Individual external doses below the lowest reference level of 1 mSv per year five years after the 2011 Fukushima nuclear accident among all children in Soma City, Fukushima: A retrospective observational study

Abstract

After the 2011 Fukushima Daiichi nuclear power plant accident, little information has been available on individual doses from external exposure among residents living in radioactively contaminated areas near the nuclear plant; in the present study we evaluated yearly changes in the doses from external exposure after the accident and the effects of decontamination on external exposure. This study considered all children less than 16 years of age in Soma City, Fukushima who participated in annual voluntary external exposure screening programs during the five years after the accident (n = 5,363). In total, 14,405 screening results were collected. The median participant age was eight years. The geometric mean levels of annual additional doses from external exposure attributable to the Fukushima accident, decreased each year: 0.60 mSv (range: not detectable (ND)-4.29 mSv), 0.37 mSv (range: ND-3.61 mSv), 0.22 mSv (range: ND-1.44 mSv), 0.20 mSv (range: ND-1.87 mSv), and 0.17 mSv (range: ND-0.85 mSv) in 2011, 2012, 2013, 2014, and 2015, respectively. The proportion of residents with annual additional doses from external exposure of more than 1 mSv dropped from 15.6% in 2011 to zero in 2015. Doses from external exposure decreased more rapidly than those estimated from only physical decay, even in areas without decontamination (which were halved in 395 days from November 15, 2011), presumably due to the weathering effects. While the ratios of geometric mean doses immediately after decontamination to before were slightly lower than those during the same time in areas without decontamination, annual additional doses reduced by decontamination were small (0.04-0.24 mSv in the year of immediately after decontamination was completed). The results of this study showed that the levels of external exposure among Soma residents less than 16 years of age decreased during the five years after the Fukushima Daiichi nuclear power plant accident. Decontamination had only limited and temporal effects on reducing individual external doses.

Fig 1. Soma City divided into four zones.

Soma City is divided into eight administrative districts; as soil contamination levels differ by district, decontamination work has been performed in multiple areas. Zones 1, 2, 3, and 4 correspond to Yamakami, Yawata, Tamano districts and the Akagi area of Nittaki-district in Soma City, respectively.

Fig 2. Trends in the annual additional doses from external exposure among children in Soma City.

(a) Distribution; (b) geometric mean. The percentage values represent the ratio of the geometric mean dose (95% CI) in each year to that in 2011.

Fig 3

Ratios of the geometric mean doses from external exposure in individual zones compared to 2011 (a) or the previous year (b); compared to doses estimated from the physical decay of radioactive Cs. The error bars represent the 95% CI.

Fig 4. Ratios of geometric mean doses after decontamination compared to those before.

* P < 0.05, *** P < 0.001. The top and bottom of the error bars represent the 95% confidence intervals.