Last year's accident at the Fukushima No. 1 nuclear power plant exposed local residents to whole-body radiation doses of up to 50 millisieverts, well below the safety threshold, the World Health Organization has estimated.
It was the first time that radiation doses from the Fukushima accident have been evaluated systematically for a broad range of regions, including neighborhoods around the crippled plant, Fukushima Prefecture, neighboring prefectures, the rest of Japan and the world.
Nowhere did the whole-body dose estimate exceed the 100-millisievert limit, which poses enhanced risks of dying from cancer.
The thyroid gland dose estimate was the largest, at 100-200 millisieverts, for infants in Namie, Fukushima Prefecture. That level was lower than the average of 490 millisieverts for evacuees from the Chernobyl nuclear power plant accident of 1986, which caused thyroid cancer in many evacuees.
Most of the WHO estimates are based on data released by the Japanese government in and before September, including concentrations of radioactive substances found in the atmosphere, soil and food. The evaluations considered exposure to radiation emanating from the soil, airborne radiation taken in through normal breathing, and radiation from ingesting contaminated foodstuffs and water.
The doses of external exposure were calculated on the assumption that the average subject stayed indoors 16 hours a day, whereas the doses of internal exposure from breathing assumed that the subject stayed outdoors 24 hours a day. The evaluations also assumed that residents living in the 20-30 kilometer zone surrounding the Fukushima No. 1 nuclear plant continued to live at their domiciles for four months after the onset of the March 11 accident.
The whole-body doses for residents of the 20-30 km zone over the four-month period were estimated at between 10 and 50 millisieverts for the inhabitants of Namie and Iitate, and at 1-10 millisieverts for the inhabitants of Katsurao. Both figures apply equally to all age brackets.
Elsewhere, the annual whole-body doses were estimated at 1-10 millisieverts for residents of the remaining parts of Fukushima Prefecture; 0.1-10 millisieverts in the neighboring prefectures of Miyagi, Tochigi, Ibaraki and Chiba; 0.1-1 millisievert in the other parts of Japan, including Tokyo and Osaka; and less than 0.01 millisievert for people living outside Japan. All those figures apply equally to all age brackets.
There has been no report of evident health impacts from radiation doses of 100 millisieverts or less, but an annual dose limit of 1 millisievert is set for members of the public in normal conditions.
The estimates indicate the range of doses that the inhabitants of the corresponding areas are most likely to have been exposed to. That does not mean that all residents of the corresponding regions fall in the indicated dose ranges.
Meanwhile, the dose in the thyroid glands of 1-year-olds, who are the most susceptible to the impact of radiation, was estimated at 100-200 millisieverts in Namie; 10-100 millisieverts in the rest of Fukushima Prefecture; 1-10 millisieverts in the rest of Japan; and less than 0.01 millisievert in other countries.
These estimates are expected to be released soon by WHO, which will evaluate health impacts on the basis of the data and compile a report this summer. The WHO report will form a basis for a separate report by the United Nations, which publishes similar reports for nuclear emergency events.
In producing the latest estimates, the WHO took care to avoid underestimation.
For example, although the WHO assumed that people in the 20-30 km zone continued to live there for four months after the accident, most of the residents actually evacuated at earlier dates. The WHO adopted the more risky scenarios because it is difficult to fully track the movements of individuals.
The estimates for areas outside Fukushima Prefecture also likely tend to overestimate dose ranges, especially for areas far removed from the prefecture, experts said.
"The effects of food safety regulations, evacuation and other measures have not been reflected fully in the estimates," a Japanese government source said.
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