Children evacuated from the Fukushima region take potassium iodide to prevent thyroid cancer after the Fukushima Daiichi nuclear disaster of 2011. Credit: The Asahi Shimbun/Getty Images

The World Health Organization (WHO) has released a relatively reassuring report about the health impacts of the 2011 disaster at the Fukushima Daiichi nuclear power plant in Japan. The accident is likely to have caused cause small, but significant, increases in cancers, but only in a few hotspots that were exposed to higher doses of the radiation, the report concludes.

The conclusions could be less comforting than they sound, however, because the pattern of prevailing winds during the accident meant that most of the radioactive materials released from the plant were blown out to sea. The results of the report therefore say little about the health risks of any future nuclear accidents.

"Had the winds been less favourable, the consequences could have been more serious than Chernobyl," says Keith Baverstock, a co-author of the report and a radiobiologist at the University of Eastern Finland in Kuopio, referring to the last major accident, in 1986.

The health-risk estimates are based on the WHO's assessment last year of the doses of radiation received by the population in the area.

Drafted by a panel of international experts in radiation risks and public health, the report concludes that there is no additional cancer risk for the population in most of Japan — even in most parts of Fukushima Prefecture — or in neighbouring countries. But the risks were slightly raised in the hotspots of Iitate village and Namie town, areas to the northwest of the plant that were contaminated by plumes of fallout.

In these hotspots, the WHO panel estimates that the risk of most cancers in children has risen by no more than 7% — although girls who were infants at the time are 70% more likely than those in other regions to develop thyroid cancer. However, because the baseline lifetime rate of thyroid cancer in women is just 0.75%, the risk in these hotspots is still just 1.25%.

Given that the projected frequency — just 3.2 cases of radiation-associated thyroid cancer per 10,000 young people — is so low, “it is unlikely that any excess would be detectable by the usual epidemiological approaches", says Roy Shore, head of research at the Radiation Effects Research Foundation in Hiroshima, Japan, and a co-author of the WHO report. Most emergency workers were estimated to have minimal increased risk, but around one-third had a small but significant increase in cancer risk.

Far from accepted

Experts are likely to argue intensely over the report over the coming weeks and months. Although many say that the report is well done, the exercise largely depended on modelling of radiation doses rather than on direct measurements of population exposures, and the data were often sub-optimal.

"One of the major problems for those of us interested in estimating the public-health consequences of the accident has been the paucity of reliable data," says Baverstock. "That problem still persists. If the WHO data are no better than we have been able to obtain, then the resultant estimates will be of limited value."

Geraldine Thomas, a radiation-health expert at Imperial College London, argues that the risks identified by the report are likely be overestimates, as the authors openly erred on the side of caution in various assumptions. "Very few members of the population will have been exposed to any more than a lifetime dose equivalent to a single whole-body CT [computed tomography] scan," she says.

Greenpeace, however, claims that the report is flawed. “The WHO report shamelessly downplays the impact of early radioactive releases from the Fukushima disaster on people inside the 20-kilometre evacuation zone who were not able to leave the area quickly,” says Rianne Teule on behalf of the environmental group. 

But the limited health effects of the accident may have been largely a fluke, Baverstock says. "The health consequences of this accident are smaller than [those of] Chernobyl because of the very favourable wind direction, out over the Pacific ocean, and the fact that there are no near neighbours [in that direction]," he says. Tokyo, for instance, is less than 200 kilometres away. "Had the winds prevailed in that direction, Fukushima would have been a whole different story."