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Following the Fukushima incident, radiation doses from external exposure accounted for the majority of the total doses. Although countermeasures are being implemented, with the aim of reducing radiation exposure, little information is available on the effects of decontamination on individual doses among the residents of radioactively contaminated areas. To evaluate the effectiveness of the decontamination measures in reducing individual doses, and to examine the influence of the timing of decontamination and the district, data were analysed for 18 392 adults and 3 650 children in Minamisoma City, Fukushima, who participated in a voluntary screening programme using individual radiation dosimeters (Glass Badge) between June 2013 and September 2016. The dose reduction rates (DRR) were calculated for one year by comparing the first and last three-month measurement results between areas with and without decontamination. Using a regression approach and Monte Carlo simulation, the dose reduction rate by decontamination eliminating the effect of physical decay (DRRd') was also estimated as a function of the timing of the decontamination and the dose at the time of starting the decontamination. The annual DRR in areas with decontamination for both adults and children were significantly higher than those in areas without decontamination, depending on the timing of decontamination: 31%-36% for 2013-14 for adults in decontamination areas and 33%-35% for children in decontamination areas, compared to 12%-23% and 13%-23% for adults and children in areas without decontamination, respectively. There was a positive correlation between DRRd' and individual doses, and DRRd' was estimated at 30%-40% for adults and children with doses of 3 mSv y-1 in 2013 and 2014. This study demonstrated that decontamination does lower individual doses from external exposure. The higher the dose at the time of starting the decontamination, the greater the dose reduction rate by decontamination, regardless of the timing of the decontamination. Our study confirms that decontamination was useful for high-dose areas in the later phases of the incident.

After the 2011 Fukushima Daiichi nuclear power plant accident, evidence on the real-life conditions of returnees to areas once designated as legal no-go zones, including their radiation dose levels, is scarce. In the present study, using a radiation dosimeter and lifestyle survey, we evaluated the lifestyle characteristics and dose levels in 2017 from external exposure among those who returned to the no-go zones after the evacuation orders were lifted. A total of 112 returnees to Odaka district, Minamisoma City, Fukushima Prefecture, were considered and compared to 266 non-returnees. The proportion of participants with annual additional doses from external exposure above 1 mSv was 7.0% for returnees, and 7.3% and 4.2% for non-returnees living in other districts or outside of the city, respectively. Although caution is required given the very small sample sizes, this implies that as of 2017 doses from external exposure among returnees in Odaka were very low and by scientific consensus would be associated with a very low likelihood of physical effects. We also found that while returnees were older on average than non-returnees, they had similar life conditions (i.e. occupation and time spent outdoors). It should be particularly emphasised that the expected lifetime doses from the incident in addition to the natural background dose are a very small among returnees. This study contributes to enhancing societal debates and risk communication regarding how government can provide returnees with the support they need, improve their outlook for radiation doses, and continue to improve crucial infrastructure in former no-go zones so that communities can be rebuilt.

Using the first WBC unit installed in Fukushima Prefecture after the accident, the radiocesium body burdens of 566 high-risk residents of Minamisoma city were measured in July 2011 at the Minamisoma Municipal General Hospital. The analysis of the data was challenging because this chair-type WBC unit did not have sufficient shielding against background gamma rays, and methods had to be developed to reliably compensate for the body-attenuated background radiation. Fortunately, data for repeated tests of hospital staff members using both the chair-type and well-shielded FASTSCAN WBC units, installed in September 2011, were available, and could be used to check the validity of the analysis. The CEDs of all subjects, estimated under the assumption of acute inhalation in March 2011, were found to be less than 1 mSv., Comment: 16 pages, 11 figures