Plutonium

"Plutonium's half life is 24,000 years ... anything released in Fukushima today could be around at dangerous levels for up to half a millon years."

There are four kinds of isotopes that are likeliest to be emitted by the crippled Fukushima Daiichi plant, as well as the other three that have been taken offline: iodine-131, cesium-137, strontium-90 and plutonium-239. Iodine-131 is, in many ways, the most dangerous of the four, because it can lead to cancer — specifically thyroid cancer — in people exposed to it in the shortest time. Epidemiologists estimate that there were 6,000 to 7,000 cases of thyroid cancer that never would have occurred as a result of the 1986 Chernobyl explosion in Russia. Most of the victims were people who were children at the time of their exposure and developed the disease later.

Strontium and cesium are the next up the danger scale. While iodine tends to concentrate its damage to the thyroid, those two are not nearly so selective. "Strontium is chemicaly similar to calcium," says Dr. Ira Helfand, a board member for Physicians for Social Responsibility. "So it gets incorporated into bones and teeth and can stay there, irradiating the body, for a long time." Strontium is most commonly linked to leukemia.

Cesium works in other ways, behaving more like potassium when it's inside the body — which means it circulates everywhere and can contaminate anything. Cesium doesn't linger as long as strontium does — it gets excreted in urine over the course of months or years — but that's more than long enough to cause cancer of the liver, kidneys, pancreas and more. "Basically all of the solid tumors," says Helfand.

More troubling, cesium and strontium linger not just in the body, but in the environment. Strontium has a half-life of 29 years; cesium's is 30. A radioactive isotope is generally considered dangerous for 10 to 20 times its half life, which in these cases tops out at about 600 years.

Most worrisome of all is plutonium-239 — for a number of reasons. First of all, the vast majority of a fuel rod is made of plutonium, which means there's just more of it in play. What's more, says Helfand, "It's extraordinarily toxic." Plutonium exposure usually comes from inhalation rather than ingestion, so it's mostly associated with lung cancer. What's more, plutonium's half life is 24,000 years, which means anything released in Fukushima today could be around at dangerous levels for up to half a millon years.