Comparison of external doses between radio-contaminated areas and areas with high natural terrestrial background using the individual dosimeter 'D-shuttle' 75 months after the Fukushima Daiichi nuclear power plant accident.

After the Fukushima Daiichi nuclear power plant accident, the air dose gradually decreases every year due to the physical decay of radioactive materials and environmental changes, as well as countermeasures. However, there is little information on personal behavioural patterns and individual dose from external exposure among the inhabitants around the nuclear power plant. To evaluate the dose from external exposure in Minamisoma city, and compare the differences with outside Fukushima Prefecture, we started the external dose assessment project in cooperation with city officials in Minamisoma and three other cities in Japan where the natural terrestrial background radiation level is relatively high. In these four cities, external dose was measured every hour for two weeks using an individual electronic dosimeter D-shuttle. The places of activity of participants were recorded every hour to compare and evaluate the dose from external exposure, and to clarify whether there is a difference in the exposure dose by behaviour. The annual effective doses from external exposure for 100 participants from four municipalities ranged from 0.566 to 1.295 with a mean value of 0.784 mSv, which was below the level where it is necessary to initiate further remedial actions. Mean external dose in Minamisoma city (0.820 mSv/year) was comparable to those in municipalities with a relatively high natural radiation background in Japan (0.793, 0.806, and 0.718 mSv/year in Fukuyama, Nanto, and Tajimi, respectively). The time spent at home and in the workplace accounted for most of the time of the participants, and this also contributed to the majority of the total dose from external exposure. The amount of exposure at times other than while at home or in the workplace was very small regardless of the indoor or outdoor location in the city. For future dose reduction and radiation protection, continuous dosimetry and countermeasures at home and in the workplace are important for individuals who present high values.