While public health officials anxiously downplayed fears Thursday that a plume from Japan's crippled nuclear reactors was descending on California, scientists at UC Berkeley declared they were already detecting radioactive particles from 5,000 miles across the ocean.

The differing accounts illustrated the confusion on the fallout from Japan's crisis as contamination continued to spew from the damaged power plants, but scientists and public health experts were united on one front: Whatever radiation may drift to California and the West Coast will be too minuscule to pose any health risks here.

"We see evidence of fission particles -- iodine, cesium, barium and krypton, a whole dog's breakfast of radiation," said Ed Morse, professor of nuclear engineering at UC Berkeley, whose students have set up a monitor on the rooftop of the campus's Etcheverry Building. A monitor at Lawrence Livermore Lab is also detecting the particles, he said.

But the concentration of radiation is so diffuse -- a picocurie per cubic meter of atmosphere, far less than reported in the Bay Area after the Chernobyl explosion in 1986 -- that it is only detectable because of its characteristic chemical fingerprint, called an isotope, he said.

"You could get mad as hell and fly to Washington, and be exposed to greater radiation on the flight," he said.


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Meanwhile, seeking to reassure worried Californians, state health officials said Thursday that increased radioactivity levels had not been detected in the state -- and they disputed a computer model, widely publicized early Thursday, that projected a threatening plume of contamination was crossing the Pacific and would arrive Friday in California.

With shifting winds and rain in the forecast, California is unlikely to see any elevated levels of radiation from the growing crisis in Fukushima, said Dr. Howard Backer, interim director of the California Department of Public Health. If it arrives, the radiation will be too diluted to pose a health threat, he said.

The Bay Area Air Quality Management District, which operates a radiation monitor in San Francisco as part of the U.S. Environmental Protection Agency's national RadNet radiation-monitoring network, said that all measurements in the Bay Area are within normal background levels so far.

Similarly, the EPA and the Federal Nuclear Regulatory Commission have indicated that there are no harmful levels of radiation expected to reach the United States.

"It would take a much larger blast than what has occurred for radiation from the power plant to get into the jet stream," said Eric Stevenson, director of technical services for the air district. "It is very unlikely that the radiation from the tragedy in Japan will impact the United States."

In a Thursday afternoon news conference, state health director Backer dismissed forecasts from the U.N.-based Comprehensive Test Ban Treaty Organization of a plume arriving along the coast by Friday, saying that rain and shifting winds makes such precision impossible.

Because of California's distance from Japan, "there will be a major dispersal effect on any radiation that reaches the atmosphere," Backer said. "Rain brings particulates out of the atmosphere."

Dr. Martin Fenstersheib, health officer for Santa Clara County, said he had been assured by state experts that "there is no plume. There isn't anything coming towards the West Coast."

"The fuel rods in the reactor are capable of dispensing material -- but it is picked up and dispersed within a 10-mile radius," he said. "For the people close to the reactor, it is a serious situation. But we are 5,000 miles away."

Radioactive particles that land into the Pacific Ocean will be diluted in the water, the state officials said.

Regarding any risk to seafood, Backer said: "It will be so dispersed so quickly there will not be a significant increase of any radioactivity in fish. Any levels will be undetectable comparable to natural radiation levels."

Similarly, the water poses no hazard to swimmers, said Dr. Jonathan Fielding, director of the Los Angeles Department of Public Health.

"We don't think anything will be problematic for surfers or bathers on the beach," he said.

Food that is already on the shelves is considered safe by the U.S. Food and Drug Administration, according to Fielding. The state will monitor levels in milk, where radiation can be concentrated by grass-eating cows.

A widely-circulated forecast, calculated Tuesday by an arm of the United Nations, showed a possible radioactive plume touching the Aleutian Islands on Thursday before hitting California late Friday.

The projection gave no information about actual radiation levels but showed how a radioactive plume would probably move and disperse -- and cautioned that the pattern could change as Pacific weather patterns shifted.

At the Massachusetts Institute of Technology, the journal Technology Review attacked the map, calling it "alarming" and "misleading."

President Barack Obama sought to reassure Americans on Thursday saying that "I want to be very clear: We do not expect harmful levels of radiation to reach the United States," including Hawaii, Alaska, and U.S. territories in the Pacific.

He said that he has asked the Nuclear Regulatory Commission to conduct a full review of American nuclear safety. But he reiterated his support of nuclear power, calling it "an important part of our own energy future, along with renewable sources like wind, solar, natural gas, and clean coal. Our nuclear power plants have undergone exhaustive studying and have been declared safe for any number of extreme contingencies."

Despite such reassurances, many people are scrambling to take precautions in the Bay Area and along the West Coast. Drug stores are reporting runs on potassium iodide pills, which can help protect the thyroid from ingested radiation. An Arizona-based company that sells Geiger counters was sold out. Its website said: "There are simply not enough detectors available to meet the overwhelming demand."

Contact Lisa M. Krieger at 408-920-5565.