February 25, 2014
From left, Prof. Koizumi, Prof. Ishikawa, Associate Professor Kouji Harada (Graduate School of Medicine), Researcher Tamon Niisoe (Disaster Prevention Research Institute)
Research group lead by Professor Akio Koizumi (Graduate School of Medicine) and Prof. Hirohiko Ishikawa (Disaster Prevention Research Institute) conducted a study, evaluating average radiation dose among municipal residents near the evacuation area of the Fukushima Daiichi Nuclear Power Plant. The survey performed exposure assessment based on personal monitoring of external and internal radiation dose among residents. Special endeavor was made to evaluate long-time dose over time of the next few decades and predict quantitatively cancer risk.
The result of this research was published in the electronic version of "Proceedings of the National Academy of Sciences of the United States of America" (February 24, 2014 U.S. Eastern Standard Time).
Comments from researchers
According to the radiological decay of radiocesium during next 10 years and 50 years, the average annual exposure is expected to be less than 1 millisievert per year, which is the ordinary exposure limit excluding medical and natural radiation exposure. In areas with high radiation exposure (2.51 millisievert per year in 2012), average risk of cancer incidence estimated from lifetime exposure dose of 2012 or later, is 1.06% for all solid cancers (other than leukemia) and 0.28% for breast cancer in women. However, continuous governing entrances to contaminated forest, distribution of contaminated food, and assignment of the hazardous zones is still needed to reduce radiation exposure of the population and thereby reducing cancer risk.
The novelty of this study includes:
- Evaluation of the external exposure and internal exposure based on personal monitoring and food duplicate method, respectively.
- Projection of the long-term exposure levels in next decades.
- Quantification of the cancer risk.
Summary
Average radiation dose of residents living outside the evacuation zone of the Fukushima Daiichi Nuclear Power Plant (Kawauchi village of Futaba county, Tamano area of Soma city, and Haramachi area of Minamisoma city of Fukushima prefecture), ranged from 0.89 millisievert per year and 2.51 millisievert per year in 2012 (taking into account of the physical decay of radioactive cesium). The dose was close to annual exposure dose due to natural radiation in Japan, 2 millisievert.
Radioactive substances were released into the environment by the nuclear accident in March 2011, and local residents near the power plant were evacuated. In this study, external exposure caused by radiation from radioactive cesium in the soil and internal exposure received from radioactive cesium in airborne dust and diet were investigated among residents of three regions which are located at 20km to 50km from the Fukushima Daiichi Nuclear Power Plant, and have borders on the evacuation area. The survey was conducted between August-September, 2012. Study participants wore personal dosimeters to survey external dose and provided food duplicate samples consisting of 24-hour meal and beverage. Airborne dust samples were collected in each area. Among three study areas, average external exposure ranged from 1.03 millisievert per year to 2.75 millisievert per year, internal exposure from the diet ranged from 0.0058 millisievert per year 0.019 millisievert per year, and internal exposure from the airborne dust was less than 0.001 millisievert per year. As a result, external exposure accounted for 99.5% or more of individual exposure.
Paper Information
[DOI] http://www.pnas.org/cgi/doi/10.1073/pnas.1315684111
Kouji H. Harada, Tamon Niisoe, Mie Imanaka, Tomoyuki Takahashi, Katsumi Amako, Yukiko Fujii, Masatoshi Kanameishi, Kenji Ohse, Yasumichi Nakai, Tamami Nishikawa, Yuuichi Saito, Hiroko Sakamoto, Keiko Ueyama, Kumiko Hisaki, Eiji Ohara, Tokiko Inoue, Kanako Yamamoto, Yukiyo Matsuoka, Hitomi Ohata, Kazue Toshima, Ayumi Okada, Hitomi Sato, Toyomi Kuwamori, Hiroko Tani, Reiko Suzuki, Mai Kashikura, Michiko Nezu, Yoko Miyachi, Fusako Arai, Masanori Kuwamori, Sumiko Harada, Akira Ohmori, Hirohiko Ishikawa, and Akio Koizumi
"Radiation dose rates now and in the future for residents neighboring restricted areas of the Fukushima Daiichi Nuclear Power Plant"
PNAS Early Edition, February 24, 2014