Radioactive iodine that exceeds the level considered safe for infants has been detected in Tokyo's tap water, a city official said Wednesday, advising parents not to give it to their babies.
On Saturday, abnormal levels of radioactive iodine-131 had already been detected in the water in Tokyo and in Fukushima prefecture, home to a nuclear power plant critically damaged in the March 11 earthquake and tsunami.
The finding in Tokyo is likely to fuel public safety fears about possible contamination from the plant. Radioactive substances have leaked into the air since the dual disaster, which knocked out the plant's reactor cooling systems.
"Under government guidelines, water containing a radioactive substance of more than 100 becquerels per kilogramme should not be used for milk for babies," the city official told reporters.
In one Tokyo ward, a water sample taken Tuesday contained more than double the legal limit, at 210 becquerels per kilogramme, the official said.
A sample taken Wednesday contained 190 becquerels per kilogramme, according to a preliminary finding, the official said.
The municipal government has advised residents throughout the city -- not just in the affected ward -- to avoid using tap water to make infant formula until further notice as a precaution.
Iodine-131 has a half-life of eight days, meaning half of it will have decayed after eight days.
The level deemed safe for adults is 300 becquerels per kilogramme.
"The amount detected in the water will not pose immediate harm to human health, and it's fine to use for daily life," Tokyo governor Shintaro Ishihara told reporters. "We will continue checking on water quality and disclose results. I want people in Tokyo to stay calm."
Traces of radioactive iodine were also found in the tap water at the weekend in the central prefectures of Gunma, Tochigi, Saitama, Chiba and Niigata.