U.S. officials from President Obama on down are trying to reassure residents of Hawaii, Alaska and the West Coast that there is little chance dangerous levels of radiation from Japan will reach them. But there is still a lot of understandable concern. Dr. Jon LaPook reports on the radiation risk, and the fear factor.
When radiation began leaking from the stricken power plants, the fallout was felt more than 5,000 miles away at a pharmacy near Los Angeles - it was soon sold out of potassium iodide.
"People are worried going from store to store trying to find anything," said Capitol Drugs pharmacist Nevin Jones.
The pills can prevent the thyroid from developing cancer caused by radiation. But are they really necessary in California?
"It's extremely unlikely there would be any risks to folks in this country," said David Brenner, Director of the Center for Radiological Research at Columbia University. "The distance is simply so large the cloud will be so dispersed by the time it reaches the U.S."
Even in Japan, not far from the reactors - the actual threat is relatively low, radiation experts say.
"There is an evacuation zone, that means very few people are going to get doses even comparable to a chest X-ray, which is a pretty low radiation dose," Brenner added.
Radiation can damage human cells. It's measured in something called millisieverts. A chest X-ray emits about 1/10th of a milisievert.
Nuclear plant workers are limited to 20 milisieverts a year. One-hundred milisieverts in one dose can increase the risk of cancer, and 100 to 500 can cause bone marrow damage, leading to infection and death.
Reports say radiation levels were as high as 400 milisieverts an hour at the plant Tuesday. But they fell dramatically -- first to 11.9, then to zero-point-six.
To put this in perspective: in Chernobyl, among people who became sick the radiation dose ranged from 800 to 1.6 million millisieverts - much higher than what's being measured so far in Japan.