Radioactive cesium was too low to detect in 99 percent of 22,000 residents examined in Fukushima Prefecture and neighboring Ibaraki, according to a University of Tokyo survey.
The team, which included professor Ryugo Hayano, examined internal radiation exposure levels in the two prefectures between March and November 2012. Their findings were unveiled Wednesday in the Transactions of the Japan Academy.
The survey found that the rate of internal exposure in the residents surveyed stood at about one-hundredth of the level detected in people living in the area around the Chernobyl plant at the time of the 1986 disaster.
Levels of cesium-137 were shown to be below the detectable threshold of 300 becquerels per kilogram of body weight for 99 percent of the residents, according to the team.
For the remaining 1 percent, or 212 people, 10 becquerels were detected. Still, their annual internal exposure would total only 0.04 millisievert, far below the government-set limit of 1 millisievert per year, the team said.
Meanwhile, cesium levels equivalent to 1 millisievert of internal exposure at an annualized rate were found in four elderly people who routinely ate wild mushrooms and boars. The team confirmed that the figures dropped after these four changed their eating habits.
Just as with the Chernobyl crisis, soil, particularly around Fukushima, was heavily contaminated with radioactive substances following the March 2011 nuclear meltdowns at the Fukushima No. 1 power plant.
The low cesium readings were attributed to the quality of the soil in the areas surveyed, which prevented food crops from absorbing radioactive materials, the conduction of radiation checks for food and the attention local residents are paying to the produce they consume, according to Hayano.
But the team, which used a whole body counter to examine the residents, concluded that checks on internal exposure and food inspections need to be continued. The study was conducted jointly with a hospital in Fukushima Prefecture.