groups; the 137Cs body content values of the G2 group were significantly higher than those of
the G1 group. One important finding (in Fig. 6) was that all of the subjects with significant 137Cs
detection among the G1 group were categorized as outliers (in accordance with the definition of a boxand-whisker plot); namely, all of the individuals other than outliers were those with negative detection.
The number of such outliers was small in the G2 group.
Table 2 provides a comparison of numbers of the G1 and G2 groups. The number of G2 groupsubjects was a total of 388 in all age groups; this was 23.7% of the 1,639 individuals. Figs 7 and 8
provide the results of our comparisons of the 137Cs intake and the CED from 134Cs and 137Cs for theG1 and G2 groups, respectively. The data corresponding to these two figures are provided as different
percentiles in Tables 3 and 4. The maximum CED was 2.3 mSv, which was detected in a subject in the
5 yr age group and the G1 group, whereas 0.72 mSv was the maximum CED in the Adult group. The
50th percentile (median) CED was determined in the 15 yr and Adult groups of the G2 group, but notfor the other groups (Table 4).
Fig. 9 compares the 137Cs body content, the 137Cs intake, and the CED from 134Cs and 137Cs for the
15 yr and Adult groups in the following four groups; the first two groups are prompt and late evacuees
who moved toward the northwest (Tsushima): G1(T) and G2(T), and the last two groups are prompt
and late evacuees toward the north (Minami-soma): G1(M) and G2(M). The numbers of G1(T), G2(T),G1(M), and G2(M) subjects were 231, 37, 64, and 24, respectively. Here, the definitions of G1 and G2
are the same as those described above. The evacuees to Tsushima and Minami-soma were categorized
by the duration of time (>24 hr during the period between 12:00 a.m. on 12 March and 12:00 a.m. on
14 March) staying in either of the two areas. Table 5 provides the subject numbers and difference
percentiles of CEDs for each of these groups. The younger age groups (5 yr and 10 yr) were notincluded because of the low Cs detection rates not enough to provide the corresponding data in thetable.

DISCUSSION
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As mentioned in the Introduction, it is crucial to clarify whether late WBC measurements taken by
the NIRS and the JAEA have a trace of the early intake of radionuclides. If not, these measurements
will result in unavailable in the thyroid dose estimation for Fukushima residents. This study focusedon residents of Namie town, one of the most affected municipalities by the FDNPP accident andanalyzed the relationship between results of the WBC measurements and the timepoint of evacuation
using available data of 1,639 residents of Namie town. The most important finding of this study was
the definite difference in the CED values between the prompt and late evacuees. Our results also
indicate that the most decisive time for the Namie residents was the afternoon of 12 March (Fig. 5).
Major release events of radionuclides from the FDNPP occurred on 12, 15–16, and 20–21 March,and so on (Tsuruta et al. 2017). Among these events, the radioactive plume caused by the hydrogen
explosion at Unit 1 on the afternoon of 12 March would be related to our above-described findings.
Several investigations determined the precise behavior of this plume; it flowed toward the northnorthwest (NNW) direction at the beginning, which was evident from the result of soil samplingadjacent to the FDNPP site, and the plume was then expected to change its direction toward the northnortheast (NNE) (Chino et al. 2016). The time trends of the airborne concentration of Cs near the
FDNPP were also revealed by analyses of radionuclides collected on filter tapes installed in suspended
particulate matter (SPM) monitors, demonstrating that the airborne concentration of Cs at sites locatedin the northern area along the coast increased shortly after the explosion event and reached the
maximum at night: >100 Bq m−3 for 7 hr after 9:00 p.m. with a maximum concentration of about 575
Bq m−3 for 137Cs at the site located 25 km north of the FDNPP (Minami-soma city) (Tsuruta et al. 2014).
Another study using the same SPM technique demonstrated that the highest 137Cs airborne
concentration of 13,600 Bq m−3 was observed at the site located 3.2 km west-northwest (WNW)(Futaba town) at 3:00 p.m. before the explosion, as a result of a vent operation at Unit 1 (Tsuruta et al.
2018). The first peak in the 137Cs airborne concentration (55.7 Bq m−3) was also observed at 9:00 a.m.
at this site.
The Cs detection rate was several times higher in our present G2 group compared to the G1 group
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(Table 2). The Cs detection rate was >60% in the G2 group adults but only ~20% in the G1 group
adults. A similar observation to this was found in our previous study (Kunishima et al. 2017). This
result may also indicate that the intake of radionuclides due to the largest release event on March 15–
16 was small in the residents of Namie town thanks to their evacuation to remote places (Fig. 3). The
90th-percentile values of the 137Cs intake for the G1 and G2 groups were 5.4  103 Bq and 1.6  104
Bq, respectively (Table 3). This difference (~104 Bq) would be an index for the magnitude of the 137Cs
intake on 12 March (for individuals with relatively high internal doses), considering that most of theG2 group evacuated outside the 20-km radius, as did most of the G1 group (Fig. 4), resulting in similarinternal exposure for the two groups on 15 March and later.
The evidence that the CED values of the evacuees who moved to Minami-soma were generally
higher than those of the evacuees who moved to Tsushima district (Fig. 9, Table 5) contributes to ourunderstanding of the intake on 12 March together with the studies described in the paragraph above.
The intake on 12 March is also important in terms of evaluating the contribution of the internal thyroid
dose from the other short-lived radionuclides (e.g., 132Te/132I, 133I) (Shinkarev et al. 2015; Ohba et al.
2017).
To estimate the internal thyroid dose from the WBC measurements which were performed since
July 2011, it is necessary to determine the intake ratio of 131I to 134Cs (or 137Cs). Our previous study
derived the intake ratio (131I/137Cs) from two independent direct measurements of residents in
Kawamata and Iitate (see Fig. 1), namely, direct thyroid measurements of children (the screening
survey by the NERLH) and the WBC measurements of adults by the JAEA (Kim et al. 2016b). The
time trend of the ambient dose equivalent rate suggested that the radioactive plume first reached
Kawamata town and Iitate village in the afternoon of 15 March, not on 12 March. Therefore, thedifference in the radionuclide composition in the plumes on these two days should be examined, when
considering the possible intake by late evacuees from Namie town. The relevant information will beobtained from SPM samples in the future. The derived intake ratio was 3, which was much smaller
than the radioactivity ratio in the core inventory, nearly 10 (Katata et al. 2015). It was considered that
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the probable main reason for this discrepancy lay in the relatively-low thyroid iodine uptake inJapanese compared to the corresponding parameter used in the iodine biokinetic model for internal
dose assessment, although further studies are needed on this regard (Kim et al. 2016b).
Finally, it should be addressed that late WBC measurement data of children are considered difficult
for use in the reconstruction of the early internal dose in the FDNPP accident at this moment. The
intake amount of radionuclides is expected to be larger in adults than in children as long as the sameintake scenario is applied to both groups. In the case of Cs, the dose coefficient due to intake isrelatively similar in the different age groups. As a result, the CED should have been higher in the Adultgroup than in the Child group in the WBC measurements; however, they were not higher (Table 4).
We suspect that the CEDs of a relatively small number of children with positive Cs detection werelikely to be overestimated due to their accidental intake via ingestion during the several months after
the accident and/or contamination on their clothes at the time of the WBC measurements (Kurihara etal. 2018a).
Such possibilities were also suggested by our findings that the intakes and the CEDs of the outliers
were not related to the timepoint of evacuation, in particular for the 5 yr age group (Figs. 7, 8).
Moreover, although food safety regulations were enforced shortly after the FDNPP accident (Hamada
and Ogino 2012), daily intake via ingestion might occur in subjects. Several surveys have reported that
dietary intake amount of Cs was 1 Bq or less per day on average in the first year after the accident(Koizumi et al. 2012; Harada et al. 2013). Even such a small intake amount could partly contribute to
the detected Cs contents if continued, resulting in overestimations of CEDs in particular for children
by applying the acute intake scenario described before. A more appropriate intake scenario is thus
needed to interpret the late WBC measurements, focusing on the difference in the Cs detection rate
between the genders (in particular for the Adult group) and the different age groups with each uniqueCs biological half-life. In conclusion, it was deduced that the trace of the early intake remained in the
WBC measurements, although this would not be necessarily true for all the individuals examined.
Further analyses on the data presented in this study are essential to find out the critical group more
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precisely.

CONCLUSION
We analyzed the WBC measurement results of 1,639 residents of Namie town together with their
behavioral data after the 11 March, 2011 nuclear power accident at the FDNPP, and our findings
revealed that the timepoint of evacuation influenced the individual internal dose from Cs. We
categorized the individuals into two groups depending on the timepoints at which they evacuated tooutside of the 20-km radius of the FDNPP. As a result, here was found an evident difference in thedetection rate and intake of Cs between the prompt and late evacuees. It was deduced that thisdifference would be caused by exposure to the radioactive plume released in the afternoon of 12 March,in particular for the late evacuees who remained within the 20-km radius as of 3:00 p.m. on that criticalday. Regarding the adults, the Cs detection rate exceeded 60% for the late evacuees, was only about
20% for the prompt evacuees. Further studies are necessary to appropriately interpret the results of theavailable WBC measurements for estimating the internal thyroid doses of Fukushima Prefecture's
residents due to their intake of radioiodine. However, the results obtained from this study would
provide useful information for the reconstruction of the early internal thyroid doses from radioiodinein the future.

FUNDING
This study was funded by the Ministry of the Environment of Japan.

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Footnotes
1
2 *National Institutes for Quantum and Radiological Sciences and Technology, 4-9-1 Anagawa, Inage-ku,
34
5 Chiba-city, Chiba, Japan
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
22
65

List of figure captions
1
2 Fig. 1 The locations of Fukushima Prefecture, Namie town, and the FDNPP in Japan with the 20-km radius
3
4
5 from the FDNPP.
67
8
9 Fig. 2 Composition of the 1,639 subjects by age and gender. The ages are as of 12 March 2011.
10
11
12
13
14 Fig. 3 The whereabouts of the 1,639 residents at 3:00 a.m. on 12 March (1/6), 9:00 a.m. on 12 March (2/6),
15
16
3:00 p.m. on 12 March (3/6), 9:00 p.m. on 12 March (4/6), 12:00 a.m. on 13 March (5/6), and 12:00 a.m. on
1718
19 15 March (6/6). The concentric circles denote 10, 20 (thick line), 30, 50, and 70 km-radii of the FDNPP
20
21 from the inside.
22
23
24
25
26 Fig. 4 Trends of the numbers of individuals (the Adult group) staying within or outside the 20-km radius or27
28 moving during the period from 12:00 a.m. on 12 March to 12:00 a.m. on 16 March.
29
30
31
32
33 Fig. 5 Trends of the 317 individuals belonging to the Adult group by separating those staying within the 2034
35
km radius (upper) and outside the 20-km radius (lower) for the period from 12:00 a.m. on 12 March to 12:00
36
37
38 a.m. on 16 March.
39
40
41
42
137
Cs body content (kBq) between the G1 and G2 evacuation groups. The boxes
43 Fig. 6 Comparison of the44
45 cover the 25th percentile (not seen) to the 75th percentile of the distribution. The right panel expands the46
47vertical axis of the left figures for easy to see the distributions in the small range. Same in Figs 7 and 8. The
48
49
50 lines in the boxes are the 50th percentile (median) of the distribution (only seen in the Adult group and the 15
51
52 yr group (G2)). The whiskers cover the first quantile –1.5  IQR (interquartile range) to the third quantile +53
54
55 1.5  IQR unless outliers (crosses) exist. The following figures with box-and-whisker plots use the same
56
57 format.
58
59
60
61
137
Cs intake (kBq) between the G1 and G2 evacuation groups.
62 Fig. 7 Comparisons of the
63
64
23
65

Fig. 8 Comparisons of the CED (mSv) from 134Cs and 137Cs between the G1 and G2 evacuation groups.

1
2
3
4
137
137
134
5 Fig. 9 Comparisons of the (a) Cs body content (kBq), (b) Cs intake (kBq) and (c) CED (mSv) from Cs
6137
Cs for the 15 yr and Adult group in the G1(T), G2(T), G1(M), and G2(M) groups. G1(T): the prompt
7 and8
9 evacuees toward the northwest, i.e. Tsushima; G2(T): the late evacuees toward the northwest, G1(M): the101112 prompt evacuees toward the north, i.e., Minami-soma; G2(M): the late evacuees toward the north.
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
24
65

APPENDIX 1
Trends of the numbers of individuals (5 yr, 10 yr, 15 yr, and All groups) staying within or moving outside

1
2 the 20-km radius or moving during the period from 12:00 a.m. on 12 March to 12:00 a.m. on 16 March.
3
4
5
6
APPENDIX 2789 Trends of individuals with positive 137Cs detection in the 5 yr, 10 yr, 15 yr, and All groups and the detection
10
11
12 rate for the period from 12:00 a.m. on 12 March to 12:00 a.m. on 16 March.
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
25
65

Table1

Table 1. The number of subjects with the positive detection of 134Cs or 137Cs for each age and gender group
5 yr

10 yr

15 yr

Adult

Number of subjects (Male and Female)With 134Cs detection
With 137Cs detection347
26 (7.5%)*40 (11.5%)

486
50 (10.3%)
79 (16.3%)

489
108 (22.1%)
145 (29.7%)

317
97 (30.6%)
116 (36.6%)

Males

179

239

241

91

With 134Cs detection
With 137Cs detection

16 (8.9%)21 (11.7%)

27 (11.3%)
44 (18.4%)

70 (29.0%)
89 (36.9%)

47 (51.6%)
51 (56.0%)

FemalesWith 134Cs detection
With 137Cs detection168
10 (6.0%)19 (11.3%)

247
23 (9.3%)
35 (14.2%)

248
38 (15.3%)
56 (22.6%)

226
50 (22.1%)
65 (28.8%)* Figures in parentheses are the detection rate in percentage.

1

Table2

Table 2. The numbers of subjects in the G1 and G2 evacuation groups
5 yr

10 yr

15 yr

Adult

Total

Number of subjects

347

486

489

317

1,639

G1 evacuation group
With 134Cs detection
With 137Cs detection279
13 (4.7%)*
19 (6.8%)

387
24 (6.2%)
42 (10.9%)363
52 (14.3%)
80 (22.0%)

222
39 (17.6%)
53 (23.9%)

1,251
128 (10.2%)
194 (15.5%)G2 evacuation group
With 134Cs detection
With 137Cs detection

68
13 (19.1%)
21 (30.9%)99
26 (26.3%)
37 (37.4%)

126
56 (44.4%)
65 (51.6%)

95
58 (61.1%)
63 (66.3%)

388
153 (39.4%)
186 (47.9%)* Figures in parentheses are the detection rate in percentage.

1

Table3

Table 3. The Cs intake (Bq) between the G1 and G2 evacuation groups134

G1 evacuation group:Max.
95th percentile
90th percentile
75th percentile
50th percentile (median)G2 evacuation group:Max.
95th percentile
90th percentile
75th percentile
50th percentile (median)

137

5 yr

Cs intake (Bq)
10 yr
15 yr

5 yr

Cs intake (Bq)
10 yr
15 yr

Adult

Adult

2.2 × 105
N.D.
N.D.

1.0 × 105
1.1 × 104
N.D.

4.5 × 104
6.9 × 103
5.2 × 103

1.4 × 104
7.2 × 103
5.5 × 103

3.1 × 105
4.7 × 104
N.D.

1.1 × 105
1.4 × 104
9.8 × 103

4.7 × 104
7.9 × 103
5.9 × 103

1.7 × 104
8.6 × 103
5.4 × 103

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

N.D.
N.D.

1.3 × 105
9.7 × 104
6.4 × 104
N.D.

6.7 × 104
3.2 × 104
2.4 × 104
1.1 × 104

2.6 × 104
1.5 × 104
1.2 × 104
7.5 × 103

5.8 × 104
1.7 × 104
1.3 × 104
7.4 × 103

1.6 × 105
1.1 × 105
8.7 × 104
4.2 × 104

8.2 × 104
3.3 × 104
2.7 × 104
1.6 × 104

3.4 × 104
1.8 × 104
1.4 × 104
8.9 × 103

7.3 × 104
1.9 × 104
1.6 × 104
8.7 × 103

N.D.

N.D.

N.D.

3.9 × 103

N.D.

N.D.

4.0 × 103

4.8 × 103

N.D.: not detected in the WBC measurements
Age range: 5 yr: more than 2 years to 7 years, 10 yr: more than 7 years to 12 years, 15 yr: more than 12 years to 17 years, Adult: more than 17 years (ICRP1995)

1

Table4

Table 4. The CEDs (mSv) of the G1 and G2 evacuation groups
5 yr

10 yr

15 yr

Adult

G1 evacuation group:Max.
95th percentile
90th percentile
75th percentile

2.3 × 100
2.1 × 10-1
N.D.
N.D.

9.7 × 10-1
1.0 × 10-1
3.8 × 10-2
N.D.

4.9 × 10-1
8.0 × 10-2
5.8 × 10-2
N.D.

1.7 × 10-1
8.2 × 10-2
6.2 × 10-2
N.D.

50th percentile (median)

N.D.

N.D.

N.D.

N.D.

G2 evacuation group:Max.
95th percentile
90th percentile
75th percentile
50th percentile (median)

1.2 × 100
8.8 × 10-1
5.5 × 10-1
2.2 × 10-1
N.D.

6.6 × 10-1
2.9 × 10-1
2.1 × 10-1
1.0 × 10-1
N.D.

3.0 × 10-1
1.6 × 10-1
1.3 × 10-1
8.7 × 10-2
2.0 × 10-2

7.2 × 10-1
2.0 × 10-1
1.6 × 10-1
8.8 × 10-2
4.5 × 10-2

N.D.: not detected in the WBC measurements

1

Table5

Table 5. The CEDs (mSv) for the 15 yr and Adult subjects among the G1(T), G2(T), G1(M) and G2(M) groups
Subjects : 15 yr and Adult groups

G1(T)

G2(T)

G1(M)

G2(M)

231

37

64

24

CED:
Max
95th percentile
90th percentile

4.9 × 10-1
1.2 × 10-1
6.4 × 10-2

6.1 × 10-1
2.9 × 10-1
1.5 × 10-1

1.2 × 10-1
1.0 × 10-1
7.7 × 10-2

7.2 × 10-1
3.0 × 10-1
2.8 × 10-1

75th percentile
50th percentile (median)

N.D.
N.D.

8.9 × 10-2
6.2 × 10-2

4.4 × 10-2
N.D.

1.4 × 10-1
4.1 × 10-2

Number of persons

N.D.: not detected in the WBC measurements1

Figure1

Fig. 1

Iitatevillage

Minamisoma
city

Fukushima Pref.

Tsushima districtof Namie town
Katsuraovillage
Tamuracity

20-km radius

Namie town
Futabatown
Okuma town FDNPP

Fig. 2

Figure2

Fig. 3Figure3

Fig. 4

Figure4350

Adult group

Number of individuals

300
250
200

20-kmwithin_20km
20-kmoutside_20km
MovingMoving

150
100
50

0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

DateFig. 5Figure5300

Individuals staying within 20-km (Adult)
Total (a)Positive detection (b)Detection rate (b/a)

200

80

60

150
40
100

Detection rate (%)

Number of individuals

250

100

20

50

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16
12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

350

Date

Individuals staying outside 20-km (Adult)

80

250

Total (a)Positive detection (b)Detection rate (b/a)

200
150

60

40

100
20

50

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16
12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Date

Detection rate (%)

Number of individuals

300

100

Fig. 6

137Cs body content (kBq)

Figure6

6

1.2

5

1.5

4

0.8

3

0.6

2

0.4

1

0.2

0

G1

G2

G1

G2

G1

G2

G1

G2

0.0

G1

G2

G1

G2

G1

G2

G1

G2

(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)

Evacuation group

Evacuation group

Fig. 7

Figure7

137Cs intake (kBq)

400

50

40

300

30
200
20

100
0

10
G1

G2

G1

G2

G1

G2

G1

G2

0

G1

G2

G1

G2

G1

G2

G1

G2

(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)

Evacuation group

Evacuation group

Fig. 8CED(mSv) from 134Cs and 137Cs

Figure8

2.5

0.25
50

2.0

0.20
40

1.5

0.15
30

1.0

0.10
20

0.5

0.05
10

0.0

G1

G2

G1

G2

G1

G2

G1

G2

0.000

G1

G2

G1

G2

G1

G2

G1

G2

(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)(5 yr) (5 yr) (10 yr) (10 yr) (15 yr) (15 yr) (Adult) (Adult)

Evacuation group

Evacuation group

Fig. 9Figure9

137Cs body content (kBq)

(a)

8
6
4
2
0

G1(T)

G2(T)

G1(M)

G2(M)

G2(T)

G1(M)

G2(M)

G2(T)

G1(M)

G2(M)

Evacuation group

137Cs intake (kBq)(b) 80
60
40
20

0

G1(T)

Evacuation group

CED(mSv) from 134Cs and 137Cs(c) 0.8
0.6
0.4
0.2
0.0

G1(T)

Evacuation groupAppendix1

Appendix 1
5yr

15yr

Number of individuals

350

600

5 yr group

300
250

20-kmwithin_20km
20-kmoutside_20km
MovingMoving

200

150
100
50

0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

Date

10yr

20-kmwithin_20km
20-kmoutside_20km
MovingMoving

300

200
100

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Date

all

1800

10 yr group

1600

500

All group

1400

400

20-kmwithin_20km
20-kmoutside_20km
MovingMoving

300
200
100

Number of individuals

Number of individuals

400

03/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

600

15 yr group500

Number of individuals

400

1200

20-kmwithin_20km
20-kmoutside_20km
MovingMoving

1000
800
600
400
200

0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

Date

Date

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Appendix 2

Appendix2

Individuals staying within 20-km (5 yr)

200
150

60

40

100

20

50

250

Total (a)Positive detection (b)Detection rate (b/a)

200
150
100

60

40

20

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Date

Date

300
250
200

40

150

100

500

80

60

Individuals staying outside 20-km (10yr)

20

100

80

400

Total (a)Positive detection (b)Detection rate (b/a)

300

60

40

200

Detection rate (%)

350

600

Detection rate (%)

Total (a)Positive detection (b)Detection rate (b/a)

100

Number of individuals

Individuals staying within 20-km (10 yr)

400

Number of individuals

80

300

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

450

100

50

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

500

Individuals staying outside 20-km (5yr)

Detection rate (%)

Total (a)Positive detection (b)Detection rate (b/a)

250

350
80

Detection rate (%)

Number of individuals

300

400

100

Number of individuals

350

20

100

50
0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

00
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Date

300

200

40

150
100

20

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

40

20

1600

60

800
40

600
400

20

200

Number of individuals

80

Individuals staying outside 20-km (All)

100

80

1400
1200

60

1000

Total (a)Positive detection (b)Detection rate (b/a)

800
600
400

40

20

200

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

00
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

Date

Date

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Detection rate (%)

Total (a)Positive detection (b)Detection rate (b/a)

1800

100

Detection rate (%)

Number of individuals

200

Date

Individuals staying within 20-km (All)

1000

60

Total (a)Positive detection (b)Detection rate (b/a)

300

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

Date

1200

400

0
0
3/12 3/12 3/12 3/12 3/13 3/13 3/13 3/13 3/14 3/14 3/14 3/14 3/15 3/15 3/15 3/15 3/16

12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00 6:00 12:00
a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m. a.m. p.m. p.m. a.m.

1400

100

80

100

50

1600

Individuals staying outside 20-km (15yr)

500

80

60

250

600

Detection rate (%)

Total (a)Positive detection (b)Detection rate (b/a)

350

Date

100

Detection rate (%)

Number of individuals

400

Individuals staying within 20-km (15 yr)

Number of individuals

450