A U.N. committee dismissed any recognizable rise in overall cancer rates among residents of Fukushima Prefecture caused by fallout from the disaster at the Fukushima No. 1 nuclear plant, sources said.
A U.N. committee dismissed any recognizable rise in overall cancer rates among residents of Fukushima Prefecture caused by fallout from the disaster at the Fukushima No. 1 nuclear plant, sources said.
In a report expected to be published on April 2, the U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) said thyroid gland cancer cases could increase recognizably among infants who were 1 year old and stayed within 30 kilometers of the nuclear plant when the disaster started in March 2011. However, it added that the matter remains inconclusive because of insufficient data.
Tests have found a number of cancer cases and lumps in the thyroid glands of children in Fukushima Prefecture, but the report said most of those cases are unrelated to radiation exposure from the nuclear accident.
UNSCEAR reports are considered the most reliable among available international health studies on nuclear accidents.
The UNSCEAR used observation data provided by the Japanese government and articles published by world scientists to estimate the doses of both external and internal exposure to radiation following the triple meltdown at the plant.
Its health study of residents both in and outside Fukushima Prefecture was split into three categories: people who were 1 year old, 10 years old and adults when the disaster started.
The report estimated the whole-body doses accumulated over the first year following the disaster at 1-10 millisieverts on average in adults across all parts of Fukushima Prefecture, including areas around the crippled nuclear plant. The estimated dose was about double those figures for 1-year-olds, who are the most susceptible to radiation.
The dose estimates for 1-year-olds fell short of 100 millisieverts, the threshold at which it is believed the risk of cancer begins to clearly rise, the report said.
Priority is placed on safety for evaluations of the health impact of radiation. Such studies assume that even modest radiation doses could increase the risk of cancer.
The report noted that one in three Japanese develop cancer irrespective of their exposures to radiation.
Any increase in cancer rates corresponding to the average whole-body doses in Fukushima residents would be a tiny fraction of the average lifetime cancer risk of 35 percent among the Japanese population, and would be statistically negligible, the report said.
Cancer of the thyroid gland is the only cancer that has been scientifically proven to be related to radiation exposure since the 1986 Chernobyl nuclear disaster.
The risk of cancer is believed to start rising when the radiation dose in the thyroid gland, not the whole-body dose, has exceeded 100 millisieverts.
According to the report, thyroid gland doses accumulated over the first year after the Fukushima disaster were 47-83 millisieverts on average in 1-year-old infants who were staying between a 20-km radius and a 30-km radius of the Fukushima No. 1 nuclear plant.
For 1-year-olds staying within the 20-km radius, the doses in their thyroid glands were estimated at 15-82 millisieverts on average.
The report added that thyroid gland cancer rates could rise significantly if a large population of infants received doses close to 80 millisieverts. But it said no conclusion could be derived because of insufficient district-by-district data available on thyroid gland doses in children.
At any rate, any increase in thyroid gland cancer cases in Fukushima would still fall short of the 6,000 or so that followed the Chernobyl disaster, the report said. That is because thyroid gland doses among Chernobyl evacuees averaged about 500 millisieverts.
The report is expected to be reviewed several years from now on the basis of new information obtained.