Basic Survey

Implementation Status

Status of estimated external radiation doses

As of March 31, 2023

  • Number of eligible persons: 2,055,236
  • Number of responses: (Cumulative total from FY2011 to FY2022: 569,438) 250 in FY2022
  • Number of results fed back: (Cumulative total from FY2011 to FY2022: 555,028) 287 in FY2022

Estimated external radiation doses

  • External exposure doses have been estimated for about 467,000 respondents, excluding radiation workers. The results suggest that 99.8% of respondents received <5mSv and the maximum value was 25mSv.

Distribution of the number of respondents in the whole prefecture, by estimated dose

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Estimated dose distribution by region

Effective Dose
(mSv)		 Total Excluding radiation workers Estimated external radiation doses (preliminary and full-scale surveys)
Kenpoku
(Note 1)		 Kenchu Kennan Aizu Minami-aizu Soso
(Note 2)		 Iwaki
< 1 296,555 290,823 62.2% 93.8% 99.8% 24,987 20.0% 58,602 51.5% 26,430 88.2% 46,300 99.3% 4,987 99.3% 55,909 77.3% 73,608 99.1%
< 2 149,998 147,648 31.6% 83,963 67.0% 46,476 40.8% 3,516 11.7% 311 0.7% 37 0.7% 12,708 17.6% 637 0.9%
< 3 26,182 25,808 5.5% 5.8% 15,740 12.6% 8,304 7.3% 18 0.1% 25 0.1% 0 － 1,691 2.3% 30 0.0%
< 4 1,587 1,504 0.3% 473 0.4% 429 0.4% 0 － 1 0.0% 0 － 597 0.8% 4 0.0%
< 5 551 505 0.1% 0.2% 40 0.0% 5 0.0% 0 － 0 － 0 － 459 0.6% 1 0.0%
< 6 442 390 0.1% 0.2% 19 0.0% 3 0.0% 0 － 0 － 0 － 367 0.5% 1 0.0%
< 7 270 231 0.0% 0.1% 10 0.0% 1 0.0% 0 － 1 0.0% 0 － 219 0.3% 0 －
< 8 155 116 0.0% 1 0.0% 0 － 0 － 0 － 0 － 115 0.2% 0 －
< 9 118 78 0.0% 0.0% 1 0.0% 0 － 0 － 0 － 0 － 77 0.1% 0 －
< 10 73 41 0.0% 0 － 0 － 0 － 0 － 0 － 41 0.1% 0 －
< 11 70 37 0.0% 0.0% 0.0% 0 － 1 0.0% 0 － 0 － 0 － 36 0.0% 0 －
< 12 52 30 0.0% 1 0.0% 0 － 0 － 0 － 0 － 29 0.0% 0 －
< 13 37 13 0.0% 0.0% 0 － 0 － 0 － 0 － 0 － 13 0.0% 0 －
< 14 36 12 0.0% 0 － 0 － 0 － 0 － 0 － 12 0.0% 0 －
< 15 27 6 0.0% 0.0% 0 － 0 － 0 － 0 － 0 － 6 0.0% 0 －
≥ 15 323 14 0.0% 0.0% 0 － 0 － 0 － 0 － 0 － 14 0.0% 0 －
Total 476,476 467,256 100.0% 100.0% 100.0% 125,235 100% 113,821 100% 29,964 100% 46,638 100% 5,024 100% 72,293 100% 74,281 100%
Maximum 66mSv 25mSv 　 　 　 11mSv 　 10mSv 　 2.6mSv 　 6.0mSv 　 1.9mSv 　 25mSv 　 5.9mSv 　
Mean 0.9mSv 0.8mSv 　 　 　 1.4mSv 　 1.0mSv 　 0.6mSv 　 0.2mSv 　 0.1mSv 　 0.7mSv 　 0.3mSv 　
Median 0.6mSv 0.6mSv 　 　 　 1.4mSv 　 0.9mSv 　 0.5mSv 　 0.2mSv 　 0.1mSv 　 0.5mSv 　 0.3mSv 　

(Note 1) Including Yamakiya District of Kawamata Town, where a preliminary survey was conducted.
(Note 2) Including Namie Town and Iitate Village, where a preliminary survey was conducted.

*Percentages have been rounded and may not total to 100%.
*Excluding those with an estimation period of less than four months.

Summary

Evaluation of the results

Since previous epidemiological studies* indicate no significant health effects at doses ≤100 mSv, we concluded that radiation doses estimated so far are unlikely to cause adverse effects on health, although this conclusion is based on external exposure doses estimated only for the first four months following the accident.

* Sources and Effects of Ionizing Radiation, United Nations Scientific Committee on the Effects of Atomic Radiation; UNSCEAR 2008 Report to the General Assembly, Vol. 2 (Japanese-language version); National Institute of Radiological Sciences

Results of the Examination on the representativeness of dose distribution

In FY2015, a comparison was made between estimated doses for people selected at random from among those who had not responded to the Basic Survey (about 3,000 people) and estimated doses for people who had responded to the Basic Survey earlier in the same districts. As a result, it was confirmed that dose levels were almost the same for both groups and that the dose distribution compiled and publicized so far well represents the situation in the prefecture as a whole.

Significance of the survey

By aggregating the doses obtained from the Basic Survey, it was possible to ascertain the level of exposure of the entire population of the prefecture due to the accident, and to provide scientific evidence as to whether or not the level of radiation was high enough to cause direct health effects.

Outline of the Basic Survey

Purpose

In consideration of radiation from the accident at TEPCO's Fukushima Daiichi NPS, caused by the Great East Japan Earthquake, the survey commenced with the aim of estimating external exposure doses of Fukushima residents based on self-reported location and activites, informing them of the results thereof, and linking such data to the improvement and maintenance of their health into the future.

Eligible persons

Within Fukushima Prefecture

  • Those who were registered as residents in Fukushima Prefecture between March 11 and July 1, 2011

Outside Fukushima Prefecture

  1. Those who lived in the prefecture between March 11 and July 1, 2011, but whose resident registration was outside the prefecture
  2. Those who lived outside the prefecture and commuted to work or school in the prefecture between March 11 and July 1, 2011
  3. Those who temporarily stayed in the prefecture between March 11 and March 25, 2011

For those who fall into categories 1 to 3 above, questionnaires sent upon their request.

Survey method

Self-administered. Estimated doses will be sent to each individual who has returned the questionnaire response. There are two types of questionnaires as follows.

Type Conditions
Detailed Version Those who changed their behavior patterns (i.e., changing residences, schools, or workplaces due to evacuation) more than twice in the four months after the earthquake, or could hardly determine their exact behaviors.
Simplified Version Only for those who changed their behavior patterns (i.e., changing residences, schools, or workplaces due to evacuation) only once or less in the four months after the earthquake.

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