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Why did Japanese officials gear up聽to distribute potassium iodide in response to the explosions at the聽nuclear power plant?

According to the聽, a nonprofit group聽in Cambridge, Mass., the radioactive materials that pose the greatest threat after a nuclear power accident are radioactive iodine聽(the聽iodine-131 isotope, in particular)聽and聽radioactive cesium (cesium-137), and news reports have said both聽have been detected outside the plant.

Radioactive iodine聽is a byproduct of the fission (splitting) of the uranium in the聽fuel rods that power a nuclear power plant.聽Once radioactive iodine聽is聽in the body, it聽concentrates mainly in the thyroid gland, which is located in the neck, just below the Adam鈥檚 apple.聽聽(See our related post on the thyroid gland and cancer from iodine-131.)聽But as聽a helpful (CDC) explains, the聽gland cannot tell the difference between聽radioactive iodine and the radioactively stable version of the mineral it needs, so it will absorb both.

Potassium iodide聽pills (sometimes abbreviated as KI: the K stands for potassium, the I for iodine)聽don鈥檛聽prevent radioactive iodine from entering the body, but they do keep it聽from getting accumulating in聽the聽thyroid gland.聽By flooding the body with non-radioactive iodine, the pills聽keep the聽gland from absorbing the radioactive聽iodine.聽Here is how the CDC聽fact sheet summed it up:

Because KI contains so much stable iodine, the thyroid gland becomes 鈥渇ull鈥� and cannot absorb any more iodine鈥攅ither stable or radioactive鈥攆or the next 24 hours.

Children and infants are more vulnerable to developing thyroid聽cancer from radioactive iodine than adults, so it鈥檚聽important that they get the pills in a radiation聽emergency. But the聽pills can be hard to swallow, especially for infants, and potassium iodide dissolved in water has a harsh, salty taste. The FDA tip:聽grind the pills up and mix them into low-fat chocolate milk, orange juice, or flat soda.

Large doses of iodine over a long period of time can be dangerous, so potassium iodide pills聽 should be聽reserved for true聽emergencies.聽(So far, no one is recommending that anyone in the United States take them, although people are apparently stocking up.)

Many varieties of table salt are聽 鈥渋odized,鈥� which means iodine has been added.聽But iodized table salt聽doesn鈥檛 contain enough of the mineral聽to saturate the thyroid gland and keep it from absorbing radioactive iodine.