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The urgency of halting the spread of radioactive contamination from the Fukushima Daiichi site was underlined on Wednesday by the health warning that infants should not drink tap water, even in Tokyo, 140 miles southwest of the stricken plant, which raised alarms about extensive contamination.

The United States Food and Drug Administration said on Tuesday that it would prohibit imports of dairy goods and produce from the affected region.

Nuclear engineers in Japan say some of the most difficult and dangerous tasks are still ahead in mitigating the damage at the Daiichi nuclear power plant.  The tasks include manually draining hundreds of gallons of radioactive water and venting radioactive gas from the pumps and piping of the emergency cooling systems. Preventing the reactors and storage pools from overheating through radioactive decay would go a long way toward limiting radioactive contamination. But that would require pumping a lot of water through them.

The Japanese government has suggested that recent rains might have washed radioactive particles from the Daiichi into the water.  But prevailing breezes for the past two weeks should have been pushing the radiation mostly out to sea. And until Wednesday, some experts had predicted that radioactive iodine would not be much of a problem, because the fission necessary to produce iodine, which breaks down quickly, with a half-life of just eight days, stopped within minutes of the earthquake on March 11. The fear is that more radiation is being released than has been understood.
 

Radioactive Iodine 131 had been detected in water samples at a level of 210 becquerels per liter, about a quart. The recommended limit for infants is 100 becquerels per liter. For adults, the recommended limit is 300 becquerels. (The unit is named for Henri Becquerel, one of the discoverers of radioactivity.)

Pregnant women, nursing mothers and fetuses, as well as children, face the greatest danger from radioactive iodine, which is taken in by the thyroid gland and can cause thyroid cancer. Children are at much higher risk than adults because they are growing, and their thyroid glands are more active and in need of iodine. In addition, the gland is smaller in children than in adults, so a given amount of iodine 131 will deliver a higher dose of radiation to a child’s thyroid and potentially do more harm.

According to the Centers for Disease Control and Prevention, if an adult and a child ingest the same amount of radioactive iodine, the thyroid dose will be 16 times higher to a newborn than to an adult; for a child under 1 year old, eight times the adult dose; for a 5-year-old, four times the adult dose.

Pregnant women also take up more iodine 131 in the thyroid, especially in the first trimester. The iodine crosses the placenta and reaches the fetus, and the fetal thyroid takes up more iodine as pregnancy progresses.

Potassium iodide can protect the thyroid by saturating it with normal iodine. People in Japan have been advised to take it. (NYT, 3/23/2011, NYT, 3/23/2011)