Government officials have confirmed that radiation has leaked from the Fukushima power plant site in Northern Japan, where workers are scrambling to prevent a meltdown at two damaged reactors. The surrounding area has been evacuated. It’s difficult to ascertain how much radiation has already leaked from the plant–or what the exposure will be if either or both of the reactors suffer a total meltdown. But one thing seems likely: Japan will soon have to ask one of the most vexing scientific questions related to public health: what is a “safe” level of radiation? When can residents return to their homes?
As my colleague Jeffrey Kluger wrote over the weekend, an overheating nuclear power plant can let off vapor packed full of high energy by-products. Scientists have a pretty firm grasp on what this radiation does to the human body in high doses: it causes agonizing and often fatal damage in a matter of days or hours. But controversy remains over the effects of low-dose radiation—the sort that does not make an individual ill but which may lead to cancer.
Most regulatory agencies around the world–and the IAEA–rely on a mathematical model to estimate radiation risk. Using this model— known as the linear-nonthreshold dose-response model (LNT model)—agencies extrapolate down in a linear fashion from high radiation dose levels known to be harmful. This method results in extremely conservative safety levels—the U.S. Nuclear Regulatory Commission (NRC), for example, sets an acceptable level of radiation exposure from any one source at 100 millirem a year. In contrast, the average level of natural background radiation in the United States is about 350 millirem a year. (A chest X-ray, for further comparison, gives the equivalent to 1 or 2 millirem to the whole body.)
But some scientists say the model is based on a false assumption. It assumes that there is no safe dose of radiation and that the risk of getting cancer or genetic damage increases in a linear fashion along with exposure. These scientists argue that there is a threshold below which radiation poses no hazard to health, as cells only slightly damaged by radioactivity can heal themselves without long-term ill effects.
Peter Zimmerman, a physicist and former chief scientist of the Foreign Relations Committee says that when his staff looked into radiation safety limits as part of legislation pertaining to nuclear terrorism, “it became pretty clear to me that the linear-nonthreshold needed to be revised.”
“Japan will have to decide what it considers an acceptable level of radiation exposure following an emergency” Zimmerman adds. “That’s a conversation it needs to have with itself.”
As an example of the controversy surrounding low-level radiation, the U.S. federal government can’t even agree on a safe dose. The Environmental Protection Agency differs from the NRC in that it advocates the more stringent standard for all radiation exposure from a single source or site at 15 millirem—rather than 100 millirem— a year. The NRC has a different set of guidelines following a severe nuclear emergency such as a meltdown or dirty bomb attack, however. In these cases, the safety threshold increases to a 25 rem per 30 days—a huge increase and right on the threshold of when people begin to show physical signs of radiation poisoning after a single dose such as changed blood work (physical symptoms usually doesn’t appear below 100 rem in a single dose). Areas receiving more radiation would be evacuated. When citizens would be allowed to return home following a clean-up operation would be decided on a “case by case” basis, according to an NRC spokesman speaking on background.
Of course one of the crucial questions following a nuclear disaster is whether an understandably jittery public will accept a higher safety threshold and eventually return home, or instead turn nearby population centers into ghost towns. Zimmerman says that the return-home level would almost certainly be higher than normally accepted standards. “When we looked at dirty bomb attacks, we didn’t contemplate throwing away much of Manhattan–maybe a few hundred meters radius but not any more than that,” Zimmerman explains.
Many supporters of the LNT model admit that there is contradictory evidence surrounding the health effects of low-dose radiation, but that it is the duty of regulatory agencies to err on the side of caution in the face of such uncertainty. A main problem in reaching consensus on a safe level is that linking cancer deaths to low-level radiation is almost impossible given that nearly 20-30% of Americans get cancer at some point in their lives anyway. Further complicating a statistical analysis is the fact that humans are exposed every day to background radiation. “We live in a radiation field. There are cities where background radiation level is extraordinarily high, such as Denver, but the cancer rates are particularly low. I don’t know why— maybe the diet is better—but the point is that no one knows why,” says Zimmerman.
What’s more, the long-term effects of the one instance of a severe radioactive meltdown and leak at a nuclear power plant—at Chernobyl in 1986—has also caused disagreement. The UN’s World Health Organization and the International Atomic Energy Agency claim that only 56 people died as a direct result of the radiation released at Chernobyl and that about 4,000 will die from it eventually. But the International Agency for Research on Cancer, another UN agency, predicts 16,000 deaths from Chernobyl; an assessment by the Russian academy of sciences says there have been 60,000 deaths so far in Russia and an estimated 140,000 in Ukraine and Belarus.
(Update: The NRC released a briefing document on radiation safety late Sunday night. It stated: “Although radiation may cause cancers at high doses and high dose rates, currently there are no data to establish unequivocally the occurrence of cancer following exposure to low doses and dose rates…Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect.”)
The field of radiation safety has become so divided in recent years that some scientists now argue that low-level radiation may actually be good for your health. They point to studies, like one of tuberculosis patients who had multiple chest X-rays and one of nuclear workers, that showed that the tubercular patients had fewer cases of breast cancer than would be expected and the nuclear workers had a lower mortality rate. (The National Council on Radiation Protection and Measurement, however, has evaluated these and other studies on the protective effects of radiation, and found that they were “insufficient to support” the hypothesis.)
The most vocal critic of the LNT model is the Oxford University physicist Wade Allison, whose book “Radiation and Reason” argued that safety levels should be increased by at least a factor of ten. His book argues that understanding of radiobiology and the ability of the body to repair itself following radiation exposure is “completely inconsistent with the current safety standards.”
“The Japenese government should respond to this crisis by building more nuclear reactors to help kick starts its economy,” Allison told me by phone from Oxford.
Whether or not the Fukushima power plant suffers a full meltdown, it’s likely that a clean-up operation in the surrounding area will eventually take place. According to Zimmerman, “The ‘good news,’ in inverted commas, is that a leakage or even meltdown can be scrubbed once the immediate emergency is over. They will need to pressure wash and scrub the streets for miles around; they will either push the dirty water to the ocean or the suds will be collected in a special machine and transported to areas designed to store low-level nuclear waste. If they find particularly “hot” areas they dig up the whole area and take the earth to a low-level waste site. It’s a big operation.”
Ferenc Dalnoki-Veress, a nuclear expert at the Monterey Institute of International Studies, says that it is precisely because radiological clean-up operations involve so much money—and because the cost associated with meeting safety standards of normally functioning nuclear power plants is so great—that he remains skeptical of all efforts to revise the safety levels of low-dose radiation.
“Here we have a high-stakes example where the science can greatly influence policy. So we have to be extra-cautious at how studies are conducted, the methods used, their funding source, and whether the conclusions drawn actually reflect the data. My own feeling is that while I realize the point of view I espouse is often criticized for being too conservative, it is prudent to assume the LNT model until proven otherwise,” he says.
It is a maxim of medicine that the greatest advances are the result of other people’s tragedies; we learn most about the human body when things go wrong. It will come as no consolation to the nearby residents of the Fukushima plant that, however this crisis unfolds, radiation experts will likely follow their cancer rates in coming years in an effort to end the controversy over where to draw the line when it comes to radiation exposure.