People exposed to the highest doses of radiation during Japan's Fukushima nuclear plant disaster in 2011 may have a slightly higher risk of cancer but one so small it probably won't be detectable, the World Health Organization said in a report released Thursday.

A group of experts convened by the agency assessed the risk of various cancers based on estimates of how much radiation people at the epicentre of the nuclear disaster received, namely those directly under the plumes of radiation in the most affected communities in Fukushima, a rural agricultural area about 240 kilometres north of Tokyo.

Some 110,000 people living around the Fukushima Daiichi nuclear plant were evacuated after the massive earthquake and tsunami on March 11, 2011, knocked out the plant's power and cooling systems, causing meltdowns in three reactors and spewing radiation into the surrounding air, soil and water.

In the new report, the highest increases in risk appeared for people exposed as infants to radiation in the most heavily affected areas. Normally in Japan, the lifetime risk of developing cancer of an organ is about 41 per cent for men and 29 per cent for women. The new report said that for infants in the most heavily exposed areas, the radiation from Fukushima would add about one percentage point to those numbers.

"These are pretty small proportional increases," said Richard Wakeford of the University of Manchester, one of the authors of the report.

"The additional risk is quite small and will probably be hidden by the noise of other [cancer] risks like people's lifestyle choices and statistical fluctuations," he said. "It's more important not to start smoking than having been in Fukushima."

Experts had been particularly worried about a spike in thyroid cancer, since iodine released in nuclear accidents is absorbed by the thyroid, especially in children. After the Chornobyl disaster, about 6,000 children exposed to radiation later developed thyroid cancer because many drank contaminated milk after the accident.

In Japan, dairy radiation levels were closely monitored, but children are not big milk drinkers there.

For people beyond the most directly affected areas of Fukushima, the cancer risks drop dramatically. (Associated Press)

WHO estimated that women exposed as infants to the most radiation after the Fukushima accident would have a 70 per cent higher chance of getting thyroid cancer in their lifetimes. But thyroid cancer is extremely rare, one of the most treatable cancers when caught early, and the normal lifetime risk of developing it is about 0.75 per cent. That risk would be half of one percentage point higher for women who got the highest radiation doses as infants.

Wakeford said the increase in such cancers may be so small it will probably not be observable.

Risks outside Fukushima 'infinitesimal'

For people beyond the most directly affected areas of Fukushima, Wakeford said the projected risk from the radiation dropped dramatically. "The risks to everyone else were just infinitesimal."

David Brenner of Columbia University in New York, an expert on radiation-induced cancers, said that although the risk to individuals is tiny outside the most heavily exposed areas, some cancers might still result, at least in theory. But they'd be too rare to be detectable in overall cancer rates, he said.

Brenner said the numerical risk estimates in the WHO report were not surprising. He also said they should be considered imprecise because of the difficulty in determining risk from low doses of radiation. He was not connected to the WHO report.

Some experts said it was surprising that any increase in cancer was even predicted.

"On the basis of the radiation doses people have received, there is no reason to think there would be an increase in cancer in the next 50 years," said Wade Allison, an emeritus professor of physics at Oxford University, who was not connected to the WHO report. "The very small increase in cancers means that it's even less than the risk of crossing the road," he said.

WHO acknowledged in its report that it relied on some assumptions that may have resulted in an overestimate of the radiation dose in the general population.

Gerry Thomas, a professor of molecular pathology at Imperial College London, accused the WHO of hyping the cancer risk.

"It's understandable that WHO wants to err on the side of caution, but telling the Japanese about a barely significant personal risk may not be helpful," she said.

WHO report used 'inflated estimates of radiation,' expert says

Thomas said the WHO report used inflated estimates of radiation doses and didn't properly take into account Japan's quick evacuation of people from Fukushima.

"This will fuel fears in Japan that could be more dangerous than the physical effects of radiation," she said, noting that people living under stress have higher rates of heart problems, suicide and mental illness.

Norio Kanno, a village chief in one of the regions hardest hit by the disaster, harshly criticized the WHO report on Japanese public television channel NHK, describing it as "totally hypothetical."

Many people who remain in Fukushima still fear long-term health risks from the radiation, and some refuse to let their children play outside or eat locally-grown food. Kanno accused the report of exaggerating the cancer risk and stoking fear among residents.

"I'm enraged," he said.