Last night, I posted a diary explaining why Japan is planning to give iodine to its residents.
As I pointed out in that diary, though, iodine is a limited remedy: there are multiple threats from nuclear fallout, but iodine only works on one of them.
Below, I'll talk about the other major problem, and why there's not much we can do about them.
As I mentioned before, in a nuclear reactor, uranium-235 is hit by neutrons, and split it into smaller nuclei.
The key thing to note is that there are multiple possible products. In that previous diary, I talked about one of them, iodine-131.
Another potential product is cesium-137. If you've never heard of cesium, it's an alkali metal, chemically similar to sodium and potassium, but bigger. It's used in some atomic clocks, but, unlike iodine-131, it has no biological role.
So why is it dangerous?
Remember in that previous diary how I explained that the body can't tell the difference between normal iodine and radioactive iodine? Well, it gets worse.
Just as the body needs iodine, it also needs potassium. So, just like iodine, the body has specialized proteins that help it absorb potassium. Unfortunately, those channels can't discriminate between potassium and cesium-137, which is not all that much bigger.
So, your body sees cesium-137, and thinks it's potassium. Thus, that radioactive cesium gets incorporated into your electrolytes, circulates throughout the body, and gets taken up by the various tissues, because potassium is the major intracellular ion in living cells. It does get slowly excreted, but it can accumulate in living organisms, and make its way up the food chain that way (in much the same way that mercury does).
Perhaps even worse is a third product, strontium-90, which is especially hazardous to us mammals. Strontium-90 resembles calcium even more closely than cesium-137 does potassium, so your body again gets fooled, and takes up strontium-90. Worse, though, is that the body does have a natural repository for calcium: the bones. So while most of the strontium-90 does get excreted, a percentage of it gets stuck in your body.
Both cesium-137 and strontium-90 have relatively long half-lives, on the order of about 30 years, so they are much slower in terms of their effects than iodine-131, but much more insidious. Cesium-137, because it isn't concentrated anywhere, tends to cause an increased risk of cancers in general. But strontium-90 concentrates in bone, so it increases risks of specific cancers: namely bone cancers and leukemias.
Now you might be wondering, if these are so harmful, why can't we do anything to mitigate the risks from cesium-131 and strontium-90 the way we do with iodine-131?
The answer to that isn't difficult. Iodine is only needed to make thyroid hormone. It serves no other purpose. So, by giving lots of iodine, it's possible to "flood" the mechanism that takes up iodine, and keep the "bad" iodine out.
Calcium and potassium, on the other hand, are critical for the functions of all cells. Since the point of prophylaxis is to prevent disease, you can't use a prophylaxis that is likely to cause more harm than good. [Would you take a vaccine that causes harmful side effects in 5 percent of recipients to prevent a disease that causes harmful side effects in 1 percent of its victims?] Any possible treatment for strontium-90/cesium-137 exposure would be likely to cause more short-term harm than long-term good. For example, the concentration of potassium in cells is 30 times higher than in the blood, so you would have to give massive doses of potassium—potentially enough to stop the heart—to keep the cesium-131 from getting into cells using the "iodine trick."
There isn't really much cause for optimism here as with iodine-131, although there is at least the fact that atmospheric flow in Japan should blow a lot of the radioactive isotopes out into the Pacific rather than inland, and that any radioactive isotopes that might have escaped are less likely to reach humans than did the fallout from Chernobyl.
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Also, for those of you wondering, I see nuclear power, at the moment, as something of a necessary evil: we can't get rid of it completely in the short term, but minimizing its use long-term would be highly beneficial.