See this article on the NPR website. Key quote:
"... Kim Kearfott, an expert on the health effects of radiation, says ... young mothers shouldn't panic.
"If it were my infant, I would make an effort to secure some water that's clean," says Kearfott, a professor of nuclear engineering and radiological sciences at the University of Michigan. But if she couldn't, she says, "I wouldn't be concerned [about using the tap water] unless this continues over a long time period."
How long? "Half a year," she says.
That's because the radiation dose limit for infants — 100 becquerels per liter of water — is based on a year's worth of exposure. The radioactivity from iodine-131 in Tokyo's water, due to the emissions of the Fukushima Dai-ichi power plant, is twice that.
So drinking the radioactive tap water for half a year would not exceed the safety standard, she says.
But parents are understandably reluctant to ignore the authorities' advice to stop giving the contaminated water to their babies. Kearfott says there are other ways to reduce the risk.
"Using a charcoal filter would dramatically reduce the amount of iodine-131 in the water, by between 50 and 100 percent," she says. These are the kinds of filters available in many grocery stores.
Another strategy is to draw tap water and keep it in a jug for eight days. That's the half-life of iodine-131, so in that time, half of the radioactivity would be gone — and the water would be within the 100-becquerel safety standard. Meanwhile, parents could use bottled water if available, or just use tap water in the knowledge that short-term use isn't likely to cause any harm."