A doctor checks uses a geiger counter to check the level of radiation on a woman while a soldier in gas mask looks on at a radiation treatment centre in Nihonmatsu city in Fukushima prefecture on March 13, 2011. Photo by Omiuri Shimbuna/AFP/Getty Images
Japan’s nuclear crisis continued Tuesday, as cooling systems continued to break down at the Fukushima Daiichi power plant in earthquake and tsunami-stricken northeastern Japan.
Residents were evacuated from a 12-mile radius around the plants and told to stay indoors in a 19-mile radius. On Tuesday, after an explosion at a third reactor and a fire at a fourth, radiation spiked to a level that, for the first time, officials said could be dangerous to human health — but it then fell back again over the next several hours.
Nuclear power plants create energy by splitting uranium atoms, and radioactive elements are the byproduct of that splitting.
“You have a whole of hodgepodge of radioactive elements that are created by the splitting,” says Fred Mettler, a professor emeritus of radiology at the University of New Mexico and the U.S representative to the U.N. scientific committee on the affects of atomic radiation.
The most dangerous of those radioactive elements, or isotopes, are forms of the elements iodine and cesium. Those elements are dangerous because they’re similar to non-radioactive elements used in the human body, and so can collect there and cause damage.
Iodine is used by the thyroid gland, so the Iodine-131 isotope can cause thyroid cancer years or decades after exposure, particularly in people who are exposed to the radiation as children or teens. (That’s why officials in Japan are, as a precaution, distributing iodine pills — Iodine-131 has a half-life of only about eight days, and if people can saturate the thyroid gland with regular iodine, the gland won’t absorb as much of the radioactive iodine.)
Cesium-137, meanwhile, is absorbed by pretty much the whole body, Mettler says, and can also cause cancer years or decades after exposure. It has a half-life of nearly 30 years, and so takes many decades to decay — it’s cesium-137 that still pollutes the area around Chernobyl.
Radiation can also have immediate health effects — called acute radiation syndrome — but only in people exposed to massive doses.
Acute radiation syndrome occurs when the radiation kills fast-growing cells like skin, blood, bone marrow and cells in the gut. It can start with symptoms like vomiting and diarrhea, and cause death within weeks or months if the radiation dose is high enough.
But even in the world’s worst nuclear accident, the Chernobyl meltdown in 1986, relatively few people were exposed to enough radiation to cause acute sickness.
“In all of Chernobyl, there were only about 130 people who had that acute syndrome” Mettner says. “So the only people you’d expect to get this are really people working inside the reactor.”
Radiation levels at the plant briefly spiked to 400 millisieverts (mSv) per hour on Tuesday — high enough to cause radiation sickness in workers after an hour or two of exposure, though not fatal — before falling again, and the plant evacuated all but 50 workers.
The chart above shows how that amount of radiation exposure compares to other exposures, such as from x-rays, CT scans and normal background exposure.
So far, for the general population, the health risk remains low from the radioactive steam already vented from the plant. But the situation remains uncertain, as the remaining workers struggle to prevent a meltdown — and a much larger release of radiation.
“it cannot necessarily be called a stable situation,” Chief Cabinet Secretary Yukio Edano said in a press conference, according to CNN.