(Updates with half-lives figures in last paragraph.)
July 4 (Bloomberg) -- Radioactive cesium-137 was found in Tokyo’s tap water for the first time since April as Japan grapples with the worst nuclear disaster in 25 years. The level was below the safety limit set by the government.
Cesium-137 registered at 0.14 becquerel per kilogram in Shinjuku ward on July 2 and none was discovered yesterday, compared with 0.21 becquerel on April 22, according to the Tokyo Metropolitan Institute of Public Health. No cesium-134 or iodine-131 was detected, the agency said on its website.
“This is unlikely to be the result of new radioactive materials being introduced” into the water supply, Hironobu Unesaki, a nuclear engineering professor at Kyoto University, said today by telephone. That’s “because no other elements were detected, especially the more sensitive iodine,” he said.
Japan is battling radiation leaks into the air, soil and water after an earthquake and tsunami on March 11 knocked out cooling systems at Tokyo Electric Power Co.’s Fukushima Dai-Ichi nuclear station, resulting in the meltdown of three of the six reactors at the plant.
Products including spinach, mushrooms, bamboo shoots, tea, milk, plums and fish have been found to be contaminated with cesium and iodine as far as 360 kilometers (224 miles) from the station. Prolonged exposure to radiation in the air, ground and food can cause leukemia and other cancers, according to the London-based World Nuclear Association.
“The reading is extremely low, on the very threshold of detection,” Unesaki said.
The Nuclear Safety Commission of Japan sets a safety limit of 200 becquerel per kilogram for cesium-134 and cesium-137. The limit for iodine-131 consumption is 300 becquerel per kilogram.
For vegetables, Japan has a limit at 2,000 becquerel of iodine per kilogram and 500 becquerel of cesium a kilogram.
Tokyo’s metropolitan government in March warned residents to avoid giving tap water to infants after radioactive iodine was found in the city’s supply at levels twice the allowable limit for infants.
The warning prompted bulk buying of bottled water at supermarkets and convenience stores even as the government said the health risks were minimal. Bottled water was available for sale in online stores listed on Amazon.com Inc. and Rakuten Inc. as of 3 p.m. in Tokyo.
Officials in the capital handed out about 240,000 bottles of water in March after iodine-131 reached 210 becquerel per kilogram on March 22 at the Kanamachi purification plant in Tokyo’s Katsushika ward. The recommended limit for infants is 100 becquerel a kilogram. The reading dropped to within safe levels the next day.
Cesium-137 has a half-life of about 30 years, compared with two years for cesium-134, according to the National Institute of Standards and Technology. Iodine-131 takes eight days for half of the substance to decay.
--Editors: Andrew Hobbs, Teo Chian Wei
To contact the reporter on this story: Pavel Alpeyev in Tokyo at email@example.com
To contact the editor responsible for this story: Amit Prakash at firstname.lastname@example.org