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Estimated association between dwelling soil contamination and internal radiation contamination levels after the 2011 Fukushima Daiichi nuclear accident in Japan
  1. Masaharu Tsubokura1,2,3,
  2. Shuhei Nomura4,
  3. Kikugoro Sakaihara2,
  4. Shigeaki Kato3,
  5. Claire Leppold2,
  6. Tomoyuki Furutani5,
  7. Tomohiro Morita3,
  8. Tomoyoshi Oikawa2,
  9. Yukio Kanazawa2
  1. 1Division of Social Communication System for Advanced Clinical Research, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo, Japan
  2. 2Department of Radiation Protection, Minamisoma Municipal General Hospital, Minamisoma, Fukushima, Japan
  3. 3Department of Radiation Protection, Soma Central Hospital, Soma, Fukushima, Japan
  4. 4Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London, Norfolk Place, London, UK
  5. 5Faculty of Policy Management, Keio University, Fujisawa, Kanagawa, Japan
  1. Correspondence to Dr Masaharu Tsubokura; tsubokura-tky{at}


Objectives Measurement of soil contamination levels has been considered a feasible method for dose estimation of internal radiation exposure following the Chernobyl disaster by means of aggregate transfer factors; however, it is still unclear whether the estimation of internal contamination based on soil contamination levels is universally valid or incident specific.

Methods To address this issue, we evaluated relationships between in vivo and soil cesium-137 (Cs-137) contamination using data on internal contamination levels among Minamisoma (10–40 km north from the Fukushima Daiichi nuclear power plant), Fukushima residents 2–3 years following the disaster, and constructed three models for statistical analysis based on continuous and categorical (equal intervals and quantiles) soil contamination levels.

Results A total of 7987 people with a mean age of 55.4 years underwent screening of in vivo Cs-137 whole-body counting. A statistically significant association was noted between internal and continuous Cs-137 soil contamination levels (model 1, p value <0.001), although the association was slight (relative risk (RR): 1.03 per 10 kBq/m2 increase in soil contamination). Analysis of categorical soil contamination levels showed statistical (but not clinical) significance only in relatively higher soil contamination levels (model 2: Cs-137 levels above 100 kBq/m2 compared to those <25 kBq/m2, RR=1.75, p value <0.01; model 3: levels above 63 kBq/m2 compared to those <11 kBq/m2, RR=1.45, p value <0.05).

Conclusions Low levels of internal and soil contamination were not associated, and only loose/small associations were observed in areas with slightly higher levels of soil contamination in Fukushima, representing a clear difference from the strong associations found in post-disaster Chernobyl. These results indicate that soil contamination levels generally do not contribute to the internal contamination of residents in Fukushima; thus, individual measurements are essential for the precise evaluation of chronic internal radiation contamination.

  • internal radiation exposure
  • whole body counter
  • transfer factor
  • countermeasure
  • Cs-134
  • Cs-137

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