Published online 14 October 2011 | Nature | doi:10.1038/news.2011.593

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Fukushima 'hot spots' raise radiation fears

But experts see little threat from patches of heightened radioactivity.

Tokyo hotspotA hot spot of radiation detected in Tokyo was traced to abandoned bottles of radium-226.Sankei via Getty Images

The discovery of 'hot spots' of radioactive material is spreading fear far beyond the damaged Japanese nuclear power plant at Fukushima. But experts say that there is no threat from the small spots of increased radioactivity now being discovered in large-scale surveys.

On 12 October, officials reported finding 195 becquerels of strontium-90 on a rooftop in Yokohama, some 250 kilometres south from the Fukushima Daiichi plant. A day later, a citizen's group in Funabashi, Chiba Prefecture, about the same distance away, made radiation readings of 5.82 microsieverts per hour (μSv h–1) at a children's theme park. The same day, an inspection of Tokyo's Setagaya district turned up a narrow strip of pavement that seemed contaminated, at 3.35 μSv h–1. Officials later traced the Tokyo contamination to an abandoned house containing bottles of radium-226, a radioactive element once used in luminous paint.

Counting the risk

Since the Fukushima accident, the Japanese government's official safety limit for radiation exposure is 20 mSv per year, which corresponds roughly to a rate of 2.28 μSv h–1. Yet the newly discovered hot spots pose no threat to human health, says Geraldine Thomas, a radiation-health expert at Imperial College in London. Total radiation dose is measured by the strength of the radioactivity in a given area and the amount of time a person spends there. The small sizes of the hot spots make it all but inconceivable that anyone would receive 20 mSv, she says. Strontium-90 can cause bone cancer if ingested, but a small patch on a roof won't cause that problem, Thomas says. "These are minuscule amounts of radiation, but the population out there is terribly nervous."

Risk is about more than radiation readings, adds Christopher Clement, the scientific secretary of the International Commission on Radiation Protection in Ottawa, Canada, an independent international organization that provides guidance on safe levels of radiation. Its 'recommended' residual dose from a nuclear accident is between 20 and 100 mSv per year for the general population. The Japanese government has chosen the lower number as its official limit, Clement says, but "it's not a magical number by any means". Clement says that most scientists believe that receiving an additional 100 mSv of radiation over a prolonged period can raise the chance of dying of cancer by 0.5% (in the general population the chance of cancer being the cause of death is about 25%).

Under the limit

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To reach that 100-mSv dose would require someone to be continuously exposed to the Japanese government's 20-mSv limit for 24 hours a day, seven days a week over a five-year period. A small amount of radioactive material on a rooftop or in a gutter poses little risk. "People don't sleep in that one spot in the gutter," he says.

Both Clement and Thomas say that more hot spots are likely to be discovered as citizens' groups, local authorities and government inspectors continue their surveys. And Clement says that everyone should be prepared for more false alarms like the one in Tokyo: "No matter where you go in the world, if you take a radiation instrument with you and look around, you'll eventually stumble across something that's above what the background for that area normally is," he says. 

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