High levels of radioactive materials were detected Monday more than 100 kilometers from the embattled Fukushima No. 1 nuclear power plant, but officials said there were no immediate dangers to health.
Despite the assurances from the Nuclear and Industrial Safety Agency, radiation experts urged close, detailed monitoring of radioactive substances in broader areas, as the radioactivity from the plant in Fukushima Prefecture appears to be spreading.
Tokyo Electric Power Co., operator of the nuclear plant, also said it has discovered high levels of radioactive substances in seawater near the plant.
Earlier, government officials said 13,000 becquerels of cesium-137 per 1 square meter per day were detected Monday in Hitachinaka, Ibaraki Prefecture, about 120 kilometers southwest of the nuclear power plant.
At a news conference Monday night, NISA officials described that level as "very weak."
"It is one-third of the safety limits level on people leaving a radiation-controlled area," a NISA official said.
Radiation-controlled areas include buildings housing nuclear reactors, where the public cannot enter and workers are required to wear protective gear.
However, Shigenobu Nagataki, professor emeritus at Nagasaki University, who specializes in radiation medicine, said the cesium-137 level detected is "considerably high."
"After the Chernobyl incident, land hundreds of kilometers away from the nuclear power plant was found contaminated with radioactive materials that flowed there," he said. "We need to expand monitoring spots and determine the whole scope of contamination."
Chief Cabinet Secretary Yukio Edano on Tuesday denied immediate health hazards from the cesium-137.
"We had experts analyze (the results), but we were told the level of cesium would not pose immediate health risks nor cause health problems in the future," he said.
He also said the results do not warrant the need to extend the area in which people are asked to stay indoors. That area is currently within a radius of 20-30 kilometers from the nuclear power plant.
People living within 20 km of the plant have been told to evacuate.
The studies also found 93,000 becquerels of iodine-131 in Hitachinaka, an amount twice the government safety level of people leaving a radiation-controlled area.
Cesium-137 and iodine-131 are not naturally existing substances; they are created through nuclear fission.
Cesium-137's long half-life of about 30 years has raised concerns that it could pollute the land, water and agricultural produce.
In addition, "internal exposure" of cesium-137 is more dangerous than "external exposure" because people cannot wash out the radiation after it accumulates in the muscles.
Iodine-131, which has a half-life of eight days, is particularly dangerous to small children and is known to cause thyroid cancer.
The two radioactive substances are believed to have been released into the air from hydrogen explosions on March 14 through March 16 and spewed steam at the nuclear reactor buildings.
The radioactive materials in the atmosphere apparently fell with the rain in eastern Japan on Monday.
The government-designated safety limit for cesium-137 is 40,000 becquerels per square meter per day for people leaving a radiation-controlled area.
According to the science ministry's radioactivity measurements nationwide on Sunday and Monday, the readings in Yamagata, Saitama, Morioka and Tokyo's Shinjuku Ward were 4,300, 790, 690 and 560 becquerels, respectively.
The levels shot up to 87 becquerels in Maebashi, where the figure was 63 becquerels a day earlier.
In Utsunomiya, the amount surged to 250 becquerels from 45 becquerels.
The readings for iodine-131 were 58,000 becquerels for Yamagata, 7,800 becquerels for Morioka and 7,200 becquerels for Saitama.
Seawater near the plant also had high levels of iodine-131.
A survey of a 0.5-liter of seawater near a drainage system of the plant at 2:30 p.m. Monday detected iodine-131 at radiation levels 126.7 times the safety limits stipulated under nuclear power plant regulations, TEPCO said Tuesday.
The finding means that if a person drinks 0.5 liter of water containing this level of radioactivity for three days, it would equal the yearly limit of permitted exposure.
Also detected in the sample water were cesium-134 at a level 24.8 times higher than safety standards, and cesium-137, at 16.5 times higher.
Cesium-134, also created through nuclear fission, has a half-life of about two years.
The contamination of the seawater could pose health risks to people eating marine products from the area.
Yukio Takizawa, professor emeritus at Akita University who studied Minamata mercury poisoning disease, agreed with the central government's view that the iodine-131 levels do not pose immediate dangers.
"Since government safety limits are very stringent, (the amount of radioactive iodine) would not cause an immediate hazardous effect," he said. "In general, radioactive materials are reduced if fish offal are removed because they are concentrated there."
Takizawa said he is more concerned about cesium-137 and strontium-90, which has a half-life of 28 years.
TEPCO was expected to examine sample waters Tuesday at four locations, including the one surveyed on Monday.
The seawater could have been contaminated with rainfall containing radioactive materials released from the stricken plant.
In addition, water sprayed on storage pools of spent nuclear fuel rods likely seeped into the ground and was carried to the sea.
TEPCO briefed NISA and Banri Kaieda, minister of economy, trade and industry, on the findings. It also notified Okuma and Futaba, the host towns of the plant, as well as the towns of Namie and Tomioka, where fisheries cooperatives are located.
Health minister Ritsuo Hosokawa promised to carry out more surveys to find out the extent of the contamination.
"We will conduct checks in various locations in the sea," he told a news conference Tuesday. "We plan to talk with prefectural officials about tests when the shipment of fish begins."
The Ministry of Health, Labor and Welfare also urged Ibaraki and Chiba prefectures to beef up their checks of marine products.
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