A full meltdown has been avoided so far at Japan's 40-year-old Fukushima Daiichi nuclear plant. Neverless, as far away as Tokyo, 240 kilometers to the south, the city government says small amounts of the radioactive iodine and cesium have been detected in the air. Higher levels of radioactive materials have been monitored closer to the plant, prompting the government to order the evacuation of residents within a 20-kilometer radius.

As a precautionary measure against radiation exposure, the Japanese have also distributed 230,000 units of potassium iodide tablets, comprising a stable form of iodine, to evacuation centers in the area around the Fukushima Daiichi and Fukushima Daini nuclear power complexes, according to the International Atomic Energy Agency (IAEA). Potassium iodide, which is available in the U.S. in 130- and 65-milligram doses (smaller doses are given to children), has been shown to protect the thyroid gland from the radioactive form of iodine released by nuclear accidents or emergencies that could lead to thyroid cancer.

Thyroid cancer ended up being the biggest negative health impact caused by the Chernobyl nuclear reactor disaster, according to a report issued last month by the United Nations Scientific Committee on the Effects of Atomic Radiation. The report (pdf) specifies that more than 6,000 Russian, Ukrainian and Belarusian residents who were children at the time of the disaster had been diagnosed with thyroid cancer as of 2006, the disaster's 20th anniversary. Fifteen of these people had died as of 2005. The incidence of thyroid cancer in contaminated areas of the Ukraine and Belarus was triple that of normal thyroid cancer incidence in the area, although the study's authors acknowledge that more attention was paid to medical examinations and improved record-keeping in those areas affected by the Chernobyl event.

As Japan struggles regain control of the Fukushima Daiichi plant, demand for potassium iodide is growing even 8,000 kilometers away on North America's west coast. Anbex, Inc., based in Williamsburg, Va., announced on its Web site Tuesday that it has sold out of its iOSAT brand of potassium iodide. Other makers of the substance have announced similar shortages.

Scientific American spoke with John Boice, a professor at Vanderbilt University's Vanderbilt-Ingram Cancer Center and a cancer epidemiologist with the International Epidemiology Institute in Rockville, Md., to better understand why potassium iodide is in demand as well as what it can (and cannot) do.

[An edited transcript of the interview follows.]

Why is potassium iodide administered to people who have been exposed to radioactive iodine?
The thyroid is like a sponge for iodine. It's been known from the 1970s that if you administer normal iodine the thyroid will absorb it and then block the uptake of subsequent exposures to radioactive iodine. Therefore, if you take potassium iodide and then are exposed to radioactive iodine, there won't be any place for it to go because your thyroid is all filled and the radioactive material will be excreted from the body.

One pill is good for 24 hours, but then you have to take another pill. You don't take two pills at once, because having too much potassium iodide isn't good for you either. Like anything else, it's not 100 percent effective, but it appears to be quite a benign thing to take, and it does block the uptake of radioactive iodine.

Is thyroid cancer the foremost risk when radioactive iodine is in the air?
With regard to radioactive iodine, it's just the thyroid gland that you're worried about; you're not concerned about anything else. Of course, in an event like Chernobyl where the reactor's containment vessel did not hold everything a number of other radioactive elements were also released, including cesium and strontium as well as some of the reactor fuel—the uraniums and plutoniums. Still, the two main elements of concern from a radiation leak would be radioactive iodine and cesium, [the latter of] which has a half-life of 30 years, so it stays around for a little while.

Would potassium iodide protect a person from other forms of cancer?
No, this is unique. These potassium iodide pills are not magic pills. They protect against thyroid cancer but they don't protect you against other possible cancers.

Assuming there is more than radioactive iodine in the air, what can people do to protect themselves?
There is no protective agent against other cancers. The protective measures are to evacuate, get as far away from the radiation exposure as you can so that your dose is much lower. Stay inside, don't go out and breathe contaminated air. If you do get some exposure to radioactive elements, take a shower and wash them off immediately.

The radiation leak at Fukushima Daiichi appears to have increased demand for potassium iodide in the U.S. Is this a necessary precaution?
I have experience studying the effects of radioactive iodine on adults and, based on that, it actually looks like the adult thyroid gland is not very sensitive to the cancer-producing effects of radiation. One of the things we have learned about studying the after-effects of Chernobyl is that the kids who lived in areas of radioactive fallout who drank contaminated milk had a huge increase in thyroid cancer related to radioactive iodine. Something on the order of 6,000 extra cases of thyroid cancer occurred among the children that had been exposed to increased radioactive iodine.

That's sort of the concern and the reason why the Japanese are distributing potassium iodide. That's why around our own nuclear power plants we have stockpiles of potassium iodide. Still, it's not recommended that adults over the age of 40 take potassium iodide. The benefit is miniscule because our thyroid glands are not that sensitive.