But the most visible example of the radiation's effect was claimed by a group of Japanese researchers who found radical physical changes in successive generations of a type of butterfly, which they said was caused by radiation exposure. They also said that the threat to humans - a much larger and longer-lived species - remains unclear.
"Our findings suggest that the contaminants are causing ecological damage. I do not know its implication to humans," Joji Otaki of the University of the Ryukyus in Okinawa, a member of the research team, said in an e-mail.
A separate study, released this week, found very low levels of radioactivity in people who were living near the Fukushima plant when it suffered the meltdowns.
The paper, published in the Journal of the American Medical Association, measured cesium levels in 8,066 adults and 1,432 children and found average doses of less than 1 millisievert, which are considered safe. It was the first such study measuring internal exposures to cesium in a large number of people from the disaster.
The research shows that contamination decreased over time, particularly among children, in part because more precautions were taken with their food, water and outdoor activity.
The research on the butterflies was published in Scientific Reports, an open-access online journal by the Nature publication group, which provides faster publication and peer review by at least one scientist.
It says that pale grass blue butterflies, a common species in Japan collected from several areas near the Fukushima plant, showed signs of genetic mutations, such as dented eyes, malformed legs and antennae, and stunted wings.
Other experts said they viewed the research as significant.
"Scientists have long known that radiation can be hazardous to human and animal health. Studies of this sort at Fukushima and Chernobyl provide invaluable information concerning just how hazardous radioactive contaminants could be for human populations living in these areas in the future," Tim Mousseau of the University of South Carolina, said in an e-mail.
"Butterflies as a group are important bio-indicators for the effects of environmental stressors like radioactive contaminants," said Mousseau, who also is not part of the Japanese research.
The results show the butterflies were deteriorating both physically and genetically, with the share of those showing abnormalities increasing from 12 percent in the first generation to 18 percent in the second and 34 percent in the third.
