Acute Intake of Radionuclides Immediately After the Incident as the Main Contributor of the Internal Radiation Exposure After Fukushima Daiichi Nuclear Disaster

The Fukushima Daiichi nuclear disaster is a series of equipment failures, nuclear meltdowns, and release of radioactive materials, raising serious health concerns in nearby residents. We recently reported that levels of internal radiation exposure (IRE) by radioactive cesium in Minamisoma, Fukushima were low¹; however, it is not as certain whether the detected levels of exposure were due to low ongoing exposure or decay from high-exposure values. To address this issue, transition of IRE among Minamisoma residents was monitored for children younger than 16 years, with biological half-lives of cesium in children up to age 15 years described as less than 93 days.²

The institutional review board of the Institute of Medical Science, University of Tokyo, approved the study, and written informed consent was obtained from all participants. A voluntary screening program for cesium exposure, cesium 134 and cesium 137, known to be representative of total IRE,³ started on September 26, 2011, for all Minamisoma residents 6 years or older using a whole-body counter (FASTSCAN model 2251; Canberra Inc). The monthly percentages of people with positive cesium exposure were calculated between September 2011 and September 2012.

A total of 3992 children (1975 girls, 49.5%) were enrolled in the study, accounting for 66% of the corresponding registered population of Minamisoma in 2010. Among them, 2831 children (71%) lived in Minamisoma at the time of the examinations. The median age was 11 years (range, 6-15 years), and 325 individuals (8.1%) had detectable levels of cesium but not the other radionuclides. The monthly detection rates of cesium are displayed in the Figure. There was a clear declining trend from September 2011 (57.5%) to September 2012 (0.0%) and sustained zero after June 2012. While total cesium exposure was converted into committed effective dose (in sieverts) based on the assumption of chronic cesium ingestion after the disaster, committed effective doses were less than 1 mSv in all participants.

This study demonstrates that levels of chronic IRE in children are marginal and vanishing 1 year after the nuclear accident. Such rapid disappearance appears attributable to the fact that the Japanese government took quick action to restrict the circulation of foods with confirmed or suspected contamination.⁴ On March 21, 2011, 10 days after the disaster, the Japanese government announced a ban on sales of spinach and raw milk from Fukushima and neighboring prefectures.⁵ Subsequently, the local governments, volunteers, and farmers conducted thorough radioactivity checks on food before shipment. These measures appear successful in reducing the chronic IRE at the moment; however, to avoid the risk of recurrent IRE by unknowingly eating contaminated food, long-term measures including continuous monitoring, individually targeted interventions for those identified at high risk of IRE, and public education are warranted.

Also, this study showed that the detected cesium was mainly derived from the acute intake of radionuclides immediately after the incident. Acute IRE is much more significant than measurable chronic IRE after a certain period in the health risk assessment of the nuclear disaster.

It is important to calculate the internal radiation doses immediately after the incident to predict future health risks, particularly for younger generations; however, we had lost the opportunity to obtain actual values of IRE at the acute stages. Although committed effective dose might be predictable based on the assumed scenarios using available environmental monitoring data or the results of in vivo measurements performed after a certain period from the incident, these estimations could vary with methods and the assumption.⁶ Our case clearly demonstrates the necessity of personal in vivo measurements as soon as possible after the nuclear disaster to make an accurate prediction for future health risks of radiation.

Corresponding Author: Masaharu Tsubokura, MD, Division of Social Communication System for Advanced Clinical Research, The Institute of Medical Science, University of Tokyo, 4-6-1, Shirokanedai, Minato-ku, Tokyo 108-8639, Japan (tsubokura-tky@umin.ac.jp).

Published Online: October 7, 2013. doi:10.1001/jamapediatrics.2013.2311.

Author Contributions: Dr Tsubokura had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Tsubokura, Shibuya, Kato, Oikawa.

Acquisition of data: Tsubokura, Oikawa, Kanazawa.

Analysis and interpretation of data: Tsubokura, Shibuya, Oikawa, Kanazawa.

Drafting of the manuscript: Tsubokura, Shibuya, Kato.

Critical revision of the manuscript for important intellectual content: All authors.

Statistical analysis: Tsubokura, Shibuya.

Administrative, technical, or material support: Tsubokura, Oikawa, Kanazawa.

Study supervision: All authors.

Conflict of Interest Disclosures: None reported.

Additional Contributions: We thank Kyohei Takahashi, MD, Hara-machi Central Obstetrics and Gynecology Clinic, for invaluable efforts in the installation of the whole-body counter, Masahiro Kami, MD, PhD, The Institute of Medical Science, University of Tokyo, for the design and analysis of the study, Ryugo Hayano, PhD, Department of Physics, University of Tokyo, for the radiation physics review, and all measurement staff at Minamisoma Municipal General Hospital.