In terms of potential health dangers from radiation from the Fukushima Daiichi Nuclear Power Station, “the people who are in the most immediate danger from acute and severe radiation doses are those people who are on site at the moment and who are desperately trying to keep the reactors under control,” says Jacqueline Williams, a radiation oncologist at the University of Rochester Medical Center.
Moving away from the immediate vicinity of the plant, radiation levels drop very rapidly. James Thrall, radiologist-in-chief at Massachusetts General Hospital, says that radiation levels are inversely proportional to the square of the distance from the source: The level at two miles from the source are one-quarter what they are at one mile, and “at 10 miles away, it’s almost an infinitesimal fraction,” he says. Individual exposure also varies widely depending on whether a person is outside or indoors, or shielded with protective clothing. Japanese authorities have evacuated the population living within a 20-kilometer radius of the plant, and have warned those living within 30 kilometers to stay indoors. Some experts say that people living beyond this range have no cause for concern at this time. “This has nothing to do with the general population,” McBride says.
The trickier question is whether lower doses of radiation—well below the threshold of acute illness—could lead to long-term health consequences for those in that area. Thrall says that epidemiological studies on survivors of nuclear attacks on Japan have found that those receiving 50 millisieverts or more had a slightly elevated cancer risk—about 5 percent higher than expected—and that risk seemed to rise with higher exposures. But scientists still vigorously debate whether that risk can be extrapolated down to even lower exposures.
After the nuclear disaster at Chernobyl, the population experienced a surge in thyroid cancers in children. However, scientists found that the culprit was not radiation in the air but radioactive contamination of the ground, which eventually found its way into cow’s milk. Thrall points out that in Japan, this is highly unlikely because the authorities are carefully monitoring the water and food supplies and keeping the public informed, which did not happen at Chernobyl.
Another important factor in determining the potential health consequences is the type of radioactive isotopes released from the plant. Different isotopes have vastly different half-lives; some decay almost instantly, while others persist for weeks or years. Iodine-131, which has been detected at the site, has a half-life of eight days, while caesium-137, also detected, has a half-life of 30 years. Japanese authorities have distributed over 200,000 doses of potassium iodine tablets, which can help prevent the risk of thyroid cancer from radioactive iodine. However, Thrall says, the pills can cause unpleasant side effects and rare serious conditions in people with allergies or thyroid problems, so they should not be taken indiscriminately.
There are major differences between the type of reactor at Chernobyl and the one at Fukushima, according to Peter Caracappa, a professor in the Radiation Measurement and Dosimetry group at the Rensselaer Polytechnic Institute. During an online Q&A hosted by Reuters, he said “the first difference is that at Chernobyl, there was a large quantity of graphite in the core which caught fire and spread contents of the reactor high into the air.” The Japanese plant uses water rather than graphite. Second, the Chernobyl reactor did not have a containment structure like the ones present at these plants. Such structures, he said, “are designed to contain the contents of the reactor even in the case of an accident. A failure of containment, if it should come to pass, may allow materials from the core to ‘leak’ out, but they would not ‘spew’ out in the same way as Chernobyl.” That could limit how far the radiation spreads.