UPDATE: Our latest news story discusses the WHO report at length, along with a second study on the exposure levels of workers and other aspects of the accident by the United Nations Scientific Committee on the Effects of Atomic Radiation.
Today, the World Health Organization (WHO) released a preliminary estimate of the radiation dose received by the public as a result of last March’s meltdowns at the Fukushima Daiichi nuclear plant in Japan. Nature has seen a draft of the final report, and it is mostly good news — the doses are very low, and very few cancers would be expected as a result.
Most residents of Fukushima prefecture received between 1–10 millisieverts (mSv) in the first year after the accident, according to the estimate. Those in neighbouring prefectures received between 0.1–10 mSv, and the rest of Japan received between 0.1–1 mSv. These levels are well below the government’s maximum recommended dose of 20 mSv and will cause a minimal increase in cancer risk.
The obvious question is how minimal. According to David Brenner, a radiation biophysicist at Columbia University in New York, a dose of 5 mSv would be estimated to lead to one excess cancer per 5,000 people exposed. Given that roughly 2,000 of those 5,000 people are going to develop cancer anyway, this is a tiny increase in risk, and Brenner emphasizes that the uncertainties in his calculations are high.
There were two areas that were above the 10-mSv range. In the town of Namie and the village of Itate, to the north-west of the plant, residents received 10–50 mSv in the first year. This is because both towns were beneath a plume of fallout from the plant, but still outside the evacuation zone set up immediately after the accident. Residents in these areas remained until a few months later, when they voluntarily left at the government’s request. As a consequence, they received a higher dose of radiation.
Even the worse case scenario — a dose of 50 mSv — poses a fairly minimal risk. However, the models showed that infants living in Namie could have got a higher dose to their thyroid, of 100–200 mSv. That higher dose would be due mainly to radioactive iodine-131 blowing from the plant immediately after the accident. Brenner says a dose of 200 mSv to a female infant under a year old might mean a 1% risk of developing thyroid cancer over her lifetime (by comparison, the lifetime risk in the United States is 0.02%).
It’s important to remember that the WHO numbers are based on models, and real doses would vary quite a bit. A survey of 1,080 infants and children in the area has shown no thyroid doses above 50 mSv thus far. Similarly, radiation surveys of Fukushima residents show very low doses. All of these measurements are consistent with the WHO model.
We’re going to have a much more detailed story on the doses received by civilians and the workers at the plant later today.
Image: Nature (data from: WHO/METI)