Radioactive iodine exceeding international health standards was found in the thyroid glands of five of 65 Fukushima Prefecture residents who were examined by a team of researchers.
The researchers at Hirosaki University’s Institute of Radiation Emergency Medicine detected radioactive iodine in 50 residents, nearly 80 percent, after examinations conducted April 11-16.
Radiation levels of five residents were above the International Atomic Energy Agency’s 50-millisievert standard for taking iodine tablets to prevent damage to thyroid glands, with a maximum reading of 87 millisieverts.
The highest reading among children was 47 millisieverts. Levels of 24 residents, or about half, were 10 millisieverts or less.
“Levels could have exceeded 100 millisieverts if infants had stayed in a district with a high iodine concentration,” said professor Shinji Tokonami, who led the research team. “The central and local governments need to provide thorough care to protect the health of children.”
Children are more vulnerable to internal radiation by radioactive iodine because their thyroid glands absorb it more actively than adults.
Thyroid radiation can lead to cancer. But little was known about the levels of radioactive iodine amid confusion in the days immediately after the Fukushima No. 1 nuclear power plant was crippled by the March 11 Great East Japan Earthquake. Radioactive iodine has a short half-life.
A government survey of 1,080 children in the three municipalities of Iwaki, Kawamata and Iitate in Fukushima Prefecture last March found a maximum level of 35 millisieverts. But the accuracy was limited because only simple measuring devices were used.
The Hirosaki University team conducted more detailed measurements, spending five minutes on each resident.
“The research is valuable because it is based on measurement data more detailed than the government’s, although iodine 132, whose half-life is two hours, is not considered,” said Yoshio Hosoi, a professor at Hiroshima University’s Research Institute for Radiation Biology and Medicine.
After the 1986 Chernobyl accident, about 6,000 people, mostly children, developed thyroid cancer, and less than 20 died. According to a U.N. report, the average levels of thyroid radiation among evacuees were 490 millisieverts.
The Fukushima prefectural government plans to conduct lifelong thyroid gland examinations for 360,000 residents who were 18 years old or younger on March 11.
Hosoi said examinations need to be considered also for adults, citing an epidemiological survey that found increased thyroid cancer risks among people 40 years old or older after the Chernobyl accident.
The government will start a project in fiscal 2012 to estimate the levels of thyroid radiation for each district in and around Fukushima Prefecture.
Officials will ask for data from other researchers who conducted examinations on residents before radioactive iodine decayed and also gather related data from air and soil monitoring.
Gen Suzuki, who heads the clinic of the International University of Health and Welfare, said it is time to re-evaluate the reality of thyroid radiation because a number of data, albeit piecemeal, have recently become available.
“The government should have conducted accurate examinations of thyroid radiation immediately after the accident,” Suzuki said. “The government failed to conduct such examinations partly because the division of responsibilities was ambiguous within its organization. It must also review its setup for examinations.”
The Hirosaki University research team examined 48 people who evacuated from around the Fukushima No. 1 plant to the city of Fukushima, and 17 people who remained in the Tsushima district of Namie, Fukushima Prefecture, around 30 kilometers northwest of the plant.
The researchers measured concentrations of radioactive iodine in their thyroid glands and calculated levels of internal exposure on the assumption that the residents inhaled iodine on March 12.
The IAEA lowered the standard for taking iodine tablets from 100 millisieverts to 50 millisieverts after a report on the Chernobyl accident said cancer risks increased when thyroid radiation levels were 50 millisieverts or higher.
The Japanese government, which uses the 100-millisievert standard, is expected to soon lower it to 50 millisieverts.
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