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Importance of external radiation exposure for individuals


Children's radiation doses to guide decontamination in Fukushima Pref. city



SOMA, Fukushima -- City authorities here will prioritize decontamination of areas where children's external radiation exposure is estimated to be 2 millisieverts or more per year, it has been learned.

The Soma Municipal Government made the decision on Feb. 22. The city will also offer medical consultations to local residents.

"It's important to detect how much radiation children were exposed to and respond appropriately," said Soma Mayor Hidekiyo Tachiya. "Based on the measured values, we will set priorities in carrying out effective decontamination work."

It is extremely rare for a city to decontaminate areas based on the levels of external radiation exposure among individual residents.

Soma lies on a stretch of Pacific coast between 30 and 50 kilometers from the crippled Fukushima No. 1 nuclear plant, and in the aftermath of the meltdowns there the city conducted an external radiation exposure survey on a total of 4,010 children -- from babies to junior high school students -- and pregnant women. The subjects were asked to carry gamma ray detectors for three months starting in October last year, and the results collected formed the basis for the annual exposure calculations.

In the survey, one resident was found to have been exposed to an annual dose of 4.3 millisieverts -- more than four times the government-set maximum of 1 millisievert. Furthermore, a total of 522 residents -- or 13 percent of subjects -- were found to have been exposed to doses exceeding the government limit, of which 33 residents (0.8 percent) were exposed to 2 millisieverts or more.

At schools in mountainous areas where the airborne radiation doses stood at 1 microsievert per hour, some 50 percent of elementary school children and approximately 70 percent of junior high school students registered 2 millisieverts or more. Even at schools in the plains, where the airborne radiation doses were relatively low at 0.2 to 0.4 microsieverts per hour, several children were found to have been exposed to 2 millisieverts or more of radiation. The results indicate that the exposure levels vary depending on areas as well as individuals.

Based on the survey results, the city decided to prioritize decontamination of environments where children were exposed to 2 millisieverts or more of radiation per year total -- a figure set in reference to a Ministry of Education, Culture, Sports, Science and Technology guideline that children should not be exposed to more than 1 millisievert of radiation a year at schools.

University of Tokyo's Institute of Medical Science doctor Masaharu Tsubokura, who cooperated with the city's survey, said, "It is important to assess the data on individual citizens' radiation exposure rather than the airborne radiation levels in each area. It is also necessary to conduct follow-up surveys to confirm whether decontamination work has steadily lessened radiation doses."

bruary 23, 2012

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