Tokyo ( CNN ) -- Japanese officials ' concerns over food contamination expanded beyond the country 's borders Tuesday as tests detected radiation in ocean water offshore . Plant operator Tokyo Electric Power Co. said that high levels of radioactive substances were found in seawater near the plant , but said that the results did not represent a threat to human health . `` There should be no immediate health impact . If this situation continues for a long period of time , some impact can occur , '' Chief Cabinet Secretary Yukio Edano told reporters . The impact such radiation could have on marine life was unclear . Japanese authorities were scheduled to measure radioactivity in waters around the plant on Tuesday and Wednesday , the International Atomic Energy Agency said . Earlier seawater radiation monitoring detected levels of iodine-131 that were 126.7 times higher than government-set standards , the electric company said on its website . Its monitors detected cesium-134 , which has a half-life of about two years , about 24.8 times higher than the government standards . Cesium-137 was found to be 16.5 times higher than the standard . The electric company detected these levels in seawater 100 meters ( 328 feet ) south of the nuclear power plant Monday afternoon . Radioactive particles disperse in the ocean , and the farther away from the shore a sample is taken , the less concentrated the contamination should be . Because of the huge amount of dilution that happens in the ocean , there 's not much chance of deep-water fish being tainted , said Murray McBride , a professor at Cornell University who studies soil and water contamination . `` I think the ocean can handle that a lot better than the physical environment and population centers , '' said Jim Walsh , an international security expert at the Massachusetts Institute of Technology and a CNN consultant . `` We do n't want any of this to happen , but better it go out to sea than stay in Japan . '' Winds have largely blown radioactive material emitted by the plant offshore since an earthquake and tsunami crippled cooling systems at the plant March 11 . But tests have detected contamination of food grown near the plant . The Japanese government has banned the sale of raw milk from Fukushima Prefecture , where the plant is located , and prohibited the sale of spinach from neighboring Ibaraki Prefecture after finding levels of radioactive iodine and cesium higher than government standards , the country 's health ministry reported . And officials in Fukushima halted the distribution of locally grown vegetables outside the prefecture . The government has also banned sales of spinach and milk from parts of Gunma and Tochigi Prefectures , according to the prime minister 's office . On Tuesday Edano said contamination had not been detected in other agricultural products . `` The products which are being grown in these areas are being monitored and the monitoring will continue , '' he said . He urged consumers to `` try not to panic , '' noting that the government had stopped shipments of any farm products they believed could be contaminated . Edano has stressed that officials believe the levels of radiation in food -- while above the legal standards -- do not pose any immediate health risk , saying they were mostly dangerous only if consumed repeatedly over one 's lifetime . On Monday a spokesman for the World Health Organization said short-term exposure to food contaminated by radiation from Japan 's damaged Fukushima Daiichi nuclear plant poses no immediate health risk . Tests have also detected radiation in tap water . On Monday , authorities in the village of Iitake urged residents to avoid drinking tap water that tests showed contained more than three times the maximum standard of radioactive iodine . Water in other jurisdictions showed lesser signs of contamination , although far below levels of concern under Japanese law , according to the International Atomic Energy Agency website . The U.N. agency said it had received reports from Japan 's government that six out of 46 samples tested positive for the iodine-131 radioactive isotope . Iodine and cesium isotopes are byproducts of nuclear fission in reactors such as the ones damaged in the March 11 earthquake and tsunami that devastated northern Honshu , Japan 's main island . Although iodine-131 has a radioactive half-life of eight days , cesium-137 's half-life is about 30 years . The decision to prohibit produce sales is another potentially devastating blow to a part of northeast Japan hit by the earthquake , tsunami and other potential fallout from the Fukushima plant . Edano has said farmers will be compensated for revenue lost by the restrictions . `` Primarily this is due to the nuclear reactor accident , so we assume ( Tokyo Electric Power Company ) will be held responsible for compensation . The government might take some supplementary action , '' he said . Fukushima ranks among Japan 's top producer of fruits , vegetables and rice . Ibaraki , south of Fukushima , supplies Tokyo with a significant amount of fruits and vegetables and is Japan 's third-largest pork producer . For radiation to be an issue for rice , the contamination would have to be more severe and prolonged that what has been seen so far , said McBride , the Cornell University professor . Soil contamination was a huge issue around Chernobyl , but the radiation emitted from the Fukushima Daiichi plant is n't anywhere near that level , he said . `` We 're not at that stage ; that 's the scenario you have to consider if contamination gets severe enough , '' McBride said . CNN 's Jo Kent , Steven Jiang , Martin Savidge , Paul Ferguson , Thom Patterson , Matt Smith and Elizabeth Landau contributed to this report .
High levels of radioactive substances have been found in seawater . The impact on marine life is unclear , and tests are expected to continue . Tests also reveal radiation in milk and spinach . Comment: