Some 99% of residents of Fukushima prefecture and neighbouring Ibaraki have barely detectable levels of internal exposure to caesium-137, a group of Japanese researchers has found. Of the remaining 1%, all showed levels well below the government-set limit.
A group of researchers headed by Professor Ryugo Hayano of the University of Tokyo studied the results of whole body scans carried out at Hirata Central Hospital in Fukushima prefecture between October 2011 and November 2012. Their finding - published in the Proceedings of the Japan Academy - show much lower internal exposure levels than estimates based on the finding of post-Chernobyl accident studies.
"These results are not conclusive for the prefecture as a whole, but are consistent with results obtained from other municipalities in the prefecture, and with prefectural data."
The team found that levels of caesium-137 were well below the detectable threshold of 300 becquerels per body for 9886 (88%) of the residents tested between October 2011 and February 2012. For the remaining 12% (1340 people), their exposure levels generally ranged between 10 and 50 bequerels per kilogram (Bq/kg). However, 21,785 (99%) of the residents tested between March and November 2012 had no detectable exposure, while the remaining 1% (212 people) mainly had exposure levels between around 10 and 25 Bq/kg. A policy introduced in March 2012 for subjects to change from their usual clothes into hopital gowns contributed to this sharp decrease.
The highest rates - about 1 millisievert per year - were found in four people, aged between 66 and 74, who were discovered to have been regularly eating unscreened food such as wild mushrooms, wild boar and freshwater fish. However, after following advice not to eat such food, their internal exposure levels significantly dropped.
A study of the results of whole body scans of nearly 1500 school children in the town of Miharu, Fukushima prefecture, found that although 54 had detectable levels of caesium-137 exposure in the winter of 2011, by the autumn of 2012 no children were found to have detectable levels.
The researchers noted, "These results are not conclusive for the prefecture as a whole, but are consistent with results obtained from other municipalities in the prefecture, and with prefectural data. This does not mean, however, that Fukushima residents are free of internal exposure risks, as evidenced by a small number of senior citizens whose body burden exceeded 100 Bq/kg."
Maintenance of the low-level of internal exposure to Fukushima residents will require continuous "conscientious and well-supervised food testing/screening and whole body counting," they suggest.
Comparison with Chernobyl
The levels of internal exposure found in people living in Fukushima prefecture are much lower than the levels found in people living near the Chernobyl plant following the 1986 accident there.
In January, the World Health Organization (WHO) said that there is only a low risk to Japan's population due to radioactivity released by the Fukushima accident.
For the general population in wider Fukushima prefecture, across Japan and beyond "the predicted risks are low and no observable increases in cancer rates above baseline rates are anticipated," WHO said.
However, it raised estimations of cancer risk for two towns near the Fukushima Daiichi plant - Namie and Iitate - where an unknown number of people may have remained at home for four months after the accident.
This is despite the fact that the environment - particularly the soil - around Fukushima was heavily contaminated with radioactive elements following the March 2011 accident at the Fukushima Daiichi plant.
A recent monitoring survey carried out by the Japanese government shows that the surface deposition density of caesium-137 totals 60,000 to 300,000 becquerels per square-metre in such densely populated cities as Fukushima and Koriyama.
The researchers said, "If we apply the knowledge of post-Chernobyl accident studies, internal exposures in excess of a few mSv/y would be expected to be frequent in Fukushima." They suggest that the proportion of food with elevated levels of contamination "are rather low" in Fukushima compared with Chernobyl.
Researched and written
by World Nuclear News