The government's rice testing program failed to detect high levels of radioactive cesium in pre-harvest tests of rice from the Onami district of Fukushima, which was found to have produced rice significantly in excess of safety standards on Nov. 15.
Pre-harvest tests of rice from Onami only detected 136 becquerels per kilogram of rice, well below the government limit of 500 becquerels per kg. The contamination was only discovered after the rice had been harvested, when 630 becquerels of radioactive cesium per kilogram was discovered.
The failure of the early tests is raising concerns about a detection regime that only looks at limited samples of rice from at-risk areas and uses the pre-harvest tests to decide which areas to subject to the most scrutiny.
"It is regrettable to see such a result after conducting so many tests," Tsukasa Iwamoto, senior vice minister of agriculture, told a news conference on Nov. 17.
Unlike vegetables and other agricultural produce, which are subject only to pre-shipment tests, rice has to go through both a pre-harvest test a week before being gathered and a main inspection after harvest.
The pre-harvest test is conducted on samples of rice grown in soil with radioactivity in excess of 1,000 becquerels per kilogram and in areas where radiation of 0.1 microsievert or more per hour has been measured.
If more than 200 becquerels of radioactivity per kilogram is detected in the pre-harvest test, the area is regarded as a priority area in the main, post-harvest inspection and more farms are subjected to testing. The fact that the Onami rice only recorded 136 becquerels per kilogram in the early test meant that it was not subjected to increased scrutiny, with rice from paddies in only two farms tested after harvest.
Rice from neither farm was found to exceed the government safety standard.
When pre-harvest tests of rice made in one district of Nihonmatsu, Fukushima Prefecture, detected radioactive cesium equal to the government's 500-becquerel safety standard in September, the prefectural government tested paddies in 47 other areas where rice was being cultivated along a river using mountain runoff, but the paddies in the Onami district were not considered at risk.
The city of Nihonmatsu has called on the central government to conduct screening of all rice bags. However, a government official indicated there would be practical problems with such a regime: "It is difficult to do so. There is room for improvement technically."
An agriculture ministry official said, "It is impossible to achieve 100 percent detection since what we do is sampling."
The contamination in the rice from Onami was detected in pre-shipment tests before reaching consumers. Chief Cabinet Secretary Osamu Fujimura announced a central government ban on shipments of rice from the district on Nov. 17.
Yoshihiro Ikeuchi, a director at Japan Chemical Analysis Center, said the contamination in the rice was not a major health hazard.
"Even if one eats three bowls of brown rice each day in which 630 becquerels of cesium per kg has been detected, annual radiation exposure would be 0.8 millisievert or less," he said. "It is unlikely it will affect human health."
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