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.

Masaharu Tsubokura,Yoshitaka Nishikawa,Claire Leppold,Shuhei Nomura,Shuhei Nomura,Tomohiro Morita,Shigeaki Kato,Michio Murakami,Masahiro Kami,Gayle E Woloschak

doi:10.1371/journal.pone.0172305

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.

Highlights

Radiation exposure can result in long-term health risks such as tumors, depending on personal doses.[1]
Individual external dose among children in Fukushima, Japan 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
The populations of residents aged less than 16 years in Soma City were 5,618, 5,350, 5,192, 5,113, and 5,002 from 2011 to 2015, respectively, and 37.0 to 71.5% of residents participated in the screenings each year

Summary

Introduction

Radiation exposure can result in long-term health risks such as tumors, depending on personal doses.[1]. In order to control doses of cumulative exposure following a major radiation-release accident, the reduction of chronic exposure, which accounts for the majority of the total radiation exposure, is of great importance.[2] Chronic exposure is classified as internal or external. While the total dose from both internal and external exposure is a crucial risk factor for radiation-related health outcomes,[1] there are several differences in characteristics between individual doses from internal and external exposure, including the distributions of doses, decreasing trends (i.e. half-life periods), high-risk behaviors for radiation exposure among residents in radioactively contaminated areas, and countermeasures such as food regulation and decontamination.[2] Internal contamination may continue for an extended period due to prolonged soil and subsequent food contamination.[4] The individual levels of internal contamination are strongly influenced by dietary habits, rather than by soil contamination levels in residential areas;[5, 6] food regulation and identification of high-risk residents who regularly consume highly contaminated local food products are of particular importance.[7]

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