International organizations such as the World Health Organization (WHO) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported public exposure doses due to radionuclides released in the Fukushima nuclear accident a few years after the event. However, the reported doses were generally overestimated due to conservative assumptions such as a longer stay in deliberate areas designated for evacuation than the actual stay. After these reports had been published, more realistic dose values were reported by Japanese scientists.
The present paper reviews those reports, including the most recently published articles; and summarizes estimated effective doses (external and internal) and issues related to their estimation.
External dose estimation can be categorized as taking two approaches—estimation from ambient dose rate and peoples’ behavior patterns—and measurements using personal dosimeters. The former approach was useful for estimating external doses in an early stage after the accident. The first 4-month doses were less than 2 mSv for most (94%) study subjects. Later on, individual doses came to be monitored by personal dosimeter measurements. On the basis of these measurements, the estimated median annual external dose was reported to be <1 mSv in 2011 for 22 municipalities of Fukushima Prefecture. Internal dose estimation also can be categorized as taking two approaches: estimation from whole-body counting and estimation from monitoring of environmental samples such as radioactivity concentrations in food and drinking water. According to results by the former approach, committed effective dose due to 134Cs and 137Cs could be less than 0.1 mSv for most residents including those from evacuated areas.
Realistic doses estimated by Japanese scientists indicated that the doses reported by WHO and UNSCEAR were generally overestimated. Average values for the first-year effective doses for residents in two affected areas (Namie Town and Iitate Village) were not likely to reach 10 mSv, the lower end of the doses estimated by WHO.
A large amount of radionuclides was released into environment due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, which caused internal and external exposure to the public. International organizations such as the World Health Organization (WHO) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported public doses due to this accident a few years after the event. However, these doses were generally overestimated, because they were based on conservative assumptions. WHO reported that adult residents of two affected areas (Namie Town and Iitate Village, their locations are shown in
The first-year doses for the same areas reported in the UNSCEAR 2013 Report [
Since the WHO and UNSCEAR 2013 Reports, more realistic dose values have been reported by Japanese scientists. Reviews on those studies have been published [
The other approach is to use personal dosimeters. In this approach, the proper use of personal dosimeters (i.e., wearing dosimeters all the time in principle) is important for accuracy of the measured dose. The personal dose equivalent, measured with personal dosimeters, obtained in the geometrical conditions of the affected areas in Fukushima Prefecture is known to be comparable with the effective dose of isotropic or rotational irradiation geometries [
In an early stage after the accident, the Japanese government adopted external dose estimation with two conservative assumptions that: (1) people stay 16 hours indoors and 8 hours outdoors; and (2) the conversion factor from ambient dose to effective dose equals one. Hereafter, this estimation method is called the “government model” [
In an early stage after the accident, personal dosimeters were not widely available for the public. As noted above, how occupancy factors (behavior patterns) are assumed is one of the influential factors for dose estimation from ambient dose rate. Some evacuees had complex behaviors including multiple moves. Therefore, obtaining records of individual behavior (such as post-disaster evacuation behavior) from residents, and combining this information with ambient dose rate was considered to estimate external dose.
This was conducted as the “Basic Survey”, a part of the Fukushima Health Management Survey [
Individual external doses for the first 4 months were estimated for about 475,000 persons. Although the maximum dose was 25 mSv, 94% of the doses were less than 2 mSv. The Basic Survey also revealed actual occupancy factors after the accident. In the case of Iitate Village, average time spent outdoors per day was around 2 hours [
The UNSCEAR 2013 Report estimated municipality-average doses using this approach for non-evacuated areas [
Dose estimation based on some typical behavior patterns by age group or occupation could give a more realistic dose than that estimated using a fixed pattern like 8 hours outdoors and 16 hours indoors. Takahara et al. [
Among the three factors affecting individual external dose assessment (
Even for the second point, the Basic Survey was considered to have a potential weakness: it relies on people’s memories for the behavior records and “recall bias” may affect the individual doses. However, a comparison between dose estimates based on behavior records collected before and those collected recently indicated that the effect of recall bias could be small [
Another issue is that the Basic Survey is a volunteer-based survey, while the dose assessment using typical behavior patterns can be applied to a whole population group. Due to this concern, representativeness of individual doses estimated by the Basic Survey was investigated [
Around a half year after the accident, measurements of external dose for residents by using integrating-type personal dosimeters (glass badges) were started on a large scale by local governments of Fukushima Prefecture. Because the measurements had an important aspect of risk communication, residents were notified of the estimated personal doses. Also, most of the municipalities have disclosed overall results to residents (e.g., Koriyama City [
Some scientific papers reported results for specific areas (Minamisoma City, Soma City, Nihonmatsu City). A paper on Soma City showed that the geometric mean levels of annual doses from external exposure decreased each year: 0.60 mSv, 0.37 mSv, 0.22 mSv, 0.20 mSv, and 0.17 mSv in 2011, 2012, 2013, 2014, and 2015, respectively [
Integrating-type personal dosimeters cannot identify the major contribution to received dose. Then, regarding this point, Nomura et al. [
As described previously, personal doses in the same areas generally decreased during the course of time (year-to-year), although the subjects for each year were not the same. In addition to physical decay of deposited radionuclides, weathering and decontamination could play a role in the decrease of personal doses. The UNSCEAR 2013 Report estimated 10-year and lifetime doses, but it did not consider effects of decontamination.
Although it was reported that decontamination decreased ambient dose rate [
A few years after the accident, the government started lifting evacuation orders in some areas. People began to return to or temporally stay in their hometown in such areas. Some studies reported personal dosimeter measurements made by such returnees. In the case of Kawauchi Village, the maximum cumulative individual dose was 3.28 mSv per year, and the median and minimum doses were 1.35 and 0.71 mSv per year for those who returned in 2013 [
Although glass badges are useful in large-scale personal monitoring, their disadvantages are that (1) they generally need a measurement period of a few months and (2) it is not possible to determine when and where the person is exposed (at home, at work, etc.). A personal dosimeter, D-shuttle, overcoming these disadvantages was developed in 2013 by AIST (National Institute of Advanced Industrial Science and Technology) and it was made commercially available by Chiyoda Technol Corporation [
Studies using the D-shuttle in combination with GPS systems allowed identification of exposure levels, locations, and times. In particular, it was possible to find the dominant exposure in the total exposure. Identifying source contributions to the total dose is important in determining effective dose reduction measures. In this respect, it has been broadly used for risk communication in Fukushima Prefecture [
As described previously, personal doses for hometown returnees were obtained using integrating-type personal dosimeters. Monitoring for those people also has been done using the D-shuttle [
Since personal dosimeters have come into large-scale use, it has become necessary to give the public a reasonable explanation for the discrepancy between external dose estimated by the government model and that estimated with personal dosimeters. Related to any explanation would be the consideration that airborne monitoring can grasp the distribution of air dose rate widely and quickly, while individual dose estimation by personal dosimeters takes time in the case of glass badges and future doses after returning to former evacuation areas cannot be predicted. Then, it would be useful for a rapid understanding of individual dose levels, if individual dose levels are estimated from airborne monitoring results. In particular, it would be useful for those who wish to return to former evacuation areas.
Thus, the relationship between ambient dose rate and personal dose has been studied by several groups of investigators. Nomura et al. [
Naito et al. [
When interpreting results obtained with personal dosimeters including the D-shuttle, several points need to be considered: (1) participant selection bias, (2) conditions of use, and (3) background dose.
First, the personal dose measurement conducted by local governments of Fukushima Prefecture was basically volunteer-based. Also, soon after the accident, the main targets for personal dosimeter measurements were children and pregnant women [
The second point affects measured values, and it is related to whether or not personal dosimeters were wore in a proper manner. Nomura et al. [
Third, it is difficult to estimate an accurate background dose (the dose due to natural radiation) for each subject. The background dose could differ from place to place, but discriminating the natural radiation dose from the dose due to the accident needs a spectroscopic measurement at each place [
Internal dose estimation can be categorized into two approaches: estimation from personal monitoring and estimation from monitoring of environmental samples such as radioactivity concentration in food and drinking water (
Immediately after the accident, no whole-body counters were available in Fukushima Prefecture. Thus, initially whole-body counters located outside the prefecture were utilized for measuring 134Cs and 137Cs body contents for residents from Fukushima Prefecture.
Morita et al. [
The Fukushima Prefecture government organized WBC measurements by a committed institute, the National Institute of Radiological Sciences, which started at the end of June 2011 [
Another committed institute, the Japan Atomic Energy Agency (JAEA), started WBC measurements on July 11, 2011. A total of 9,927 subjects were measured until the end of January 2012. Most of these subjects were residents of evacuated areas. The median CED values were 0.02 mSv and 0.025 mSv for subjects aged 13–17 years and for subjects aged >17 years, respectively [
Several months after the accident, installation of whole-body counters in Fukushima Prefecture was started. Some of them were a mobile type. The installed whole-body counters were operated either by (i) Fukushima Prefecture including its commissioned organizations such as JAEA; (ii) local governments (municipalities); or (iii) organizations independent of the prefectural or local governments. Data on measurements conducted by operators of category (i) have been collected by the Fukushima Prefectural government and they are periodically reported on webpages of Fukushima Prefecture [
Some results obtained by operators for categories (ii) and (iii) have been reported as scientific papers: these papers included subjects of Minamisoma City [
As mentioned before, personal dosimeters were utilized for estimating external doses for returnees to the former evacuation order areas. Most municipalities in such areas organize opportunities for persons to undergo WBC so that they can check their own internal contamination, if they wish. WBC results for returnees to Kawauchi Village were reported by Tsubokura et al. [
The UNSCEAR 2013 Report adopted this approach. It estimated ingestion dose based on a food database with some conservative assumptions. For example, many 134Cs and 137Cs concentrations in the database were shown as below the limits of detection and in these cases, it was generally assumed that 134Cs and 137Cs concentrations were each 10 Bq·kg−1. The 2013 Report estimated inhalation dose based on atmospheric transport dispersion and deposition models for radioactive materials released from the FDNPP accident. The first-year dose due to ingestion was estimated to be 0.94 mSv for adults in non-evacuated areas. The first-year dose due to inhalation was estimated to be from 0 to 0.47 mSv for adults in non-evacuated areas, depending on location. Thus, the municipality-average first-year dose due to internal exposure ranged from 0.94 to 1.41 mSv for adults. Comparison with the WBC results mentioned in previous section indicates the dose was likely to be overestimated.
Actually, restriction orders for food supplies such as contaminated vegetables and milk, and intake of tap water were implemented within several days after the major release of radionuclides on March 15, 2011 [
Harada et al. [
In this general situation that ingestion of highly contaminated food was not likely to occur, attention should be directed to residents who consume homegrown produce without radiation inspection, and who often collect wild mushrooms or cultivate their own mushrooms on bed-logs. Tsubokura et al. [
Technical issues raised for internal dose assessment by WBC of Fukushima Prefecture residents were well summarized by Kurihara et al. [
The first issue is associated with how to estimate the intake amount from 134Cs and 137Cs body content (see
Kunishima et al. [
On the other hand, Nomura et al. [
As mentioned in previous subsection, “interpretation of personal dose measurement results”, representativeness of subjects is one of the concerns about volunteer-based monitoring. Representativeness of WBC subjects was investigated by Nomura et al. [
Doses reported by the publications reviewed in the present paper were summarized in
Internal doses were basically shown as CEDs due to intake within the periods of interest. The CEDs in the reviewed publications were calculated based on either acute or chronic intake scenario. The scenario adopted for each publication is shown in
The different methodologies shown in these tables can be compared in the following way. Regarding external dose estimation, personal dosimeters were not available on a large scale for up to around 6 months from March 2011. Thus, external dose up to then had to be estimated from ambient dose rate. As described before, Ishikawa et al. [
As described in previous section, internal dose estimation can be categorized into two approaches: estimation from personal monitoring (WBC) and estimation from monitoring of environmental samples such as radioactivity concentration in food and drinking water. If the latter approach is adopted with conservative assumptions, it results in overestimation of dose. This was the case for UNSCEAR and WHO’s dose estimations. That is, WHO assumed that all the food monitored was on the market although the monitoring data set included the results of food samples that were collected for monitoring purposes and were not allowed on the market [
The doses based on the most reliable methodology discussed above can be used to estimate the first-year effective doses in the following way. As examples of two affected areas, Namie Town and Iitate Village, effective doses for the first year can be inferred as follows: as the first four-month doses, the Basic Survey results showed that average doses for residents in the two affected areas were around 4 mSv and 1 mSv for all age groups, respectively [
Thus, the average values for the first-year effective doses for residents in the two affected areas are not likely to reach 10 mSv, the lower end of doses estimated by the WHO’s first report appearing in 2012. They would be even lower than the district averages of the UNSCEAR estimations (7.8–8.0 mSv for adult evacuees in Iitate Village and 5.0–7.0 mSv for adult evacuees in Namie Town, depending on evacuation routes) [
For non-evacuated areas, the maximum municipality-average first-year effective dose could be around 3 mSv (4-month external dose, 1.5 mSv; the subsequent 8-month external dose, 1.5 mSv; internal dose due to 134Cs and 137Cs, less than 0.1 mSv). The maximum dose of 3 mSv due to external exposure for non-evacuated areas was consistent with that estimated by the UNSCEAR 2013 Report (adults). The municipality-average internal doses for the first year estimated by UNSCEAR (0.94–1.41 mSv, adults in non-evacuated areas) must be overestimated.
Since the accident, individual external dose levels in inhabitant areas have changed due to human factors as well as radioactive decay and weathering. For example, (1) decontamination of soil has been conducted in the inhabitant areas in Fukushima Prefecture and (2) some residents in former evacuation areas have begun to return to their hometown. Also, people’s daily behaviors (occupancy factors) may have changed (or returned to their normal behaviors before the accident) over time. Due to these human factors, individual external doses in a rehabilitation phase cannot be correctly predicted only by model calculations [
Some of the personal dosimeter/WBC measurements conducted in Fukushima Prefecture (e.g., [
No potential conflict of interest relevant to this article was reported.
Locations of municipalities mentioned in the present review.
Two approaches for external dose estimation.
Two approaches for internal dose estimation.
Doses Estimated for Periods within 1 Year after the Accident
Methodology | Reference | Target population | Additional doses by area (mSv) | Remarks | ||||||
---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||
Statistical parameter | Namie | Iitate | Other evacuated areas | Non-evacuated areas | Detailed places (if simply described) | |||||
External | Ambient dose rates and actual behavior patterns | Ishikawa et al. [ |
All age groups | Average | 1 | 4 | 1 | 0.1–1.5 | - | First 4-month dose |
Ambient dose rates and assumed behavior patterns | UNSCEAR [ |
Adults (20 yr) | Average | - | - | - | 0–3.0 | - | First-year dose | |
Children (10 yr) | Average | - | - | - | 0–4.3 | - | ||||
Infants (1 yr) | Average | - | - | - | 0–5.0 | - | ||||
Personal dosimeter | Kamiya et al. [ |
No description | Median | - | - | - | <1 | 22 municipalities | A few-month doses measured from 6 months (or later) after the accident were converted to annual doses | |
Tsubokura et al. [ |
Children (<16 yr) | Geometric mean | - | - | - | 0.6 | Soma | |||
Fujimura et al. [ |
Children (0–15 yr) | Average | - | - | - | 1.5 | Nihonmatsu | |||
| ||||||||||
Internal | Whole-body counting | Morita et al. [ |
Adults | 90th percentile | 0.06 | Various places in Fukushima | CED based on acute intake scenario | |||
Kim et al. [ |
Adults (>17 yr) | 90th percentile | 0.12 | 0.085 | 0.07 | - | - | |||
Children (<18 yr) | 90th percentile | 0.12 | 0.095 | - | - | |||||
Momose et al. [ |
Children (13–17 yr) | Median | 0.02 | - | - | |||||
Adults (>17 yr) | Median | 0.025 | - | - | ||||||
Tsubokura et al. [ |
Adults and children (>5 yr) | Maximum | - | - | 1.07 | - | Minamisoma | CED based on acute intake scenario for adults and chronic intake scenario for children | ||
Hayano et al. [ |
Adults and children (>15 yr) | Maximum | - | - | <1 | - | Minamisoma | CED based on acute intake scenario | ||
Atmospheric dispersion modeling and food database | UNSCEAR [ |
Adults (20 yr) | Average | - | - | - | 0.94–1.41 | - | First-year dose | |
Children (10 yr) | Average | - | - | - | 1.16–1.94 | - | ||||
Infants (1 yr) | Average | - | - | - | 1.90–2.82 | - | ||||
| ||||||||||
Internal (ingestion) | Food and water analysis | Harada et al. [ |
Adults | Median | - | - | - | 0.023 | - | CED due to 1-year chronic ingestion |
Koizumi et al. [ |
Adults | Median | - | - | - | 0.003 | - | |||
Sato et al. [ |
Adults | Maximum | - | - | - | <0.1 | - | |||
| ||||||||||
Total | External dose: ambient dose rates and assumed behavior patterns. Internal dose: atmospheric dispersion modeling and food database | WHO [ |
All age groups | Range | 10–50 | 10–50 | 1–50 | 1–10 | - | First-year dose |
WHO [ |
All age groups | Range | 12–25 | 12–25 | 1–5 | 1–5 | - | |||
UNSCEAR [ |
Adults (20 yr) | Average | 5.0–7.0 | 7.8–8.0 | 1.1–9.3 | - | - | |||
Children (10 yr) | Average | 7.0–8.9 | 8.7–9.0 | 1.3–10.2 | - | - | ||||
Infants (1 yr) | Average | 8.8–11.1 | 11.2–11.5 | 1.6–13.1 | - | - |
UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation; CED, committed effective dose; WHO, World Health Organization.
Doses Estimated for Periods after the First Year
Methodology | Reference | Target population | Additional doses by area (mSv, year of measurement) | |||||
---|---|---|---|---|---|---|---|---|
| ||||||||
Statistical parameter | Evacuated areas | Non-evacuated areas | Detailed places (if simply described) | Remarks | ||||
External | Personal dosimeter | Tsubokura et al. [ |
Children (<16 yr) | Geometric mean | - | 0.37–0.17 (2012 to 2015) | Soma | Annual additional dose |
Tsubokura et al. [ |
Children (6–15 yr) | Median | 0.66 (2012 to 2013) | - | Minamisoma | |||
Fujimura et al. [ |
Children (0–15 yr) | Average | - | 1.5–0.65 (2012 to 2014) | Nihonmatsu | |||
Orita et al. [ |
Adults | Median | 1.35 (2013) | - | Kawauchi | Annual dose (background dose is included) | ||
Nomura et al. [ |
All age groups | Median for returnees | 0.4 (2017) | - | Minamisoma | Annual additional dose | ||
Nomura et al. [ |
Adults | Average | 0.93 (2019) | - | 10 municipalities | Annual dose (background dose is included) | ||
| ||||||||
Internal (ingestion) | Whole-body counting | Fukushima Prefecture [ |
All age groups (>3 yr) | Maximum | <1 (2012 to present) | <1 (2012 to present) | All areas of Fukushima | CED due to 1-year chronic ingestion except for Orita et al. [ |
Orita et al. [ |
All age groups (>0 yr) | Maximum | - | 0.06 (2012 to 2013) | Iwaki | |||
Akiyama et al. [ |
All age groups (>3 yr) | Maximum | - | 0.029 (2012 to 2014) | Iwaki | |||
Hosokawa et al. [ |
All age groups | Average for detected persons | 0.025 (2012 to 2015) | - | Namie | |||
Hayano et al. [ |
Children (6–15 yr) | Maximum | - | <0.04 (2012 to 2013) |
Miharu | |||
Hayano et al. [ |
Children and infants (0–11 yr) | Maximum | <0.016 (2013 to 2015) | <0.016 (2013 to 2015) | Koriyama, Iwaki, Minamisoma, etc. | |||
Tsubokura et al. [ |
Adults | Median for detected persons | 0.011 (2012 to 2013) | - | Kawauchi | |||
| ||||||||
Internal (ingestion) | Food and water analysis | Sato et al. [ |
Adults | Maximum | - | <0.1 (2012) | All areas of Fukushima | CED due to 1-year chronic ingestion |
Orita et al. [ |
All age groups | Range | 0.02–0.04 (2013 to 2014) | - | Kawauchi |
CED, committed effective dose.
Orita et al. [
Namie residents were evacuated to Nihonmatsu City and measurement was done there.
CED due to 137Cs only.