Fukushima people eating more cesium but not in danger, says study

January 19, 2012

The median daily intake of radioactive cesium from meals eaten by families in Fukushima Prefecture is more than 11 times the level in the Kanto region near Tokyo but still well within safety standards, according to a study.

The median intake from three daily meals in Fukushima Prefecture, home to the crippled Fukushima No. 1 nuclear power plant, was 4.01 becquerels compared with 0.35 becquerel in the Kanto region around Tokyo, according to the joint study by The Asahi Shimbun and Kyoto University's Department of Health and Environmental Sciences.

In western Japan, well away from the nuclear disaster, the level of radioactivity was mostly below the detection limit.

But researchers pointed out that the higher intake by Fukushima residents would still mean that annual internal radiation exposure was about 40 times lower than a new, stricter annual maximum to be introduced by the government in April.

"Even the cesium level in Fukushima Prefecture is sufficiently low," said Akio Koizumi, a professor at Kyoto University's Graduate School of Medicine who conducted the study. "It did not turn out to be so high as to give rise to concern about its health effects.

"The total cesium content in meals tends to be thinned out, because food ingredients circulating in the market come from a variety of regions. The latest figures are not so large that one should immediately review the choice of food ingredients. It is essential to have a well-balanced diet to disperse the risks," Koizumi added.

However, there is currently no clear-cut boundary below which health effects from low radiation doses have been ruled out by scientists.

The Asahi Shimbun and Kyoto University conducted the study on the basis of samples, provided by 53 households, of all food and drink items consumed by a household member on Dec. 4. Twenty-six of the partner households were from Fukushima Prefecture, whereas 16 were from the Kanto region. The other 11 were from western Japan, including the Chubu, Kansai and Kyushu regions.

The partner households were asked to cook the samples using the same ingredients that they normally use. Many of the households studied in Fukushima Prefecture were using locally produced vegetables.

The median daily cesium intake from food among the 26 households in Fukushima Prefecture was 4.01 becquerels. A median was used instead of an average because no accurate figures were available for samples below the detection limit.

The annual dose from such a diet was calculated at 0.023 millisievert, far below the maximum of 1 millisievert under the government's new food safety standard.

The highest daily intake among the samples from Fukushima was 17.30 becquerels. Even that level translated to an estimated annual dose of only 0.1 millisievert, or one-10th of the new safety standard.

Even before the Fukushima nuclear disaster, food has always contained radioactive potassium-40 of natural origin, accounting for an average annual dose of 0.2 millisievert for people living in Japan. The radiation from cesium from the Fukushima No. 1 nuclear plant fell short of that background level.

"This apparently demonstrates the effectiveness of the food safety regulations that have been implemented so far," said Michiaki Kai, a professor of radiation protection at the Oita University of Nursing and Health Sciences. "Even in Fukushima Prefecture, the annual dose estimated from the latest outcomes is much lower than the dose from background radiation due to potassium-40, which is contained in food by nature and is irradiating everybody. It is essential to continue monitoring and controlling the radioactive substances contained in the diet."

The science ministry conducted a similar study between 1963 and 2008. The median cesium content in the Japanese diet was 2.03 becquerels in 1963, because the United States, the Soviet Union and China were conducting numerous nuclear tests in the atmosphere in the 1960s. The figure from the latest study in Fukushima Prefecture is about twice that level.

Cesium was below the detection limit in seven of the 16 households in the Kanto region, including Tokyo, Chiba and Gunma prefectures. The median was 0.35 becquerel, and the annual dose was estimated at 0.002 millisievert, or one-500th of the new safety standard. Cesium was below the detection limit in 10 of the 11 households in western Japan.

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Graphic by The Asahi Shimbun

Graphic by The Asahi Shimbun

  • Graphic by The Asahi Shimbun
  • A member of a household in Fukushima Prefecture, partnering in the joint study by The Asahi Shimbun and Kyoto University, pours miso soup for supper into a sample container on Dec. 4. (Hiroki Endo)

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