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GENEVA (Reuters) - Spikes in radiation caused by the Fukushima nuclear disaster were below cancer-causing levels in almost all of Japan, but infants in one town appear to be at a higher risk of developing thyroid cancer, the World Health Organization said on Wednesday.
In a preliminary report, independent experts said that people in two locations in Fukushima prefecture may have received a radiation dose of 10-50 millisieverts (mSv) in the year after the accident at the power station operated by TEPCO.
Separately on Wednesday, a U.N. scientific body said that several TEPCO-related workers were "irradiated after contamination of their skin", but that no clinically observable health effects had been reported.
"Six workers have died since the accident but none of the deaths were linked to irradiation," said a statement issued in Vienna on the interim findings of a study by the United Nations Scientific Committee on the Effects of Radiation (UNSCEAR).
The 9.0 magnitude earthquake and tsunami on March 11, 2011 wrecked the plant, triggering nuclear meltdowns that contaminated food and water and forced mass evacuations.
Nearly 16,000 people were killed in the earthquake and the tsunami and 3,300 remain unaccounted for.
The areas estimated to have received the highest doses of radiation were Namie town in Futaba county and Iitate village in Soma county, northwest of the stricken plant, the report said.
Infants in Namie were thought to have received thyroid radiation doses of 100-200 mSv, it added. The thyroid is the most exposed organ as radioactive iodine concentrates there and children are deemed especially vulnerable.
"That would be one area because of the estimated high dose that we would have to keep an eye on," WHO spokesman Gregory Hartl told Reuters. "Below 100 mSv, the studies have not been conclusive."
Populations exposed to radiation typically stand a greater chance of contracting cancers of all kinds after receiving doses above 100 mSv, according to the United Nations agency. The threshold for acute radiation syndrome is about 1 Sv (1000 mSv).
The local government said in December that the highest exposure levels were in Iitate, where residents were allowed to take their time to leave. It is located 40 km (25 miles) northwest of the plant and outside the 20-km evacuation zone.
The average annual dose from natural background radiation is about 2.4 mSv globally, with a typical range of 1-10 mSv in various regions, according to the 124-page report.
In the rest of Fukushima prefecture, the effective dose was estimated to be within that band of 1-10 mSv, while effective doses in most of Japan were put at just 0.1-1 mSv.
In the rest of the world, doses were below 0.01 mSv or less, including neighbouring Indonesia, the Philippines, South Korea, far eastern parts of Russia, and southeast Asia.
A dose of 0.01 mSv is equivalent to one tenth of the radiation received on a one-stop flight from New York to Tokyo, half the dose received during a chest X-ray, or equal to a dose received during a one-hour visit to one of Egypt's pyramids.
The report did not deal with radiation exposure suffered by emergency workers or people closest to the disaster site.
"Doses have not been estimated for the zone within 20 kilometres from the Fukushima Daiichi site because most people in the area were evacuated rapidly and an accurate estimation of dose to these individuals would require more precise data than were available," the report said.
The experts did not examine the short- and long-term health risks for the emergency response workers who worked on the site - that will be part of a wider WHO report due from a separate group of experts in July. That report will also assess the prospect for long-term increases in cancer cases.
The experts based their assessment on data available up to last September on the amount of radioactivity in air, soil, water and food supplies after the disaster.
Referring to the world's worst nuclear disaster at Chernobyl in Ukraine in 1986, the report said: "The experience of the Chernobyl accident was that about 30 percent of the lifetime dose was delivered during the first year and about 70 percent during the first 15 years.
"On the basis of environmental activity concentration data, it can be expected that the fraction of the lifetime dose beyond the first year will be lower for the Fukushima Daiichi nuclear power plant accident than for the Chernobyl accident," it said.
Reporting by Stephanie Nebehay and Tom Miles; additional reporting by Fredrik Dahl in Vienna; Editing by Michael Roddy