Hours after the Japanese government announced a plan to import bottled water from overseas and distribute it to tens of thousands of parents, officials said that levels of radioactive iodine in Tokyo tap water were once again safe for infants. Yesterday, levels of iodine-131 (which can cause thyroid cancer in excessive doses) at a downtown plant that provides water to 23 wards in Tokyo along with as five other cities hit 210 becquerels per kilogram. The acceptable limit for infants is 100 becquerels. Early-morning tests today were down to 79 becquerels. Based on the new numbers, Tokyo governor Shintaro Ishihara lifted the recommendation that babies not drink tap water, but bottled water will continue to be distributed to households with young children. Japan's leading ob-gyn organization said pregnant and nursing women should continue to drink tap water even if it reached 500 becquerels, since that level of radioactive iodine wouldn't be that harmful and dehydration is worse.
But evidence from the Center of Disease Control and Prevention seem to point to the need for more caution in pregnant and nursing women. Radioactive iodine can cause thyroid cancer because it's absorbed in the thyroid gland. Like growing children, pregnant women also take up more iodine-131 in the thyroid, especially in the first trimester. According to the CDC, women who are breast-feeding will secrete about a quarter of the iodine they ingest into their milk.
Doctors in the U.S. and the World Health Organization have both stressed that Japan's guidelines for ingesting radiation are conservative and err on the side of caution. But weather conditions and the ongoing struggle to contain the situation at the Fukushima Daiichi nuclear plant make the situation unclear. Officials said yesterday that rain was partly responsible for the spread of radiation so far from the plant. But a senior Western nuclear executive told the Times that prevailing breezes actually seemed to be pushing the radiation out to sea, not the rivers that feed into the water supply: “The contamination levels are well beyond what you’d expect from what is in the public domain."
As the Times notes, the nuclear disaster in Chernobyl caused an epidemic of thyroid cancer caused by drinking milk from cows who grazed on grass laced with the fallout. If people had been told not to drink milk and been given potassium iodide (Japan has been distributing the pill), the epidemic might have been prevented.