As you may have read, the Japanese government plans to distribute tablets of potassium iodide to deal with potential exposure to radioactivity as a result of the radiation leaks at nuclear power plants there.
But you might be wondering: why iodine?
Well, it has to do with how nuclear reactors and our bodies work. Read on for a quick primer.
All nuclear power plants operate on the idea of nuclear fission.
In other words, by bombarding them with particles (in this case, neutrons), a radioactive isotope breaks down into smaller nuclei of other elements (not surprisingly, this process is called transmutation—after the goal of early alchemists!).
Also, let's define a couple of terms quickly. Nuclei contain positively-charged particles, protons, and neutral particles, neutrons. The electrons in an atom orbit the nucleus.
What determines whether an atom is an atom of hydrogen, helium, etc., is the atomic number—that is, the number of protons. An atom with one proton is always a hydrogen atom, an atom with two protons is always a helium atom, and so on.
Isotopes are atoms of the same element with different numbers of neutrons. This means they have different mass numbers, which is the number after the element name. So, for example, when I talk about uranium-235 (U-235 for short), I'm referring to an atom of uranium, which always has 92 protons, and 143 neutrons (235 - 92 = 143). [For the record, protons and neutrons are about 2,000 times heavier than electrons, so we ignore electrons in calculating mass numbers.]
Two things determine whether an isotope is radioactive:
# The atomic number. Any isotope with more protons than lead (82) is radioactive. [You may have learned that bismuth is the heaviest stable isotope. Theoretical calculations actually suggested that it should be radioactive, a point proven just a few years ago. But don't run to your bathroom to get rid of it! Bismuth decays so slowly that it poses zero risk to you.]
# The ratio of the mass number to the atomic number. If that ratio is too high or too low, an atom is subject to decay.
Nuclear reactors make use of uranium-235. When you bombard it with neutrons, it decays into various isotopes, at least some of which are radioactive. As it turns out, one of the more common isotopes is iodine-131, which is radioactive (the stable isotope of iodine is iodine-127). This is a highly unstable isotope that decays quickly (unlike bismuth, with a half-life measured in billions of billions of years (no, that's not a typo), iodine-131 decays with a half-life of 8 days. [A half-life is the amount of time it takes for half the atoms in a sample to decay.]
Our bodies need iodine, though, especially our thyroid glands. Our bodies can absorb iodine into our bloodstream from both our digestive tracts and our lungs. Unfortunately, though, our bodies can't tell the difference between the "safe" iodine-127 and the "dangerous" iodine-131; it sees them all as "iodine" and absorbs them all.
So, what happens is that this radioactive iodine is concentrated in the thyroid, where it suddenly decays to produce high-energy gamma rays that can cause DNA mutations . . . which later in life can lead to thyroid cancer. They can also lead to hyperthyroidism and other problems.
The one saving grace, as it were, though, is that since iodine-131 decays so quickly, it doesn't stay in the body all that long. Plus, the thyroid gland is relatively small, and can only hold so much iodine at any given moment (on the order of 10 milligrams). So, if you can keep the iodine-131 from accumulating in the thyroid over the exposure period, which is normally just a few days, you can minimize the risk.
So, that's where the potassium iodide tablets come in. They contain about 100 milligrams of safe iodine-127, which effectively "floods" the body with safe iodine, and minimizes the likelihood that the dangerous iodine-131 will be taken up by the thyroid before it can be excreted by the body. The risk of getting cancer from exposure to iodine-131 is not even close to 100%, but, obviously, the lower the better.
Sadly, that prophylaxis was not administered to those exposed to radiation from the Chernobyl disaster, but Japan is taking steps to make sure its citizens get it.
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I hope you found this useful. That said, I should point out that iodine-131 is a potential danger, but it's probably not the biggest danger. If there's interest, I'd be glad to write another diary explaining what that other danger is—and why we can't treat that danger the way we do iodine-131.
Updated by Samer at Sun Mar 13, 2011, 01:13:23 PM
Part 2, on the bigger danger, is now posted.
Updated by Samer at Sun Mar 13, 2011, 05:19:02 PM
Deep Harm noted below that I didn't really address the issue of how to protect yourself in the possible event that iodine-131 makes its way to the U.S.
As he noted (and I found independently while responding to another post), the NY State Department of Health has an excellent site explaining the details of when and how much to take, as well as who shouldn't take it (and why).