Also, heavier radiation particles would fall out of a radioactive plume, so "by the time anything reached us it would be no more than we experience normally," she said. "Everyone is being irradiated all the time."
This so-called "background" radiation isn't harmful and comes from sources in the Earth, solar rays in the atmosphere, and even drinking water.
But it's not inconceivable that the health risk in the United States might have to be reassessed if the Japan disaster reached catastrophic proportions, Rosenstein said.
"In harkening back to Chernobyl, where there was a tremendous release of radioactive isotopes, it did affect a good portion of Europe, certainly well beyond the Chernobyl area," he said.
Still, he pointed out, Europe and the Ukraine are much closer to each other than are Japan and the United States, even when talking about Hawaii or Alaska.
At this point, however, there is "no concern" about harmful effects from radioactivity this side of the Pacific, Rothstein said.
The Nuclear Regulatory Commission concurs. All the available information indicates that "weather conditions have taken the small releases from the Fukushima reactors out to sea away from the population. Given the thousands of miles between the two countries, Hawaii, Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity," the agency said on its Web site.
Still, not everyone is so sanguine.
Dr. Ira Helfand, a Massachusetts-based nuclear safety expert and past-president of Physicians for Social Responsibility, told CBS News that if fuel at one of the four crippled reactors in Japan were to melt down and break through the containment vessel, it could cause a massive explosion as superheated fuel mixed with the water coolant.
"The fuel rods contain enormous amounts of radioactive material
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