The Tokyo Metropolitan government announced Tuesday that the drinking water in Tokyo is above the safe radioactivity limit for infants.  The water at one of the city purification plants measured 210 becquerels on Tuesday and 190 becquerels today.

The safe limit for adults is 300 becquerels per liter, while the limit for infants is much lower: 100 becquerels.

Infants are more susceptible to radiation damage than adults.  Unborn children suffer the worst damage, closely followed by infants.   Radiation damage is similar to aging, so the elderly are least at risk.

The water for Tokyo comes from the Okutama Reservoir, the largest of it's kind in the world. The runoff from radiation-carrying rain has increased the level or radioactive contamination to the point of danger for children under 12 months.

Iodine-131

The radiation source is Iodine-131 from the Fukushima power plant disaster.  It is a radioactive pollutant which has drifted 200 km (120 miles) driven by the winds.   It got airborne when radioactive reactor water got so hot it turned to steam and wafted away.  Officials believe it got in the Tokyo water from the rain.

A major danger from Iodine-131 is that it is identical chemically with dietary iodine.  If a person is low on iodine when they are exposed to the radioactive isotope of iodine, their bodies accumulate the isotope in the thyroid, where the radiation damages it.

Tokyo stressed that even over the long haul it is safe for adults.

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

Bottled mineral water sold out in minutes when the public learned the tap water was radioactive.

The Metropolitan Waterworks Bureau announced Wednesday that they would deliver bottled water to families with infants. There are about 80,000 households with infants in the effected region.