Wednesday, May 23, 2012
Today, the World Health Organization (WHO) released a preliminary estimate of the 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 City, 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 10mSv range. In Namie town and Itate village, to the north-west of the plant, residents received between 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, 50 mSv, poses a pretty minimal risk. However, the models showed that infants living in Namie town could have gotten a higher dose to their thyroid, of between 100-200 mSv. That higher dose would be mainly due to radioactive iodine-131 blowing from the plant immediately following 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 US 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)
Posted by Michele Kearney at 9:03 AM