"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.
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.
In Japan, Norio Kanno, the chief of Iitate village, 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.
Some restrictions have been lifted on a 12-mile (20-kilometer) zone around the nuclear plant. But large sections of land in the area remain off-limits. Many residents aren't expected to be able to return to their homes for years.
Kanno accused the report's authors of exaggerating the cancer risk and stoking fear among residents.
"I'm enraged," he said.
Mari Yamaguchi in Tokyo and AP Science Writer Malcolm Ritter in New York contributed to this report.
WHO report: http://bit.ly/YDCXcb
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