Tokyo officials warned today that infants should not drink city tap water because radioactive iodine exceeded legal limits at one purification facility, even as hopes rose that the source of that contamination – the damaged Fukushima Daiichi nuclear plant – may soon be under control.

Infants are much more vulnerable than adults to iodine-131, which officials have measured at 210 becquerels per liter. The limit for infants is 100 becquerels per liter, while for adults the limit is 300 becquerels.

“Even if you drink this water for one year, it will not affect people’s health,” Chief Cabinet Secretary Yukio Edano said.

At Fukushima, external power lines have now reached all six reactor buildings and lights went on in one central control room at the plant late Tuesday night. Tests monitoring equipment in buildings No. 1, 3, and 4 have also been completed and cooling pools in buildings No. 5 and 6 are reported to be stable.

But late Wednesday afternoon workers were once again evacuated as dark smoke rose from building No. 3. The normal cooling systems in buildings No. 1 through 4 have not yet started up because plant operator TEPCO is still inspecting and repairing machinery there.

In the meantime firemen, plant employees, and Japan's self-defense force members pumped and sprayed huge amounts of water into the volatile spent fuel pool in building No. 4 and also into the No. 1 reactor, which rose 150 degrees above its normal temperature early this morning. Normal pumps in building No. 3 – the first to regain power in its control room – could start up as early as tonight or tomorrow, a company representative says.

But even if cooling is restored and the plant stops emitting contaminated steam, low levels of radioactive substances will keep raining down on farms, cities, and oceans for years, says Ikue Kanno, who designs radiation detectors at Kyoto University

“Radioactive materials stick to dust floating in the atmosphere. When it rains, we see a rise in cesium-137 levels resulting from atomic weapons tests done by the Soviet Union and United States decades ago, although not at dangerous levels,” he says.

Emissions of cesium-137 and iodine-131 from Fukushima may be "approaching emissions of these isotopes from the Chernobyl accident in 1986, according to estimates from Austria’s Central Institute for Meteorology and Geodynamics reported in a blog at Nature.com Tuesday. "Fukushima is no Chernobyl, however," the report said, adding that wind has blown much of the cesium over the ocean, rather than toward cities.

Food and economic concerns

Radioactive contamination is a growing public safety concern and an economic problem for Japan. In addition to the Tokyo tap water warning, Prime Minister Naoto Kan took yesterday’s existing domestic ban on shipping vegetables from contaminated areas a step further, warning the public not to eat leafy vegetables from Fukushima Prefecture, even those already on store shelves.

On one type of leafy green collected Monday, radioactive cesium exceeded the legal limit by 164 times. Cesium-137 has a half-life of 30 years and cesium-134 of 2.1 years.

The government insisted its measures are extremely precautionary.

“Consuming these products in the short term won’t cause health problems, but since this situation is unfortunately predicted to continue for a long time, we feel it’s best to act at an early stage,” said Mr. Edano.

According to the US Environmental Protection Agency website, “There is no firm basis for setting a “safe” level of [radiation] exposure above background” below which the risk of developing cancer definitely does not increase.

The United States Food and Drug Administration, meanwhile, has banned imports of all dairy products and fresh produce from four prefectures near the plant: Fukushima, Ibaraki, Tochigi, and Gunma. Hong Kong also banned food and milk imports from five prefectures in Japan.