The metropolitan government of Tokyo has advised residents not to allow infants under the age of one to drink tap water (or powdered milk made with tap water) after radioactive iodine was detected in a purification plant in Katsushika Ward.

The amounts of radioactive iodine exceeded the government’s regulated level for babies.

‘‘The standards are set by considering damage to human health from intake over a long period of time. It is all right to drink the water if there is no substitute drinking water,’’ a metropolitan government official said.

Parents are advised to use bottled water to make powdered milk.

According to Ei Yoshida, manager of Tokyo's Waterworks Bureau, the iodine-131 was detected in water taken from the Kanamachi Purification Plant. The level was measures at 210 becquerels per liter of water, which is more than double the recommended level of 100 becquerels for infants as designated by the Food Sanitation Act.

The water from the Kanamachi plant goes out to all 23 wards, including the suburbs of Musashino, Mitaka, Machida, Tama and Inagi in western Tokyo.

The level is not dangerous unless you keep drinking the water for a long period of time, Yoshida told a news conference. If there is nothing else to drink, you can let babies drink the water every once in a while.

The government’s Health, Labor and Welfare Ministry supported Yoshida.

The regulated level, 100 becquerels per liter, is a level at which people can have babies drink for a long time without worrying about radiation, said Kazuya Kumagai of the ministry's Water Supply Division.

Kumagai added that for people over the age of 1, 300 becquerels per liter of water is the standard regulated level, although pregnant and breast-feeding women may want to reduce the standard to 100 becquerels per liter.

Moreover, Chief Cabinet Secretary Yukio Edano assured that adults can drink tap water without any problems.

A team of nuclear engineers and physicists at the University of Tokyo said on a Twitter account: Iodine-131, when contained in water, can be removed to some extent by boiling it.”