Concerns also spread to Europe. In Iceland, officials said they measured trace amounts of radioactive iodine in the air but assured residents it was "less than a millionth" of levels found in European countries in the wake of the 1986 Chernobyl disaster.
Radioactive iodine is short-lived, with a half-life of eight days — the length of time it takes for half of it to break down harmlessly. However, experts say infants are particularly vulnerable to radioactive iodine, which can cause thyroid cancer.
In Tokyo, government spokesman Yukio Edano pleaded for calm. Officials urged residents to avoid panicked stockpiling, sending workers to distribute 240,000 bottles — enough for three small bottles of water for each of the 80,000 babies under age 1 registered with the city.
That didn't stop Reiko Matsumoto, mother of 5-year-old Reina, from rushing to a nearby store to stock up.
"The first thought was that I need to buy bottles of water," the Tokyo real estate agent said. "I also don't know whether I can let her take a bath."
New readings showed Tokyo tap water was back to safe levels Thursday but the relief was tempered by elevated levels of the cancer-linked isotope in two neighboring prefectures: Chiba and Saitama. A city in a third prefecture, just south of the nuclear plant, also showed high levels of radioactive iodine in tap water, officials said.
Tap water in Kawaguchi City in Saitama north of Tokyo contained 210 becquerels of radioactive iodine — well above the 100 becquerels considered safe for babies but below the 300-becquerel level for adults, Health Ministry official Shogo Misawa said.
In Chiba prefecture, the water tested high for radiation in two separate areas, said water safety official Kyoji Narita. The government there warned families in 11 cities in Chiba not to feed infants tap water.