Web edition: March 17, 2011
Radioactive iodine released by the Chernobyl nuclear accident has left a legacy of thyroid cancers among downwinders — one that shows no sign of diminishing. Details of one at-risk population emerges in a new study of some 12,500 Ukraine residents who were children at the time of the April 1986 accident.
The new data also point to what could be in store if conditions at Japan’s troubled Fukushima Daiichi nuclear-power complex continue to sour. Since March 12, public fears have been building about the possibility that people living near the earthquake-ravaged electrical-generating complex could be hit by substantial fallout from radioactive plumes that might be emitted with little notice.
On Wednesday, March 16, the U.S. Nuclear Regulatory Commission seemed to justify those fears when it issued a statement saying it “believes it is appropriate for U.S. residents within 50 miles of the Fukushima reactors to evacuate.”
Such a recommendation is warranted, NRC said, “when projected doses could exceed 1 rem to the body or 5 rem to the thyroid.” A rem is a dose of radiation, one that’s equal to 0.01 sievert (the corresponding SI unit).
Japanese officials in recent days have reported dose rates as high as 0.4 sievert per hour near damaged reactors and used-fuel pools at the Fukushima facility. But doses at the entrance gate have only peaked at values about two percent as high. So to understand the new NRC announcement, it would appear the agency’s intel points to the potential not only for large plumes of radioactive materials — but also ones that might cause harm well beyond the 18.6-mile (30 kilometer) perimeter that Japanese officials have identified as constituting a high-risk zone.
The iodine risk
The six-reactor Fukushima complex lost power following the earthquake and accompanying tsunami. As a result, those reactors that were up and running lost their cooling, which allowed their fuel to overheat. To relieve the pressure which might compromise a reactor’s protective vessel, safety crews periodically vented a little of the gas building up inside those vessels. Some iodine-131, a radioactive byproduct of the fissioning in a nuclear reactor, escaped during those controlled releases.
That’s not a health concern. What is: a catastrophic crack in one of the reactor vessels — or a melting of overheated fuel through the vessel’s floor. Either might allow the wholesale release of radioactive gases and particles. That’s what happened during the Chernobyl accident, almost exactly 25 years ago.
Working together, scientists at the National Cancer Institute, three U.S. universities and a medical institute in Kyiv, Ukraine, uncovered 65 new cases of thyroid cancer between 2000 and 2007 among their study’s recruits. The participating men and women have undergone extensive and repeated monitoring since the accident. Each individual’s radiation dose was also well characterized shortly after the accident.
Because the thyroid uses iodine, people who incurred the biggest iodine-131 doses would be expected to experience the highest cancer risk. And they did, NCI’s Alina Brenner and her colleagues report in a paper posted early online March 17 in Environmental Health Perspectives.
For each rem of I-131 their bodies took in, their risk of developing thyroid cancer almost tripled. And the body’s response to those doses appeared linear, which means that proportionately smaller or larger doses resulted in proportionately smaller or larger risks of developing the cancer.
During earlier surveys of these people “we were able to detect 45 cancers,” Brenner says, “but because these thyroid cancers are slow growing, we don’t know when they first occurred.” The new study identified additional malignancies that first emerged 15 to 22 years after the I-131 exposures.
Role of possible iodine deficiency
Participants for the new study came from three Ukraine states, and rates of thyroid cancer per unit of radiation varied a bit by geographic region. The suspicion, Brenner says, is that those who appeared most sensitive to iodine’s effects might have been particularly iodine deficient at the time of the accident. Indeed, some of these regions have had a chronic iodine-deficiency problem.
Iodine status is an important determinant of I-131’s risk, which explains why government officials often hand out iodine tablets to people at risk of I-131 exposures. By loading up the body’s thyroid with benign iodine, the organ will ignore much of the radioactive form that might be ingested later.
Some of the participants of the new study had taken such prophylactic iodine, Brenner says. But knowing for sure who got it and whether it was taken in time to be effective is hard to assess at this late date. “These individuals were children,” the epidemiologist observes. “So how well they could remember 14 years later what they had taken — well, there is a lot of uncertainty here.”
As for the current environment in Japan, “If there is some kind of significant exposure, one should also be concerned with radioactive iodine — especially in children,” says Kiyo Mabuchi, acting head of the Chernobyl unit at NCI in Bethesda, Md.
The prognosis for surviving thyroid cancer treatment is very good, Mabuchi notes — if treated in time. That’s why downwinders of any Fukushima catastrophic releases warrant careful monitoring for decades, he says.
A.V. Brenner, et al. I-131 dose-response for incident thyroid cancers in Ukraine related to the Chornobyl accident. Environmental Health Perspectives, in press. DOI: 10.1289/ehp.1002674. Abstract at: [Go to]
Office of Public Affairs. NRC provides protective action recommendations based on U.S. guidelines. Nuclear Regulatory Commission: Washington D.C. [Go to]
International Chernobyl Radiological Portal of the ICRIN project, a website developed as a joint effort by the International Atomic Energy Agency, the United Nations Development Programme, the United Nations Children's Fund, and the World Health Organization. [Go to]