Scientists said high doses of radiation found in the thyroid glands of evacuees from areas near the Fukushima No. 1 nuclear power plant were actually well within international safety limits.
A team of Japanese and Russian researchers led by Shinji Tokonami, a professor at Hirosaki University’s Institute of Radiation Emergency Medicine, revised the previous estimates they presented in March (ajw.asahi.com/article/0311disaster/fukushima/AJ201203090071). The latest results were published July 12 in British scientific journal Scientific Reports.
The latest study estimated the maximum dose at 33 millisieverts. The previous estimates, based on assumptions that maximized the doses, were 87 millisieverts for adults and 47 millisieverts for children. Those dose estimates exceeded the international 50-millisievert reference level for taking iodine tablets to prevent a negative health impact in five of the subjects.
No dose estimate exceeded that level in the latest results.
To produce the latest estimates, the scientists teamed up with Mikhail Balonov, a professor of radiation protection at the Institute of Radiation Hygiene in St. Petersburg, and evaluated the doses under more realistic assumptions. Balonov has experience in dose evaluation for residents in the Chernobyl area.
The scientists re-analyzed the radioactive iodine concentration data taken between April 12-16 last year in the thyroid glands of 62 residents and evacuees in Fukushima Prefecture. Forty-five of the subjects had evacuated from coastal areas around the damaged nuclear plant to Fukushima city, the prefectural capital, while the remaining 17 continued to live in Namie, a town close to the nuclear plant.
During the re-analysis, the researchers evaluated the thyroid gland doses using international standards and took into account up-to-date information, such as the movement of radioactive plumes and dosimetry data immediately following the onset of the nuclear disaster on March 11, 2011.
The previous estimates were based partly on domestic standards of the Nuclear Safety Commission of Japan.
The revised dose estimates exceeded 20 millisieverts in only seven subjects, and were under 10 millisieverts in 70 percent of all subjects.
However, the scientists warned that children could have been exposed to thyroid doses of 40-60 millisieverts if they had been in the part of Namie where the doses were high.
The scientists plan to communicate the revised analysis results to residents of Fukushima Prefecture in the near future.
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