Mr. Yoshida said that infants in Tokyo and surrounding areas should not drink tap water. He said iodine-131 had been detected in water samples at a level of 210 becquerels per liter, about a quart. The recommended limit for infants is 100 becquerels per liter. For adults, the recommended limit is 300 becquerels. (The measurement unit is named for Henri Becquerel, one of the discoverers of radioactivity.)
The announcement added to the growing anxiety about public safety posed by the Fukushima Daiichi Nuclear Power Station, which was severely damaged by the March 11 earthquake and tsunami. Prime Minister Naoto Kan said earlier Wednesday that the public should avoid additional farm produce from areas near the power station because of contamination, according to the Japanese news media.
The Health Ministry said in a statement that it was unlikely that there would be negative consequences to infants who did drink the water, but that it should be avoided if possible and not be used to make infant formula. The warning applied to the 23 wards of Tokyo, as well as the towns of Mitaka, Tama, Musashino, Machida and Inagi to the west of the city.
“It’s unfortunate, but the radiation is clearly being carried on the air from the Fukushima plant,” said Yukio Edano, the chief cabinet secretary.
“Because it’s raining, it’s possible that a lot of places will be affected. Even if people consume the water a few times, there should be no long-term ill effects.” He said the government and Tokyo officials were discussing measures to help families with children.
But it was unclear why the levels of iodine were so high, said a senior Western nuclear executive, given that the prevailing breezes seem to be pushing radiation out to sea.
“The contamination levels are well beyond what you’d expect from what is in the public domain,” said the executive, who insisted on anonymity and has broad contacts in Japan. “There is no way that stored fuel did not burn in a very significant way.”
At the Lawson’s convenience store in the Tsukiji neighborhood of central Tokyo, the shelves were about half-stocked with water. But a clerk said he had just restocked them an hour before.
"People came in and cleared us out in the first hour after the announcement," he said, saying he didn’t want to be identified because he didn’t want to anger his boss. "They were taking 20 or 30 bottles at a time."
Outside the store a man struggling to load more than 30 half-liter bottles onto his bicycle said he had bought the water for his wife, who is seven months’ pregnant.
"Tap water is O.K. for me," he said, asking that he be identified only by his family name, Takahashi. "But all they said was that babies shouldn’t drink it. They haven’t said anything about what pregnant women should do."
"We’re going to stay in Tokyo for now," Mr. Takahashi, 31, said, "unless the reactor problem gets worse."
Around the corner at the AM/PM convenience store the bottled water section of the shelves was bare but for nine half-liter bottles of sparkling lemon-flavored water.
Outside Tokyo the government said it had found radioactive materials at levels exceeding legal limits in 11 vegetables in Fukushima Prefecture, the Kyodo news agency reported. Shipments of the affected vegetables from there ended on Monday.
On Wednesday Prime Minister Kan also suspended shipment of raw milk and parsley from neighboring Ibaraki Prefecture, Kyodo reported.
The United States Food and Drug Administration said on Tuesday that it would prohibit imports of dairy goods and produce from the affected region.