Radioactive iodine exceeding official levels for infants was detected Wednesday in water in a purification plant in Katsushika Ward, Tokyo, prompting the metropolitan government to advise residents not to let babies younger than 1 year old drink tap water or powdered milk made with it in the 23 wards and five cities.

The news immediately emptied mineral water at supermarkets. This prompted the Tokyo Metropolitan Government to announce later in the day it will distribute a total of 240,000 550-milliliter bottles of water to Tokyo households with infants.

The iodine-131 was detected in water taken Tuesday from the Kanamachi Purification Plant. The level was 210 becquerels per liter of water, more than double the recommended level of 100 becquerels for infants stipulated in the Food Sanitation Act, according to Ei Yoshida, manager of Tokyo’s Waterworks Bureau.

“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.”

Water from the plant goes to all of the capital’s 23 wards as well as the suburbs of Musashino, Mitaka, Machida, Tama and Inagi in western Tokyo.

The Health, Labor and Welfare Ministry backed Yoshida’s remark.

“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.

He advised parents to use bottled water to make powdered milk, but added that they shouldn’t panic even if babies have drunk tap water.

Later in the day, Chief Cabinet Secretary Yukio Edano offered assurances that adults can drink tap water and use it in everyday life.

If stores run out of bottled water, boiling faucet water will help reduce toxicity to some extent, radiation experts said.

Immediately after the announcement by the metropolitan government, a team of radiotherapists, physicists and nuclear engineers at the University of Tokyo released their take on the contamination of tap water on the group’s Twitter account.

“Iodine-131, when contained in water, can be removed to some extent by boiling it,” the team said.

Iodine-131’s half-life — the period by which the radiation level is halved — is eight days.

Three hundred becquerels per liter of water is the standard regulated level for all people over the age of 1, but pregnant and breast-feeding women may want to tighten the standard to 100 becquerels per liter, Kumagai said.

The metropolitan government collected water from its Kanamachi, Asaka and Ozaku purification plants Tuesday to check the density of radioactive substances.

It found no iodine-131 at the Asaka facility in Saitama Prefecture, but 32 becquerels per liter were found at the Ozaku facility in Hamura, western Tokyo.

Officials also checked for cesium-137, another radioactive substance, at the three purification plants, and did not find any traces, Yoshida said.

It collected water from the same three plants Wednesday and the preliminary report showed that water at the Kanamachi plant had 190 becquerels per liter of iodine-131, he said.

The high density “undoubtedly results from the Fukushima nuclear plant accident. We just don’t know the details as to how we got this high level,” Yoshida said.

The metropolitan government has more than three purification plants but chose the three to check water because they cover all three sources of water supply for Tokyo residents. The Kanamachi plant uses water from the Edo River watershed, Asaka uses the Ara River watershed and Ozaku uses the Tama River watershed.

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