Tiny amount of radioactive particles on U.S. West coast

VIENNA (Reuters) - Minuscule amounts of radioactive particles believed to have come from Japan's crippled Fukushima nuclear power plant have been detected on the U.S. west coast, diplomatic sources said on Friday.

The level of radioactivity was far too low to cause any harm to humans, they said.

One diplomat, citing information from a network of international monitoring stations, described the material as "ever so slight," consisting of only a few particles.

"They are irrelevant," the diplomat said. "There have been measurements, and they are magnitudes below what might be potentially causing harm."

Another diplomatic source gave a similar picture.

A senior official at the United Nations' International Atomic Energy Agency (IAEA) separately said there was no reason to believe there was any danger to human health, even in Japanese cities near the accident.

"Dose rates in Tokyo and other cities remain far from levels which would require action; in other words, they are not dangerous to human health," Graham Andrew told a news conference.

"If the situation changes dramatically, then we will have to make a reassessment."

MONITORING STATIONS

The Comprehensive Test Ban Treaty Organization (CTBTO), a Vienna-based U.N. body for monitoring possible breaches of the atom bomb test ban, has 63 stations worldwide for detecting such particles, including one in Sacramento, California.

They can pick up very small amounts of radioactive particles such as iodine isotopes.

"Even a single radioactive atom can cause them to measure something and this is more or less what we have seen in the Sacramento station," the diplomat said.

Asked if they were believed to originate from the Fukushima plant in Japan, which has leaked radioactivity since being damaged by last week's massive earthquake and tsunami, he said: "That is the obvious assumption."

The CTBTO continuously provides data to its member states, but does not make the details public. Since the Japanese crisis it has agreed to share it with the IAEA as well.

Detectable radiation is also likely to spread far further.

"We expect that maybe in seven days from now we could detect some atoms, with very, very exact instruments, here in Sweden," said Klas Idehaag at the Swedish Radiation Safety Authority.

On Thursday, the U.S. Nuclear Regulatory Commission said on Thursday radioactivity would disperse as it traveled long distances, and it did not expect any harmful amounts to reach the United States.

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.

"I don't think the fact of the mere detection of particles of radioactivity ... is something people should be concerned about," the IAEA's Andrew said.

(Additional reporting by Sylvia Westall and Michael Shields in Vienna and by Alister Doyle in Oslo; Editing by Kevin Liffey)