Low concentrations of radioactive particles are heading eastwards from Japan's disaster-hit nuclear power plant and are expected to reach North America in days, a Swedish official said on Thursday.
Lars-Erik De Geer, research director at the Swedish Defence Research Institute, a government agency, was citing data from a network of international monitoring stations established to detect signs of any nuclear weapons tests.
Stressing that the levels were not dangerous for people, he predicted the particles would continue across the Atlantic and eventually also reach Europe.
"It is not something you see normally," he said by phone from Stockholm. But, "it is not high from any danger point of view."
He said he was convinced it would eventually be detected over the whole northern hemisphere.
"It is only a question of very, very low activities so it is nothing for people to worry about," Mr. De Geer said.
"In the past when they had nuclear weapons tests in China ... then there were similar clouds all the time without anybody caring about it at all," he said.
Before he spoke, the U.S. Nuclear Regulatory Commission advised any Americans living near Japan's crippled Fukushima nuclear plant to move at least 80 kilometres away but it played down the risks of contamination to the United States.
"All the available information continues to indicate Hawaii, Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity," it said in a statement on Wednesday.
Mr. De Geer was commenting on data from the Comprehensive Test Ban Treaty Organization (CTBTO), a Vienna-based independent body for monitoring possible breaches of the test ban.
He said he believed the radioactive particles would "eventually also come here".
The CTBTO has more than 60 stations around the world which can pick up very low levels of radioactive particles such as caesium and iodine isotopes.
It continuously provides data to its member states, including Sweden, but does not make the details public.
The New York Times said a CTBTO forecast of the possible movement of the radioactive plume showed it churning across the Pacific, and touching the Aleutian Islands on Thursday before hitting southern California late on Friday.
It said health and nuclear experts emphasized that radiation would be diluted as it travelled and at worst would have extremely minor health consequences in the United States.
In a similar way, radiation from the Chernobyl disaster in 1986 spread around the globe and reached the west coast of the United States in 10 days, its levels measurable but minuscule, the newspaper said. The CTBTO projection gave no information about actual radiation levels but only showed how a radioactive plume would probably move and disperse, it said.
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