Fukushima tuna 'pose little health risk'

Bluefin tuna Pacific bluefin tuna are regularly caught to assess their Fukushima-derived radioactivity

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The radiation dose someone would get from eating tuna tainted with pollution from Fukushima is far below the levels that should cause health concern.

This is the conclusion of researchers who have measured the radioactivity in marine organisms, notably tuna, caught in the Pacific Ocean since 2011.

The scientists have assessed the likely health impacts from fish consumption in a paper published in the Journal PNAS.

They say the numbers ought to be reassuring to the public.

For all the marine organisms analysed, the team found that the Fukushima-derived doses to be hundreds of times lower than the doses from naturally occurring radioactivity already in the food.

Nick Fisher and colleagues provide the example of a typical 200g, restaurant-sized serving of bluefin tuna contaminated with the same levels of Fukushima-derived radioactivity they measured in fish caught off California in August 2011.

The team says this dish would provide only about 5% of the dose acquired from eating one uncontaminated banana and absorbing its potassium-40, a radioactive element that is naturally occurring and ubiquitous in the environment.

This week's PNAS paper is a follow-up to the team's report in the same journal in May last year.

That earlier report presented the measured levels of radioactivity in Pacific bluefin tuna that could be traced directly to the Fukushima Daiichi nuclear accident in March 2011.

These were tuna whose parents would have spawned in Japanese waters and spent one to two years locally before heading to feeding grounds off California.

All the fish caught at that time showed elevated levels of radioactive caesium - the isotopes 137 and 134.

"That earlier report seemed to raise a lot of anxiety," conceded Prof Fisher from Stony Brook University, New York.

"Part of the problem I think was that we presented radioactivity concentrations but did not convert them into doses that human consumers would get from eating those fish, and we didn't present the attendant risk associated with those doses.

"We've now done that for this new paper. It provides the doses human consumers would get and the likelihood that would lead to cancer, both for consumers in the US and people in Japan, where the fish are more radioactive and people tend to eat more seafood on average than they do in the United States."

Someone eating that 200g portion of tuna would receive a dose equivalent from the radioactive caesium of less than eight nano-sieverts (the sievert is a standard measure of the biological impacts of radiation). This is about a thousand times less than the dose someone would receive from a typical dental X-ray. This is roughly five micro-sieverts, which carries an increased probability of developing a fatal cancer of about two in 10 million.

What is more, this applies to fish caught in the months following the Fukushima accident. Tuna caught off California since have shown substantially reduced levels of radioactive caesium.

For average Japanese consumers, the doses are also very low.

Even if a Japanese person were to ingest their entire annual average consumption of fish as tuna contaminated with the higher radio-caesium levels expected in local Japanese waters in April 2011 - it would produce a dose just above 30 micro-sieverts. This is not dissimilar from the dose one would get from cosmic rays during a single transcontinental flight.

"We hope this study is reassuring," said Prof Fisher.

"I personally do not believe these radioactivity levels are harmful to the fish or to human consumers, even if they eat quite a bit of it.

"We have presented this latest set of data to show that the anxiety on the part of some members of the public - as I understand it - is not commensurate with the real risk."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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