TY - JOUR
AU - Takamura, Noboru
AB - Abstract Tomioka Town is located within a 20-km radius of the Fukushima Daiichi Nuclear Power Station. Radiation dose rates due to radiocesium in residents’ living spaces were evaluated from the measurements of ambient dose rates and environmental samples after returning home. The mean ambient dose rates were 0.15–0.18-μSv/h indoors and 0.23–0.26-μSv/h outdoors during 2018 and 2019, and the additional radiation dose rates were calculated to be 1.4 mSv/y in 2018 and 1.1 mSv/y in 2019. Ambient dose equivalent from surface soils within housing sites were estimated to be 0.66 mSv/y in 2018 and 0.54 mSv/y in 2019. Moreover, committed effective doses from local foods were calculated in 19–74 μSv/y for children and 39–100 μSv/y for adults during 2018 and 2019. These findings suggest that current radiation exposure doses have been controlled at the levels close to the public dose limit (1 mSv/y) in residents’ living spaces. INTRODUCTION Ten years have almost passed since 11 March 2011, the date of the 9.0-magnitude Great East Japan Earthquake, subsequent tsunami and nuclear accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), which is operated by the Tokyo Electric Power Company. During the FDNPS accident, various radionuclides including iodine-131 (131I, half-life: 8 d), cesium-134 (134Cs, 2.1 y) and cesium-137 (137Cs, 30 y) were emitted into the atmosphere, eventually being deposited on land, in rivers and at sea in the surrounding areas(1,2). The United Nations Scientific Committee on the Effects of Atomic Radiation reported that the total releases of radionuclides for 131I, 134Cs and 137Cs were 120.0, 9.0 and 8.8 petabecquerels, respectively(2), and reported that the amount of released radionuclides was one-sixth the amount released during the Chernobyl accident in 1986(2). Although the Japanese government and local municipalities have carried out decontamination efforts in affected areas, a ‘difficult-to-return zone’ (areas which residents are prohibited from returning to their homes) remains, affecting > 43 000 residents of Fukushima(3–5). Even after decontamination of areas around the FDNPS, radiocesium with a long half-life still exists in environmental samples including soils, plants and local foods such as agricultural products(3,4,6,7). Thus, residents pay attention to the radiation exposure due to radiocesium(2,8–11). On 1 April 2017, the Japanese government stated that residents could return to their homes in ~88% of the gross area of Tomioka Town, which is located within a 20-km radius of the FDNPS, because the annual doses were expected to be < 20 mSv/y (Figure 1)(3,5,6). Although 3 y have passed since this declaration, as of 1 April 2020, among the 11 236 evacuees of Tomioka Town, 2258 (20.1%) were still living outside of Fukushima Prefecture, 8978 (79.9%) were living somewhere in Fukushima Prefecture, and only 1292 (10.3%) had actually returned home(7). The reason for this low number of returned residents is reflected to be anxieties regarding radiation exposure by the accident(12,13). Our previous study on residents of Tomioka Town showed that specific factors such as ‘living with children < 18-y old’, ‘reluctance to drink tap water’ and ‘anxiety regarding genetic effects for the next generation due to radiation exposure’ were significantly associated with the intention to return home(14). Because risk communication based on monitoring of environmental radioactivity and the potential effects of radiation on health is important, the external and internal exposure doses due to daily activities of residents remain a matter of public attention and dissemination of scientific and timely information to the public is needed. Figure 1 Open in new tabDownload slide Location of Tomioka Town, Fukushima Prefecture and survey points. Black dots and red dots indicate the measuring points in the evacuation-order-lifted area and difficult-to-return zone, respectively. Figure 1 Open in new tabDownload slide Location of Tomioka Town, Fukushima Prefecture and survey points. Black dots and red dots indicate the measuring points in the evacuation-order-lifted area and difficult-to-return zone, respectively. In Japan, edible wild plants and mushrooms have traditionally been consumed in various ways as part of countryside culture(15–18), especially in regions of Eastern Japan, including Fukushima Prefecture(15,19). In previous studies conducted after the Fukushima accident, it has been reported that internal exposure doses from the intake of edible wild plants and mushrooms collected in the affected areas are extremely limited(15,20). However, internal exposure from the intake of local foodstuffs such as edible wild plants, mushrooms, vegetables and fruits produced and/or collected in Tomioka Town near the FDNPS has not been evaluated sufficiently, based on the data which the municipal government have reported by literature, database and website. Also, it is extremely important to evaluate the external exposure in the areas in which residents spend a lot of their time, such as housing sites, over the long term. In the present study, we aimed to investigate ambient dose rates and the radiocesium concentration in surface soil collected in Tomioka Town to evaluate the external exposure for returned residents. Additionally, we analyzed the radiocesium concentration in local foods produced and/or collected in Tomioka Town to evaluate the internal exposure for returned residents. MATERIALS AND METHODS Sampling areas; measurement of ambient dose rates and collection of surface soil The FDNPS (37°25′ N, 141°02′ E) is located on the east seaside of Fukushima Prefecture, Japan. Tomioka Town (Town office: 37°20′ N, 141°0′ E) is located 8.5-km south of the FDNPS. The east side in Tomioka Town is a densely populated area with major buildings such as the public office and schools. On the other hand, the west side of Tomioka Town mainly consists of farmland and forests. In the present study, we measured ambient dose rates and collected surface soils during 2018–2019. In Tomioka Town, the ambient dose rates at 64 locations (residents’ homes and assembly halls for residents) in 2018 and 30 locations in 2019 were measured in the air at 1-m above the ground using a NaI (Tl) scintillation survey meter (thallium doped sodium iodide, ∅1 × 1 inches), which can measure gamma rays (Energy range; 50 keV–3 MeV, ambient dose equivalent rate (H* (10)): 0–30.0 μSv/h at 1-cm depth; TCS-172B, Hitachi-Aloka Medical, Ltd., Tokyo, Japan; Figure 1). Soil samples were also collected at all dose measurement points (in the yards of housing sites within ~5 m of the front entrance; Figure 1). These monitoring areas ranged from 37°31′ N to 37°37′ N and 140° 95′ E to 141°02′ E, and the distances from the FDNPS were between 5.5 and 17.2 km (Figure 1). Measurement of radionuclides The radioactivity in soils of Tomioka Town was analyzed in 128 samples (64 sampling sites) in 2018 and 60 samples (30 sampling sites) in 2019. Surface soil (0–5 and 5–10-cm layers below the surface) was collected using a core sampling technique (two core samples at each sampling site). The size of the soil sample was 18.2 cm2 (diameter: 4.8 cm), the density of the soil layers ranged from 0.35 to 1.6 g/cm3-dry and the mass ranged from 37.5 to 356.6 g. After collection, all samples were dried at 105°C for 24 h and sieved for gravel and organic materials (>2 mm). Then, fine particles (<2 mm) were prepared before measuring radioactivity. After preparation, the surface soil samples were put into polypropylene containers (U8), and analyzed using a high-purity germanium detector (ORTEC GMX series, Ortec International Inc., Ltd., Oak Ridge, TN, USA) coupled to a multi-channel analyzer (MCA7600, Seiko, EG&G Co., Ltd., Chiba, Japan) for 3600 s. Measuring time was set to a level at which the target radionuclide (radiocesium) was detectable at 4–5 Bq/kg and more. The gamma-ray peaks adopted for the measurements were 604.66 keV for 134Cs (2.1 y) and 661.64 keV for 137Cs (30 y). Decay corrections were set to the sampling date. Detector efficiency calibration for different measurement geometries including the density and thickness of samples was performed using mixed activity standard volume sources (Japan Radioisotope Association, Tokyo, Japan). The relative detection efficiency of this apparatus was 37.7%, with 1332.47 keV for cobalt-60 (5.3 y). All soil samples were processed and analyzed at Nagasaki University, Nagasaki, Japan. Then, the concentration of objective radionuclide (radiocesium) samples (730 samples in 2018 and 503 samples in 2019) of foods produced and/or collected at Tomioka Town throughout the year (January–December) was analyzed. The Tomioka Town Food Inspection Department of the Tomioka Town Office carried out the monitoring of foods (screening against the standard limit) to avoid unnecessary internal exposure due to radiocesium derived from the FDNPS accident(21). In the present study, foodstuffs brought to this department by residents were analyzed. Vegetables, fruits and tuber samples were grown in decontaminated private gardens on residents’ home sites and edible wild plant and mushroom samples were collected in non-decontaminated areas such as boundary sites, forests or roadsides. After the FDNPS accident, the Japanese government established a new standard value (100-Bq/kg radiocesium for general foods) and food produced in Fukushima has been confirmed to be below this standard limit before being shipped(22,23). All samples were cleaned with fresh water and impurities such as gravel, tree stems and branches were removed. After preparation, these local food samples (>300 g-fresh) were put directly into a polyethylene bag and measured using the NaI (Tl) detector (thallium doped sodium iodide, ∅5 × 5 inches) coupled to a multi-channel analyzer (AFT-NDA2, Advanced Fusion Technology, Co., Ltd., Tokyo, Japan) for 600 s. This detector covers a wide range of energy peaks (100–3000 keV) and can analyze various radionuclides in food samples. The gamma-ray peaks adopted for measurements were 605 keV for 134Cs (2.1 y) and 662 keV for 137Cs (30 y). The resolution of this apparatus was < 7.5% on 661.64 keV for 137Cs. The energy and efficiency calibration of this detector were performed using a standard source of 134Cs and 137Cs. Food sample collection was performed by residents of Tomioka Town; processing and measurements of radionuclides were performed by the Tomioka Town Office; and analysis of radionuclides was performed at Nagasaki University. Sampling, processing and analysis were carried out using the standard methods series of radioactivity measurement certified by the Ministry of Education, Culture, Sports, Science and Technology and the Nuclear Regulation Authority in Japan(24,25). External radiation exposure by ambient dose equivalent rate (H*(10)) In areas of Tomioka Town where the evacuation order was lifted, the additional radiation dose rates (Aext) due to radiocesium excluding the natural radiation dose rates were estimated with the following equation: $$\begin{eqnarray} {A}_{\mathrm{ext}}\left(\mathrm{mSv}/\mathrm{y}\right) = \left\{\left({C}_{\mathrm{int}}-0.05\kern0.5em \mu \mathrm{Sv}/\mathrm{h}\right)16\;\mathrm{h}\right.\nonumber\\ +\left.\left({C}_{\mathrm{ext}}-0.04\kern0.5em \mu \mathrm{Sv}/\mathrm{h}\right)8\;\mathrm{h}\right\}365\;\mathrm{d}0.001 \end{eqnarray}$$(1) where Cint is the mean ambient dose rate indoors (entrance hall) (μSv/h) and Cext is the mean ambient dose rate outdoors (yard) (μSv/h). The fixed values (0.05 μSv/h indoors and 0.04 μSv/h outdoors) in Equation (1) are defined as the natural radiation dose rates (external terrestrial radiation and cosmic radiation)(26,27), and 16 and 8 h (24 h/d) are considered as representing the indoor and outdoor activities of daily living based on the guideline of the Ministry of the Environment (MOE), Japan, respectively(28). According to environmental monitoring data from Fukushima Prefecture, the ambient dose rates (background radiation) before the FDNPS accident were around 0.04 μGy/h in Tomioka Town, using a 2-inch NaI (Tl) detector(29). The methodology in Equation (1) was decided based on our previous investigation(10). The calculation for obtaining additional exposure doses after the FDNPS accident is under the assumption that a person stays outdoors for 8 h and stays in a traditional Japanese house (wooden house) for 16 h with the natural radiation exposure dose at 0.05-μSv/h indoors and 0.04-μSv/h outdoors(30). According to the International Commission on Radiological Protection (ICRP), three exposure situations are categorized(30). ‘The emergency exposure situation’ immediately after the nuclear accident is set in the range of 20–100 mSv/y, ‘the existing exposure situation’ in the recovery and reconstruction period is set in the range of 1–20 mSv/y and ‘the planned exposure situation’ is set at 1 mSv/y. In the existing exposure situation, the reference level is a lower dose range within 1–20 mSv/y, with a long-term goal of 1 mSv/y (the public dose). External radiation exposure from surface soils After the measurements of soil samples, the external effective doses rates were estimated from the radiocesium concentration with the following equation: $$\begin{equation} {H}_{ext}=C\;{D}_{ext}S \end{equation}$$(2) where C is the radiocesium (134Cs and 137Cs) inventory (mean: radiocesium inventory in kBq/m2 calculated from the radiocesium concentration in Bq/kg, including fine particles (<2 mm) of surface soils (0–5 cm) collected with a size of 0.00182 m2) based on the in-situ gamma-ray spectrometry(31–34), Dext is the dose conversion coefficient reported as the ambient dose equivalent rate at 1-m above the ground per unit activity per unit area {(μSv/h)/(kBq/m2)}, supposing the air-kerma rate and the absorbed dose rate in air were equivalent, for radiocesium with the relaxation mass per unit area (β: g/cm2) adopted as 5.0 based on the measurement data in Fukushima Prefecture after the FDNPS accident (3.41 × 10−3 (μSv/h)/(kBq/m2) for 134Cs and 1.25 × 10−3 (μSv/h)/(kBq/m2) for 137Cs)(31–34), and s is the occupancy-shielding factor (0.2 fractional time outdoors +0.8 fractional time indoors × 0.2 building shielding = 0.36)(35). The methodology in Equation (2) was decided based on our previous investigations(9,10). Internal radiation exposure by ingesting local foods (agricultural products) After the measurements of local foods produced and/or collected in Tomioka Town, the committed effective doses of four food items (vegetables, edible wild plants, mushrooms and fruits) in all local food samples were estimated from the radiocesium concentration with the following equation: $$\begin{equation} {H}_{\mathrm{int}}=C\;{D}_{\mathrm{int}}e \end{equation}$$(3) where C is the radiocesium concentration (median) of radiocesium (134Cs and 137Cs) (Bq/kg-fresh), Dint is the dose conversion coefficient for child intake (age: 0–18 y, 1.3 × 10−5 to 2.6 × 10−5 mSv/Bq for 134Cs and 9.6 × 10−6 to 2.1 × 10−5 mSv/Bq for 137Cs) and for adult intake (age: > 19 y, 1.9 × 10−5 mSv/Bq for 134Cs and 1.3 × 10−5 mSv/Bq for 137Cs) based on ICRP Publication 72(36), and e is quoted from the average daily intake survey (g/person/d) for age and sex issued by the Ministry of Health, Labour and Welfare of Japan in 2018(37). For this methodology, average daily intake was estimated at the internal exposure doses in case of ingesting food samples during activities of daily living. To calculate the committed effective doses, the undetected radioactivity of radiocesium in the local foods was defined as one-fourth of the detection limit for vegetables (134Cs in 2018–2019 and 137Cs in 2019) and fruits (134Cs in 2018–2019); one-half of the detection limit for vegetables (137Cs in 2018), edible wild plants (134Cs in 2019) and fruits (137Cs in 2019); and equal to the detection limit itself for edible wild plants (134Cs in 2018 and 137Cs in 2018–2019), for mushrooms (134Cs in 2018–2019 and 137Cs in 2018–2019) and for fruits (137Cs in 2018), certified by the World Health Organization’s Global Environmental Monitoring System(38). The methodology in Equation (3) was decided based on our previous investigations(15,20,39). Ethics statement The present study was approved by the ethics committee of Nagasaki University Graduate School of Biomedical Sciences (No. 17030212), and written informed consent was obtained from the house owners where all samples were collected. RESULTS External radiation exposure The ambient dose rates in Tomioka Town in 2018 and 2019 are shown in Table 1. In the evacuation-order-lifted areas, the mean ambient dose rates inside the returned residents’ homes were 0.18 (1.5) μSv/h (mSv/y) indoors in 2018, 0.15 (1.4) μSv/h (mSv/y) indoors in 2019, 0.26 (2.3) μSv/h (mSv/y) outdoors in 2018, 0.23 (2.0) μSv/h (mSv/y) outdoors in 2019, 0.34 (3.0) μSv/h (mSv/y) in the backyard in 2018 and 0.29 (2.5) μSv/h (mSv/y) in the backyard in 2019. Therefore, the additional radiation exposure dose in 2018 and 2019 was stable at 1.1–1.4 mSv/y based on Equation (1). On the other hand, in the difficult-to-return zone, the mean ambient dose rates were 1.4 (12) μSv/h (mSv/y) outdoors in 2018, 1.6 (14) μSv/h (mSv/y) outdoors in 2019, 1.8 (16) μSv/h (mSv/y) in the backyard in 2018 and 1.5 (13) μSv/h (mSv/y) in the backyard in 2019. Table 1 Ambient dose equivalent rates (H* (10)) around living spaces in Tomioka Town, Fukushima Prefecture, in 2018 and 2019 Measurement pointa . Year . n . Ambient dose rate . Additional radiation dose rate in mSv/y . Shielding factor . . . . . Mean in μSv/h (mSv/y) . Range in μSv/h . Median in μSv/h . . . Evacuation-order-lifted area Indoorsb 2018 35 0.18 (1.5)c 0.098–0.30d 0.16 (0.26)e 1.4f 0.68g Outdoors 59 0.26 (2.3) 0.088–0.48 0.25 (0.37) Backyard 59 0.34 (3.0) 0.12–1.2 0.29 (0.51) Indoors 2019 10 0.15 (1.4) 0.11–0.26 0.14 (0.19) 1.1 0.69 Outdoors 25 0.23 (2.0) 0.084–0.44 0.22 (0.32) Backyard 25 0.29 (2.5) 0.13–0.68 0.24 (0.53) Difficult-to-return zone Outdoor 2018 5 1.4 (12) 0.30–2.4 1.2 (2.1) Backyard 5 1.8 (16) 0.36–2.8 2.2 (2.6) Outdoor 2019 5 1.6 (14) 0.32–2.2 1.8 (2.1) Backyard 5 1.5 (13) 0.27–2.0 1.6 (1.9) Measurement pointa . Year . n . Ambient dose rate . Additional radiation dose rate in mSv/y . Shielding factor . . . . . Mean in μSv/h (mSv/y) . Range in μSv/h . Median in μSv/h . . . Evacuation-order-lifted area Indoorsb 2018 35 0.18 (1.5)c 0.098–0.30d 0.16 (0.26)e 1.4f 0.68g Outdoors 59 0.26 (2.3) 0.088–0.48 0.25 (0.37) Backyard 59 0.34 (3.0) 0.12–1.2 0.29 (0.51) Indoors 2019 10 0.15 (1.4) 0.11–0.26 0.14 (0.19) 1.1 0.69 Outdoors 25 0.23 (2.0) 0.084–0.44 0.22 (0.32) Backyard 25 0.29 (2.5) 0.13–0.68 0.24 (0.53) Difficult-to-return zone Outdoor 2018 5 1.4 (12) 0.30–2.4 1.2 (2.1) Backyard 5 1.8 (16) 0.36–2.8 2.2 (2.6) Outdoor 2019 5 1.6 (14) 0.32–2.2 1.8 (2.1) Backyard 5 1.5 (13) 0.27–2.0 1.6 (1.9) aResidences and assembly halls. bIndoors mean the inside of the entrance (entrance hall) and outdoors mean the outside of the entrance (yard). cCalculated annual dose rate from mean value. dMinimum–maximum. eParentheses show 90th percentile. fCalculated using Equation (1). gAmbient dose ratio (indoors/outdoors) calculated from mean values. Open in new tab Table 1 Ambient dose equivalent rates (H* (10)) around living spaces in Tomioka Town, Fukushima Prefecture, in 2018 and 2019 Measurement pointa . Year . n . Ambient dose rate . Additional radiation dose rate in mSv/y . Shielding factor . . . . . Mean in μSv/h (mSv/y) . Range in μSv/h . Median in μSv/h . . . Evacuation-order-lifted area Indoorsb 2018 35 0.18 (1.5)c 0.098–0.30d 0.16 (0.26)e 1.4f 0.68g Outdoors 59 0.26 (2.3) 0.088–0.48 0.25 (0.37) Backyard 59 0.34 (3.0) 0.12–1.2 0.29 (0.51) Indoors 2019 10 0.15 (1.4) 0.11–0.26 0.14 (0.19) 1.1 0.69 Outdoors 25 0.23 (2.0) 0.084–0.44 0.22 (0.32) Backyard 25 0.29 (2.5) 0.13–0.68 0.24 (0.53) Difficult-to-return zone Outdoor 2018 5 1.4 (12) 0.30–2.4 1.2 (2.1) Backyard 5 1.8 (16) 0.36–2.8 2.2 (2.6) Outdoor 2019 5 1.6 (14) 0.32–2.2 1.8 (2.1) Backyard 5 1.5 (13) 0.27–2.0 1.6 (1.9) Measurement pointa . Year . n . Ambient dose rate . Additional radiation dose rate in mSv/y . Shielding factor . . . . . Mean in μSv/h (mSv/y) . Range in μSv/h . Median in μSv/h . . . Evacuation-order-lifted area Indoorsb 2018 35 0.18 (1.5)c 0.098–0.30d 0.16 (0.26)e 1.4f 0.68g Outdoors 59 0.26 (2.3) 0.088–0.48 0.25 (0.37) Backyard 59 0.34 (3.0) 0.12–1.2 0.29 (0.51) Indoors 2019 10 0.15 (1.4) 0.11–0.26 0.14 (0.19) 1.1 0.69 Outdoors 25 0.23 (2.0) 0.084–0.44 0.22 (0.32) Backyard 25 0.29 (2.5) 0.13–0.68 0.24 (0.53) Difficult-to-return zone Outdoor 2018 5 1.4 (12) 0.30–2.4 1.2 (2.1) Backyard 5 1.8 (16) 0.36–2.8 2.2 (2.6) Outdoor 2019 5 1.6 (14) 0.32–2.2 1.8 (2.1) Backyard 5 1.5 (13) 0.27–2.0 1.6 (1.9) aResidences and assembly halls. bIndoors mean the inside of the entrance (entrance hall) and outdoors mean the outside of the entrance (yard). cCalculated annual dose rate from mean value. dMinimum–maximum. eParentheses show 90th percentile. fCalculated using Equation (1). gAmbient dose ratio (indoors/outdoors) calculated from mean values. Open in new tab Radiocesium distributions in soil layers (0–5 and 10 cm) in Tomioka Town in 2018 and 2019 are shown in Tables 2 and 3 (Supplementary Tables S3 and S4). Dose-contributing artificial radionuclides 134Cs (2.1 y) were prevalent in most soil samples although measurements were below the detection level in 16 of 128 (12.5%) samples in 2018 and 6 of 60 (10.0%) samples in 2019. Dose-contributing artificial radionuclides 137Cs (30 y) were prevalent in all soil samples. In the evacuation-order-lifted areas, the mean radiocesium concentration on the site of resident’s homes were 535 (7.4–4352) Bq/kg-dry in 2018 and 315 (10–2406) Bq/kg-dry in 2019 for 134Cs (0–5 cm), and 4938 (13–44 676) Bq/kg-dry in 2018 and 4040 (19–32 061) Bq/kg-dry in 2019 for 137Cs (0–5 cm). Annul ambient dose equivalent from the surface soil (0–5 cm) were estimated to be 0.076 (0.66) μSv/h (mSv/y) in 2018 and 0.061 (0.54) μSv/h (mSv/y) in 2019 based on Equation (2). In the difficult-to-return zone, on the other hand, the mean radiocesium concentration on the site of the assembly halls for residents were 3012 (19–5720) Bq/kg-dry in 2018 and 1805 (20–5873) Bq/kg-dry in 2019 for 134Cs (0–5 cm) and 31 603 (243–58 719) Bq/kg-dry in 2018 and 25 658 (223–83 001) Bq/kg-dry in 2019 for 137Cs (0–5 cm). Annul ambient dose equivalent from the surface soil (0–5 cm) were estimated to be 0.35 (3.1) μSv/h (mSv/y) in 2018 and 0.44 (3.9) μSv/h (mSv/y) in 2019 based on Equation (2). Table 2 Radiocesium distribution in soil layers (0–5 and 10 cm) in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Sampling sitea . Year . Depth in cm . n . Radionuclide concentration in Bq/kg-dry . Concentration ratio in Bq/kg . . . . . Mean . Range . Median . . . . . . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs/137Cs . Evacuation-order-lifted area 2018 0–5 59 535 4938 7.4–4352b 13–44 676 301 (1035)c 2635 (11 054) 0.093d 5–10 567 5342 8.4–2410 12–24 715 318 (1545) 2913 (14 354) 0.093 2019 0–5 25 315 4040 10–2406 19–32 061 126 (709) 1656 (8976) 0.070 5–10 235 2780 13–1344 25–18 246 134 (422) 1502 (5965) 0.072 Difficult-to-return zone 2018 0–5 5 3012 31 603 19–5720 243–58 719 2992 (5157) 31 479 (53 730) 0.092 5–10 925 9739 254–2716 2596–28 443 335 (2047) 3522 (21 498) 0.095 2019 0–5 5 1805 25 658 20–5873 223–83 001 469 (4430) 6892 (62 785) 0.073 5–10 1135 13 036 80–2889 157–41 171 786 (2461) 1650 (33 153) 0.070 Sampling sitea . Year . Depth in cm . n . Radionuclide concentration in Bq/kg-dry . Concentration ratio in Bq/kg . . . . . Mean . Range . Median . . . . . . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs/137Cs . Evacuation-order-lifted area 2018 0–5 59 535 4938 7.4–4352b 13–44 676 301 (1035)c 2635 (11 054) 0.093d 5–10 567 5342 8.4–2410 12–24 715 318 (1545) 2913 (14 354) 0.093 2019 0–5 25 315 4040 10–2406 19–32 061 126 (709) 1656 (8976) 0.070 5–10 235 2780 13–1344 25–18 246 134 (422) 1502 (5965) 0.072 Difficult-to-return zone 2018 0–5 5 3012 31 603 19–5720 243–58 719 2992 (5157) 31 479 (53 730) 0.092 5–10 925 9739 254–2716 2596–28 443 335 (2047) 3522 (21 498) 0.095 2019 0–5 5 1805 25 658 20–5873 223–83 001 469 (4430) 6892 (62 785) 0.073 5–10 1135 13 036 80–2889 157–41 171 786 (2461) 1650 (33 153) 0.070 aResidences and assembly halls (same site as Table 1). bMinimum–maximum. cParentheses show 90th percentile. dCalculated from mean values. Open in new tab Table 2 Radiocesium distribution in soil layers (0–5 and 10 cm) in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Sampling sitea . Year . Depth in cm . n . Radionuclide concentration in Bq/kg-dry . Concentration ratio in Bq/kg . . . . . Mean . Range . Median . . . . . . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs/137Cs . Evacuation-order-lifted area 2018 0–5 59 535 4938 7.4–4352b 13–44 676 301 (1035)c 2635 (11 054) 0.093d 5–10 567 5342 8.4–2410 12–24 715 318 (1545) 2913 (14 354) 0.093 2019 0–5 25 315 4040 10–2406 19–32 061 126 (709) 1656 (8976) 0.070 5–10 235 2780 13–1344 25–18 246 134 (422) 1502 (5965) 0.072 Difficult-to-return zone 2018 0–5 5 3012 31 603 19–5720 243–58 719 2992 (5157) 31 479 (53 730) 0.092 5–10 925 9739 254–2716 2596–28 443 335 (2047) 3522 (21 498) 0.095 2019 0–5 5 1805 25 658 20–5873 223–83 001 469 (4430) 6892 (62 785) 0.073 5–10 1135 13 036 80–2889 157–41 171 786 (2461) 1650 (33 153) 0.070 Sampling sitea . Year . Depth in cm . n . Radionuclide concentration in Bq/kg-dry . Concentration ratio in Bq/kg . . . . . Mean . Range . Median . . . . . . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs/137Cs . Evacuation-order-lifted area 2018 0–5 59 535 4938 7.4–4352b 13–44 676 301 (1035)c 2635 (11 054) 0.093d 5–10 567 5342 8.4–2410 12–24 715 318 (1545) 2913 (14 354) 0.093 2019 0–5 25 315 4040 10–2406 19–32 061 126 (709) 1656 (8976) 0.070 5–10 235 2780 13–1344 25–18 246 134 (422) 1502 (5965) 0.072 Difficult-to-return zone 2018 0–5 5 3012 31 603 19–5720 243–58 719 2992 (5157) 31 479 (53 730) 0.092 5–10 925 9739 254–2716 2596–28 443 335 (2047) 3522 (21 498) 0.095 2019 0–5 5 1805 25 658 20–5873 223–83 001 469 (4430) 6892 (62 785) 0.073 5–10 1135 13 036 80–2889 157–41 171 786 (2461) 1650 (33 153) 0.070 aResidences and assembly halls (same site as Table 1). bMinimum–maximum. cParentheses show 90th percentile. dCalculated from mean values. Open in new tab Table 3 Radiocesium distribution in surface soil (0–5 cm) in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Sampling sitea . Year . n . Radiocesium inventory in kBq/m2 . External effective dose rate in μSv/h (mSv/y) . Ambient dose rate in μSv/h (mSv/y) . Mean . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . Evacuation-order-lifted area 2018 59 15 135 0.25–146b 0.26–1498 8.2 (34)c 72 (363) 0.076 (0.66)d 0.26 (2.3)e 2019 25 8.8 114 0.17–55 0.99–726 2.7 (23) 37 (316) 0.061 (0.54) 0.23 (2.0) Difficult-to-return zone 2018 5 58 615 1.0–121 13–1301 62 (105) 650 (1109) 0.35 (3.1) 1.4 (12) 2019 5 58 820 0.86–165 9.7–2328 21 (135) 307 (1917) 0.44 (3.9) 1.6 (14) Sampling sitea . Year . n . Radiocesium inventory in kBq/m2 . External effective dose rate in μSv/h (mSv/y) . Ambient dose rate in μSv/h (mSv/y) . Mean . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . Evacuation-order-lifted area 2018 59 15 135 0.25–146b 0.26–1498 8.2 (34)c 72 (363) 0.076 (0.66)d 0.26 (2.3)e 2019 25 8.8 114 0.17–55 0.99–726 2.7 (23) 37 (316) 0.061 (0.54) 0.23 (2.0) Difficult-to-return zone 2018 5 58 615 1.0–121 13–1301 62 (105) 650 (1109) 0.35 (3.1) 1.4 (12) 2019 5 58 820 0.86–165 9.7–2328 21 (135) 307 (1917) 0.44 (3.9) 1.6 (14) aResidences and assembly halls (same site as Table 1). bMinimum–maximum. cParentheses show 90th percentile. dCalculated using Equation (2). eMean value of outdoors (See Table 1). Open in new tab Table 3 Radiocesium distribution in surface soil (0–5 cm) in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Sampling sitea . Year . n . Radiocesium inventory in kBq/m2 . External effective dose rate in μSv/h (mSv/y) . Ambient dose rate in μSv/h (mSv/y) . Mean . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . Evacuation-order-lifted area 2018 59 15 135 0.25–146b 0.26–1498 8.2 (34)c 72 (363) 0.076 (0.66)d 0.26 (2.3)e 2019 25 8.8 114 0.17–55 0.99–726 2.7 (23) 37 (316) 0.061 (0.54) 0.23 (2.0) Difficult-to-return zone 2018 5 58 615 1.0–121 13–1301 62 (105) 650 (1109) 0.35 (3.1) 1.4 (12) 2019 5 58 820 0.86–165 9.7–2328 21 (135) 307 (1917) 0.44 (3.9) 1.6 (14) Sampling sitea . Year . n . Radiocesium inventory in kBq/m2 . External effective dose rate in μSv/h (mSv/y) . Ambient dose rate in μSv/h (mSv/y) . Mean . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . 134Cs (2.1 y) . 137Cs (30 y) . Evacuation-order-lifted area 2018 59 15 135 0.25–146b 0.26–1498 8.2 (34)c 72 (363) 0.076 (0.66)d 0.26 (2.3)e 2019 25 8.8 114 0.17–55 0.99–726 2.7 (23) 37 (316) 0.061 (0.54) 0.23 (2.0) Difficult-to-return zone 2018 5 58 615 1.0–121 13–1301 62 (105) 650 (1109) 0.35 (3.1) 1.4 (12) 2019 5 58 820 0.86–165 9.7–2328 21 (135) 307 (1917) 0.44 (3.9) 1.6 (14) aResidences and assembly halls (same site as Table 1). bMinimum–maximum. cParentheses show 90th percentile. dCalculated using Equation (2). eMean value of outdoors (See Table 1). Open in new tab In the evacuation-order-lifted areas, the radiocesium concentration exceeded 8000 Bq/kg-dry, which is the standard value for storing decontamination waste according to the guidelines by the national government in Japan(40), at 11 of 59 (18.6%) sampling sites for 137Cs (0–5 cm) in 2018, 15 of 59 (25.4%) sampling sites for 137Cs (5–10 cm) in 2018, 5 of 25 (20.0%) sampling sites for 137Cs (0–5 cm) in 2019 and 2 of 25 (8.0%) sampling sites for 137Cs (5–10 cm) in 2019. In the difficult-to-return zone, the radiocesium concentration exceeded this value at 4 of 5 (80%) sampling sites for 137Cs (0–5 cm) in 2018, 2 of 5 (40%) sampling sites for 137Cs (5–10 cm) in 2018, 2 of 5 (40%) sampling sites for 137Cs (0–5 cm) in 2019 and 2 of 5 (40%) sampling sites for 137Cs (5–10 cm) in 2019. The external dose rates in residents’ living spaces in several years after restrictions in Tomioka Town were lifted are shown in Figure 2. In the present study, external exposure doses, containing natural background radiation, were slightly higher in the backyard than those outdoors at the front entrance. Moreover, external exposure doses due to radiocesium in surface soils were slightly lower or with only minor changes (Supplementary Figures S1–S6). Figure 2 Open in new tabDownload slide External dose rates in Tomioka Town residents’ living spaces in 2018 (a) and 2019 (b). The red bar shows the ambient dose equivalent due to radiocesium from surface soils. Other bars show the ambient dose rates including natural and artificial radionuclides. Figure 2 Open in new tabDownload slide External dose rates in Tomioka Town residents’ living spaces in 2018 (a) and 2019 (b). The red bar shows the ambient dose equivalent due to radiocesium from surface soils. Other bars show the ambient dose rates including natural and artificial radionuclides. Internal radiation exposure by ingestion A summary of the radioactive contaminants in local foods produced and/or collected in Tomioka Town is shown in Figure 3, Supplementary Tables S6–S8 and Supplementary Figures S8–S10. The dose-contributing artificial radionuclides in the foodstuff samples were radiocesium (134Cs and 137Cs). The detected rates of all local foods that exceeded the standard limit of radiocesium (100 Bq/kg for general foods) were 17.3% (126 of 730) in 2018 and 17.5% (88 of 503) in 2019 (Figure 3). Additionally, the detected rates of vegetables, edible wild plants, mushrooms and fruits exceeding this limit were 0.56% (1 of 179) in 2018 and 0.79% (1 of 126) in 2019 for vegetables, 50.9% (81 of 159) in 2018 and 43.5% (37 of 85) in 2019 for edible wild plants, 72.7% (8 of 11) in 2018 and 83.3% (5 of 6) in 2019 for mushrooms, and 7.9% (22 of 277) in 2018 and 7.4% (14 of 190) in 2019 for fruits (Supplementary Tables S6–S8). Figure 3 Open in new tabDownload slide Detected rates of the radiocesium contaminants in local foods in Tomioka Town, Fukushima Prefecture in 2018 and 2019. Blue bars show the detected rates of food samples in 2018. Grey bars show the detected rates of food samples in 2019. N.D., ‘not detected’. Figure 3 Open in new tabDownload slide Detected rates of the radiocesium contaminants in local foods in Tomioka Town, Fukushima Prefecture in 2018 and 2019. Blue bars show the detected rates of food samples in 2018. Grey bars show the detected rates of food samples in 2019. N.D., ‘not detected’. Radiocesium distribution in local foods produced and/or collected in Tomioka Town is shown in Table 4. Radiocesium concentration of all samples showed a wide range throughout the year. Radiocesium (134+137Cs) concentration of edible wild plants and mushrooms widely ranged: < 6851 in 2018 and <4588 in 2019 in edible wild plants and <4139 in 2018 and <99 653 in 2019 in mushrooms. On the other hand, the radiocesium concentration of vegetables and fruits did not show a wide range: < 165 in 2018 and <233 in 2019 in vegetables, and <1844 in 2018 and <246 in 2019 in fruits. Table 4 Radiocesium distribution in local foods produced and/or collected in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Classification (sampling method) . Year . n . Radiocesium concentration in Bq/kg-fresh . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . Vegetables (produced) 2018 179 <20a <145 <165 2.5 (2.5)b 5.0 (23) 7.5 (26) 2019 126 N.D.c <230 <233 2.5 (2.5) 2.5 (2.5) 5.0 (5.0) Edible wild plants (collected) 2018 159 <772 <6079 <6851 10 (57) 99 (495) 109 (550) 2019 85 <372 <4216 <4588 5.0 (30) 30 (276) 35 (297) Mushrooms (collected) 2018 11 <378 <3760 <4139 40 (141) 422 (1399) 463 (1540) 2019 6 <8467 <91 186 <99 653 27 (4251) 350 (45 811) 385 (50 054) Fruits (produced) 2018 277 <146 <1698 <1844 2.5 (2.5) 15 (70) 18 (72) 2019 190 <19 <228 <246 2.5 (2.5) 5.0 (42) 7.5 (44) Meatsd (captured) 2018 9 <243 32–1929 42–2195 29 (240) 381 (1935) 410 (2177) 2019 14 <48 34–854 44–864 24 (45) 319 (570) 351 (617) Seafoods (collected) 2018 12 N.D. <39 <39 2.5 (2.5) 5.0 (14) 7.5 (17) 2019 8 N.D. <97 <100 2.5 (2.5) 5.0 (67) 7.5 (70) Seeds (collected) 2018 26 <76 <763 <839 2.5 (2.5) 66 (119) 69 (127) 2019 12 <101 <1152 <1253 5.0 (49) 80 (652) 86 (701) Tubers (produced) 2018 35 <22 <81 <103 2.5 (2.5) 5.0 (23) 7.5 (25) 2019 31 N.D. <80 <83 2.5 (2.5) 2.5 (13) 5.0 (16) Other (produced/collected) 2018 22 N.D. <247 <247 2.5 (2.5) 20 (62) 23 (65) 2019 31 <16 <135 <151 2.5 (2.5) 5.0 (78) 7.5 (81) Classification (sampling method) . Year . n . Radiocesium concentration in Bq/kg-fresh . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . Vegetables (produced) 2018 179 <20a <145 <165 2.5 (2.5)b 5.0 (23) 7.5 (26) 2019 126 N.D.c <230 <233 2.5 (2.5) 2.5 (2.5) 5.0 (5.0) Edible wild plants (collected) 2018 159 <772 <6079 <6851 10 (57) 99 (495) 109 (550) 2019 85 <372 <4216 <4588 5.0 (30) 30 (276) 35 (297) Mushrooms (collected) 2018 11 <378 <3760 <4139 40 (141) 422 (1399) 463 (1540) 2019 6 <8467 <91 186 <99 653 27 (4251) 350 (45 811) 385 (50 054) Fruits (produced) 2018 277 <146 <1698 <1844 2.5 (2.5) 15 (70) 18 (72) 2019 190 <19 <228 <246 2.5 (2.5) 5.0 (42) 7.5 (44) Meatsd (captured) 2018 9 <243 32–1929 42–2195 29 (240) 381 (1935) 410 (2177) 2019 14 <48 34–854 44–864 24 (45) 319 (570) 351 (617) Seafoods (collected) 2018 12 N.D. <39 <39 2.5 (2.5) 5.0 (14) 7.5 (17) 2019 8 N.D. <97 <100 2.5 (2.5) 5.0 (67) 7.5 (70) Seeds (collected) 2018 26 <76 <763 <839 2.5 (2.5) 66 (119) 69 (127) 2019 12 <101 <1152 <1253 5.0 (49) 80 (652) 86 (701) Tubers (produced) 2018 35 <22 <81 <103 2.5 (2.5) 5.0 (23) 7.5 (25) 2019 31 N.D. <80 <83 2.5 (2.5) 2.5 (13) 5.0 (16) Other (produced/collected) 2018 22 N.D. <247 <247 2.5 (2.5) 20 (62) 23 (65) 2019 31 <16 <135 <151 2.5 (2.5) 5.0 (78) 7.5 (81) aMinimum–maximum or < maximum (including not detected data). bParentheses show 90th percentile. cNot Detected (detection limit: 10 Bq/kg-fresh at 134Cs and 10 Bq/kg-fresh at 137Cs). dBirds and boars captured as noxious wildlife (not for edible). Open in new tab Table 4 Radiocesium distribution in local foods produced and/or collected in Tomioka Town, Fukushima Prefecture in 2018 and 2019 Classification (sampling method) . Year . n . Radiocesium concentration in Bq/kg-fresh . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . Vegetables (produced) 2018 179 <20a <145 <165 2.5 (2.5)b 5.0 (23) 7.5 (26) 2019 126 N.D.c <230 <233 2.5 (2.5) 2.5 (2.5) 5.0 (5.0) Edible wild plants (collected) 2018 159 <772 <6079 <6851 10 (57) 99 (495) 109 (550) 2019 85 <372 <4216 <4588 5.0 (30) 30 (276) 35 (297) Mushrooms (collected) 2018 11 <378 <3760 <4139 40 (141) 422 (1399) 463 (1540) 2019 6 <8467 <91 186 <99 653 27 (4251) 350 (45 811) 385 (50 054) Fruits (produced) 2018 277 <146 <1698 <1844 2.5 (2.5) 15 (70) 18 (72) 2019 190 <19 <228 <246 2.5 (2.5) 5.0 (42) 7.5 (44) Meatsd (captured) 2018 9 <243 32–1929 42–2195 29 (240) 381 (1935) 410 (2177) 2019 14 <48 34–854 44–864 24 (45) 319 (570) 351 (617) Seafoods (collected) 2018 12 N.D. <39 <39 2.5 (2.5) 5.0 (14) 7.5 (17) 2019 8 N.D. <97 <100 2.5 (2.5) 5.0 (67) 7.5 (70) Seeds (collected) 2018 26 <76 <763 <839 2.5 (2.5) 66 (119) 69 (127) 2019 12 <101 <1152 <1253 5.0 (49) 80 (652) 86 (701) Tubers (produced) 2018 35 <22 <81 <103 2.5 (2.5) 5.0 (23) 7.5 (25) 2019 31 N.D. <80 <83 2.5 (2.5) 2.5 (13) 5.0 (16) Other (produced/collected) 2018 22 N.D. <247 <247 2.5 (2.5) 20 (62) 23 (65) 2019 31 <16 <135 <151 2.5 (2.5) 5.0 (78) 7.5 (81) Classification (sampling method) . Year . n . Radiocesium concentration in Bq/kg-fresh . Range . Median . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . 134Cs (2.1 y) . 137Cs (30 y) . 134 + 137Cs . Vegetables (produced) 2018 179 <20a <145 <165 2.5 (2.5)b 5.0 (23) 7.5 (26) 2019 126 N.D.c <230 <233 2.5 (2.5) 2.5 (2.5) 5.0 (5.0) Edible wild plants (collected) 2018 159 <772 <6079 <6851 10 (57) 99 (495) 109 (550) 2019 85 <372 <4216 <4588 5.0 (30) 30 (276) 35 (297) Mushrooms (collected) 2018 11 <378 <3760 <4139 40 (141) 422 (1399) 463 (1540) 2019 6 <8467 <91 186 <99 653 27 (4251) 350 (45 811) 385 (50 054) Fruits (produced) 2018 277 <146 <1698 <1844 2.5 (2.5) 15 (70) 18 (72) 2019 190 <19 <228 <246 2.5 (2.5) 5.0 (42) 7.5 (44) Meatsd (captured) 2018 9 <243 32–1929 42–2195 29 (240) 381 (1935) 410 (2177) 2019 14 <48 34–854 44–864 24 (45) 319 (570) 351 (617) Seafoods (collected) 2018 12 N.D. <39 <39 2.5 (2.5) 5.0 (14) 7.5 (17) 2019 8 N.D. <97 <100 2.5 (2.5) 5.0 (67) 7.5 (70) Seeds (collected) 2018 26 <76 <763 <839 2.5 (2.5) 66 (119) 69 (127) 2019 12 <101 <1152 <1253 5.0 (49) 80 (652) 86 (701) Tubers (produced) 2018 35 <22 <81 <103 2.5 (2.5) 5.0 (23) 7.5 (25) 2019 31 N.D. <80 <83 2.5 (2.5) 2.5 (13) 5.0 (16) Other (produced/collected) 2018 22 N.D. <247 <247 2.5 (2.5) 20 (62) 23 (65) 2019 31 <16 <135 <151 2.5 (2.5) 5.0 (78) 7.5 (81) aMinimum–maximum or < maximum (including not detected data). bParentheses show 90th percentile. cNot Detected (detection limit: 10 Bq/kg-fresh at 134Cs and 10 Bq/kg-fresh at 137Cs). dBirds and boars captured as noxious wildlife (not for edible). Open in new tab Moreover, committed effective doses from local foods produced and/or collected in Tomioka Town are shown in Figure 4. We calculated the committed effective doses due to the intake of agricultural products (four food items) in Tomioka Town using Equation (3). The committed effective doses were 30–74 μSv/y for children and 62–100 μSv/y for adults (30–100 μSv/y for males and 32–95 μSv/y for females) in 2018, and 19–45 μSv/y for children and 39–62 μSv/y for adults (19–62 μSv/y for males and 19–59 μSv/y for females) in 2019 (Figure 4). Figure 4 Open in new tabDownload slide Committed effective doses calculated for inhabitants by age and sex, in 2018 (a) and 2019 (b) by agricultural products (vegetables, edible wild plants, mushrooms and fruits in order from the top). Figure 4 Open in new tabDownload slide Committed effective doses calculated for inhabitants by age and sex, in 2018 (a) and 2019 (b) by agricultural products (vegetables, edible wild plants, mushrooms and fruits in order from the top). DISCUSSION External radiation exposure In the present study, the ambient dose rates around the returned residents’ homes in the evacuation-order-lifted areas ranged from 0.084 to 0.48 μSv/h outdoors and from 0.12 to 1.2 μSv/h in the backyard (Table 1). According to environmental radioactivity and radiation surveys conducted by the government and municipal authorities in Japan, there was no significant change in radiation levels in the environment at least for several decades before the FDNPS accident, though artificial radionuclides derived from atmospheric nuclear testing elsewhere in the world were detected slightly(24). Immediately before the FDNPS accident, the ambient dose rates in Tomioka Town were monitored in the range of 0.035–0.074 μGy/h using a NaI (Tl) detector and ionization chamber during 1–10 March 2011, and doses in other areas around the FDNPS were also at low levels(24,41,42). After the FDNPS accident, however, 134Cs and 137Cs were detected in all surface soils collected in Tomioka Town by gamma spectrometry, and the 134Cs/137Cs ratios in these samples decreased from 0.092–0.093 in 2018 to 0.070–0.073 in 2019 (Table 2). According to other reports, the 134Cs/137Cs ratios in the environmental samples were estimated to be 0.9–1.1 in areas to the south and southwest of the FDNPS immediately after the accident(43,44). More than three or four times the half-life (2.1 y) of 134Cs has passed since the FDNPS accident, and it was confirmed that the 134Cs/137Cs ratios have been decreasing due to this short half-life of 134Cs. On the other hand, 137Cs (30 y) remains in the environment around FDNPS. In the present study, the radiocesium concentration gradually lowered over time in the surface soil (0–10 cm) in Tomioka Town (Table 2). Our previous investigations reported that there was a positive correlation between soil radioactivity and ambient dose rates and that removing surface soil through decontamination techniques was efficient(10). The main reason for the decreasing dose rates over time in Tomioka Town including the difficult-to-return zone is considered to be the various decontamination techniques applied, such as removing surface soil and deposits left on roofs, decks and gutters; weeding; high-pressure washing of walls and roads; and wiping off roofs(45). Therefore, it is suggested that the external radiation exposure doses among residents’ living spaces in Tomioka Town have been stably decreasing due to these decontamination countermeasures and the external exposure risk due to radiocesium driven from the FDNPS accident was extremely limited (Tables 1 and 3 and Supplementary Figures S3–S6). Our previous investigations also reported that an additional radiation exposure dose in Tomioka Town was estimated to be 1.6 mSv/y in 2017(10). In the present study, an additional radiation exposure dose for those living in the evacuation-order-lifted areas was estimated to be 1.1–1.4 mSv/y in 2018 and 2019 (Table 1). These values were close to the public dose limit and it was suggested that the external exposure dose was decreasing and the external exposure risk was obviously limited. In other words, the external exposure doses were the lower limit of the ‘existing exposure situations’ (1–20 mSv/y) in the recovery and reconstruction period after the FDNPS accident. Also, it was reported that the external exposure dose measurement of residents using individual radiation dosemeters was 0.46–1.6 mSv/y in Tomioka Town in 2019 and this result was extremely close to the estimated doses from the environmental monitoring in the present study(46). Thus, it is suggested that the estimation of external exposure using environmental radiation monitoring of living spaces worked well and played an important role in allowing residents to return to areas around the FDNPS. Even at low dose rates, however, health risks due to chronic radiation exposure have become a large concern among Japanese residents(47). Moreover, the ambient dose rates outside residents’ homes (the front entrance and backyard) and the external effective dose rates from soil samples in Tomioka Town showed a positive relationship (r ranged from 0.53 to 0.74, Supplementary Figure S7). These findings suggest that the environmental radiation doses were mainly derived from surface soil and areas around vegetation, including fallen leaves, because the decontamination of boundary areas including the area 20-m away from the living space and/or spots with relatively higher radiation doses (hot spots) such as backyards with overgrown plants may be limited(5). On the other hand, the ambient dose rates indoors and the external effective dose rates from soil samples or the ambient dose rates indoors and the ambient dose rates outdoors showed a negative relationship (r ranged from 0.15 to 0.36, Supplementary Figure S7). Additionally, the shielding effect of residents’ houses continued to be comparatively high because there were a lot of new houses with high airtightness in areas where restrictions were lifted in Tomioka Town (shielding factor ranged from 0.68 to 0.69, Table 1). Internal radiation exposure by ingestion Committed effective doses by ingestion were calculated from local foodstuffs in Tomioka Town. The internal exposure by the intake of some foods produced and/or collected in the local area (especially agricultural products such as vegetables, edible wild plants, mushrooms and fruits) is a subject of concern for residents in daily life. In the present study, radiocesium was not detected in many food samples, and 17.3–17.5% of all samples (<20%) exceeded 100 Bq/kg-fresh in 2018 and 2019, although radiocesium contamination above the standard limit (100 Bq/kg) was widespread in the edible wild plants in 2018 and mushrooms in 2018 and 2019 (Table 4). Moreover, radiocesium concentration that includes local food samples slightly decreased between 2018 and 2019 (Table 4). On the other hand, it is well known that wild mushrooms accumulate radiocesium(20,39,48). Additionally, the radiocesium concentration of mushrooms depends on the species and habitats vary between species(49). Our previous reports also suggested that the radiocesium concentration in some species such as Aralia elata, Eleutherococcus sciadophylloides and Osmunda japonica in Kawauchi Village, located next to Tomioka Town (Figure 1), remain higher than in other edible wild plants around the FDNPS(15,50). According to other reports, mycelia are distributed on the surface, and mycorrhizal fungi absorb radiocesium in the surface soil layer(51,52). However, residents who returned to their homes around the FDNPS have avoided unnecessary internal exposure due to radiocesium by following the food monitoring system(53). Although a difference in radiocesium concentration between sampling sites was not shown clearly in the present study (Supplementary Figures S8 and S9), it was suggested that differences depend on whether samples were produced by the resident or grown in specific sites. In other words, it depends on whether the sampling sites were decontaminated or not (Table 4). Additionally, in the present study, daily consumption of vegetables, edible wild plants, mushrooms and fruits slightly contributed to residents’ committed effective doses, but contaminated levels of radiocesium in foodstuffs produced and/or collected in Tomioka Town were much lower than the standard limit (>100 Bq/kg for radiocesium) or the public dose limit (1 mSv/y; Table 4 and Figure 4)(35). It was suggested that water-rich foods such as vegetables and fruit were not contaminated due to radiocesium driven from the FDNPS accident because most of the radiocesium in the environment exists in insoluble chemical form(23). On the other hand, the radiocesium concentration in wild edible plants and mushrooms contributed to committed effective doses because the excess rate against the standard limit was relatively higher than those of vegetables and fruits (Table 4 and Figure 4)(22,23). In the present study, moreover, it was suggested that dietary habits between generations (age groups) contributed to committed effective doses, and that sex was not always an important factor (Figure 4, Supplementary Tables S6–S8 and Supplementary Figures S8–S10)(39). Usually, dietary intake of vegetables, edible wild plants, mushrooms and fruits for people aged over 50 y are higher than that for those under 40 y(35). Interestingly, in a previous study, the majority (83.2%) of residents, especially men (68.2%), showed an internal body burden of radiocesium based on whole body counter (WBC) measurements, but they were not concerned about contaminated food products(54). In the present study, it was confirmed that the internal radiation exposure doses among older residents were comparatively higher than those of younger residents, according to differences in food consumption between generations. It was also shown that the committed effective doses calculated by Equation (3) were under 100 μSv/y (0.10 mSv/y) in 2018 and under 62 μSv/y (0.062 mSv/y) in 2019. Committed effective doses from the intake of vegetables, edible wild plants, mushrooms and fruits were calculated in the range of 19–74 μSv/y for children and 39–100 μSv/y for adults in 2018 and 2019. Although potent radiation exposure still exists, especially internal doses for adults, estimated internal exposure doses were far below 1 mSv (ICRP, 1991) (Table 4 and Figure 4)(30). In fact, specific radiation monitoring in the environment and the evaluation of radiation exposure doses have been extremely effective in areas affected by the FDNPS accident(55,56). Thus, a long-term follow-up study with detailed conditions is extremely important for residents to evaluate the external and internal exposure doses due to radiocesium in areas around the FDNPS. Limitations The present study has several limitations. First, the sampling sites of surface soil were limited, especially in the difficult-to-return zone (n = 5), because residential areas were investigated as much as possible to help those who want to return home in the evacuation-order-lifted areas. Additionally, in the present study, dosimetry was performed using an environmental monitoring survey. In our previous study in Tomioka Town, however, the estimated annual external dose was measured at 1.7 mSv in 2016 using a personal dosemeter(57). In another previous study, the annual additional external exposure in this town was estimated at 1.6 mSv in 2017 using the same methodology as the present study(10). These values were extremely similar. Therefore, it was confirmed that the validity of the present methodology was suitable for the evaluation of the external exposure doses from the ambient dose survey and soil sampling, although this calculation for external exposure may be conservative without considering physical attenuation of radionuclides or the transfer of radioactive materials by weathering effects such as wind and rain. Second, the fluctuation factors in ambient dose rates might result from radiocesium driven from surface soils being resuspended in the air with dust particles due to dump truck traffic performing decontamination work or meteorological events(45). Even in the evacuation-order-lifted area, however, it was clear that high concentrations of radiocesium driven from surface soils were not detected from dust samples based on the monitoring survey conducted by the municipal government(3). Thus, the possibility of internal exposure due to inhale radiocesium driven from soils is low. Therefore, in the present study, it was assumed that radiocesium from soil samples contributed to the external exposure only. Third, the sample size of local foods may be unbalanced because these samples were brought into the Tomioka Town Office by residents. Although follow-up investigations are needed to determine the future distribution of radiocesium with consideration of detailed conditions such as ecosystem components, hot spots and decontamination effects, this case of Tomioka Town is the first report to evaluate the external exposure doses around residents’ living spaces and internal exposure doses due to the intake of local foodstuffs containing radiocesium simultaneously for residents’ homes after the FDNPS accident. CONCLUSIONS In the present study, we evaluated current environmental contamination and external and internal exposure derived from the FDNPS accident in Tomioka Town, Fukushima Prefecture. These findings suggest that the current external doses are gradually decreasing compared to our previous studies, though ambient dose rates in backyards remain relatively high. Moreover, the current internal doses are sufficiently low compared to the public dose limit (1 mSv/y)(30). Overall, these findings suggest that current external and internal exposure doses due to radiocesium have been controlled at a low level in the evacuation-order-lifted areas. However, long-term follow-up investigations such as radiation monitoring in the environment, countermeasures for further decontamination in the difficult-to-return zone and forested areas, and restrictions on the intake of local foods are needed to reduce unnecessary radiation exposure for residents, because radiocesium derived from the FDNPS accident still exists in the environment in some areas around the FDNPS. Tomioka Town will serve as the model for returning residents with radiation exposure. ACKNOWLEDGEMENTS We would like to thank the staff of Tomioka Town Office for their assistance with our research plan and sample collection. FUNDING This work was supported by Research on the Health Effects of Radiation organized by the Ministry of the Environment, Japan. CONFLICT OF INTEREST STATEMENT None of the authors has any conflicts of interest to declare. References 1. The World Health Organization . Preliminary dose estimation from the nuclear accident after the 2011 Great East Japan Earthquake and Tsunami . Geneva : (WHO) ; ( 2012 ) https://apps.who.int/iris/bitstream/handle/10665/44877/9789241503662_eng.pdf;jsessionid=C67A8A30873EA8EC37484D20942B760C?sequence=1 (27 February 2021 accessed). 2. The United Nations Scientific Committee on the Effects of Atomic Radiation . Sources, effects and risks of ionizing radiation: United Nations Scientific Committee on the Effects of Atomic Radiation 2013 Report vol. 1 . 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TI -  LOCAL LEVELS OF RADIATION EXPOSURE DOSES DUE TO RADIOCESIUM FOR RETURNED RESIDENTS IN TOMIOKA TOWN, FUKUSHIMA PREFECTURE
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncab049
DA - 2021-05-17
UR - https://www.deepdyve.com/lp/oxford-university-press/local-levels-of-radiation-exposure-doses-due-to-radiocesium-for-eT820qmqz3
SP - 207
EP - 220
VL - 193
IS - 3-4
DP - DeepDyve
ER -