Fukushima tuna study finds minuscule health risks

(CNN) -- Go ahead, order the sushi.

Levels of radioactivity found in Pacific bluefin tuna that spawned off Japan around the time of the Fukushima Daiichi nuclear accident are far below anything that would pose a health risk and have dropped in fish caught the following year, U.S. researchers reported Monday.

The latest findings follow up on a 2012 study that found radioactive cesium, a nuclear reactor byproduct, in tuna caught off California in the months after the triple meltdown at Fukushima Daiichi. The attention that study received led scientists to take another look at the data, said Nicholas Fisher, a marine science professor at New York's Stony Brook University.

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"People did not know how to translate that into a dose, or into what risk do I have from eating that tuna," Fisher said. "The paper that's coming out today addresses that."

They found that anyone who eats the bluefin -- highly prized for sushi and sashimi -- would get about 5% of the radiation they'd get from eating one typical banana, a fruit high in naturally radioactive potassium. The results were released Monday by the journal Proceedings of the National Academy of Sciences.

Even subsistence fishermen, who eat far more fish than the typical American, would receive a dose of radiation from the cesium isotopes released in the meltdown equivalent to a single dental X-ray, Fisher and his colleagues reported. That translates to a "worst-case scenario" of two additional cancer deaths for every 10 million people in that category, he said.

The doses were calculated from fish caught off San Diego in August 2011. A follow-up study with fish caught in 2012 found the amount of cesium-134 and -137 dropped by about half in those tuna, Fisher said.

"Even if we use the higher concentrations, the concentrations we measured in 2011, the doses to human consumers are very low, and lower than the naturally occurring radionuclides," he said.

Cesium-137 has a radioactive half-life of about 30 years, and traces of the isotope still persist from above-ground nuclear bomb tests in the 1950s and '60s. But cesium-134, which has a half-life of only two years, clearly came from the meltdowns, the researchers say.

There are other reasons to worry about the Pacific bluefin, such as overfishing and mercury contamination. Anyone who eats a large quantity of bluefin over a long period would be more likely to see ill effects from mercury -- which can damage the brain and nervous systems of young or unborn children -- before facing the threat of cancer from radioactive cesium, Fisher said.

The three operating reactors at Fukushima Daiichi melted down after the earthquake and tsunami that struck Japan in March 2011, creating the worst nuclear disaster since Chernobyl in 1986. The crippled plant's owner is still trying to manage hundreds of thousands of tons of radioactive water used to cool the reactors, and scientists believe some of that is still seeping into the Pacific.

The waters near the plant remain closed to fishing, and Japan has tightened its standards to keep any contaminated catch from reaching consumers.

While the earthquake killed more than 15,000 people, no deaths have been blamed on the nuclear disaster that followed. In May, the World Health Organization concluded that only a small group of people will face an additional cancer risk from the meltdowns, but more than 100,000 people have had to evacuate towns surrounding the plant.