Children in Fukushima Prefecture likely received thyroid gland doses of internal radiation, despite earlier government assurances that the levels of such doses were zero, according to an independent study.
The estimated lifetime radiation doses among the children are still low, but they do exist, the National Institute of Radiological Sciences (NIRS) said at an international symposium in Chiba Prefecture on July 10.
The findings run counter to the government’s assertion that they effectively received zero thyroid gland doses.
However, the government has no plans to notify the children’s parents of the latest analysis results, citing their large error margins and the fear of causing anxiety.
"Researchers derived them for scientific purposes," a government source said. "We have no plan, now or in the future, to notify individuals of their doses."
The government measured the hourly thyroid doses of 1,080 children under 16 years old in Fukushima Prefecture from March 24 to 30 last year, shortly after a disaster began to unfold at the Fukushima No. 1 nuclear power plant on March 11.
Five months later, the government's Nuclear Emergency Response Headquarters told the parents of 55 percent of the children that they had zero internal thyroid doses, which included doses below the detection limit of the testing equipment.
A team of researchers led by Toshikazu Suzuki, a section head at the NIRS Research Center for Radiation Emergency Medicine, conducted an independent study of the same measurement data.
The team estimated the lifetime thyroid doses of the 1,080 children at 12 millisieverts on average and 42 millisieverts at the maximum.
The dose estimates were 30 millisieverts or more for four of the subjects. The children tested were from at least 10 municipalities, and the dose estimates tended to be higher for those from Iitate and Iwaki.
Experts say the lifetime thyroid doses, which indicate impact on health, are low because none of the children exceeded the International Atomic Energy Agency's reference level for the intake of stable iodine tablets to block thyroid irradiation. The level is 50 millisieverts in the case of 1-year-old infants.
Last year, the government calculated lifetime doses for some of the children with high hourly doses, but notified the parents only about the hourly dose values.
Officials at the time said the tests were low in accuracy because the objective was to find doses exceeding the reference level for taking stable iodine tablets.
The latest study by the NIRS researchers evaluated the lifetime doses of all 1,080 subjects from their hourly dose measurements by using conditions that maximized them.
Lifetime thyroid doses can be estimated from the measurement of the hourly radiation level emitted by radioactive iodine in the thyroid gland. The calculation takes into account the age of the subject, the timing of irradiation and other factors.
Toward the end of March 2011, the Nuclear Safety Commission of Japan advised the Nuclear Emergency Response Headquarters to conduct additional research on children with high doses, but the headquarters rejected the proposal, citing the fear of causing anxiety.
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