The scope of radiation-contaminated tap water expanded Thursday, with radioactive iodine detected in tap water in Chiba, Saitama and Ibaraki prefectures, while the Tokyo Metropolitan Government, which said the day before its drinking water was contaminated, scurried to distribute 240,000 bottles of water to households with babies.

Chiba authorities urged parents not to give tap water to infants less than 1 year old. Saitama refrained, at least initially, from taking similar measures after the level of iodine fell below the regulated limit later Thursday.

Hitachi, Ibaraki Prefecture, started Thursday distributing water to make formula for infants.

The moves followed the metro government’s finding Wednesday that the level of radioactive iodine in tap water samples taken Tuesday exceeded the state’s recommended limit for infants.

On Thursday, however, the metropolitan government lifted the alert after radioactive iodine in tap water at the purification plant in Katsushika Ward dropped below the alert level for infants, officials said. Depending on the course of events, however, the alert could be reinstated, they said.

The rise and fall of radioactive iodine in tap water is believed to be affected by several factors, including its half-life of eight days, and the amount of rainfall.

Despite health authorities’ reassurances that the tap water was still relatively safe, people in Tokyo and the neighboring prefectures rushed to supermarkets to stock up on bottled mineral water, leaving the shelves bare.

To respond to the dramatic rise in demand, Chief Cabinet Secretary Yukio Edano said the government has asked bottlers to bump up production, even though they were already in full operation since the deadly March 11 earthquake and tsunami in the Tohoku region.

Edano added other countries may be asked to help supply water.

At a morning news conference, he once again urged the public to remain calm.

“I can say there is almost no possibility that (tap water) would affect those apart from infants and would like people without infants to keep calm,” Edano said. “To ensure that families with infants are not impacted . . . I have asked for an increase in production and instructed authorities to provide them appropriately.”

According to data from Tokyo’s Waterworks Bureau, the level of iodine was dropping — reaching 79 becquerels per liter Thursday morning from 210 on Tuesday and 190 on Wednesday.

In Chiba, two water treatment plants detected radioactive iodine Wednesday — a level of 220 becquerels per liter at one and 180 at the other. In Kawaguchi, Saitama Prefecture, the level of iodine dropped from what was 120 becquerels per liter Tuesday to 46 as of Thursday morning.

In Ibaraki’s Hitachi, 298 becquerels per liter were detected, while 170 becquerels were registered at Kasama, in the same prefecture.

The government’s recommended limit is 100 becquerels per liter for infants and 300 for anyone older.

Edano said the government was conducting large-scale monitoring of tap water and promised it has and will continue to report the results to the public as fast as possible.

“Unfortunately, radioactive materials have been leaking from the (Fukushima No. 1) nuclear plant and I believe it is possible that (iodine will be detected) to a certain extent,” Edano said.

“And that is why the government has been strengthening its monitoring and taking firm measures to ensure that objective information is provided to the public even though there is no effect on their health.”

While Tokyo began to hand out 550-ml bottled mineral water for families with babies in the 23 wards and five cities Thursday, Minato Ward also distributed drinking water it had stored for disasters.

To secure safe water, parents living in Minato Ward lined up with their babies at ward office branches Thursday. The ward prepared 12 bottles holding 500 milliliters per baby.

“I’m grateful we can get safe water,” said Junko Matsuo, mother of a 3-month-old boy, who went to a ward branch in the Shibaura district in the afternoon. “Bottled water near my place is all sold out. What I’m worried about right now is how long this situation will continue.”

A mother of a 6-month-old girl who only gave her family name, Sawahara, said she wants accurate, practical information on water contamination.

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