In the absence of data, the debate over low-level radiation is largely one of faith. Some researchers believe in the "linear no-threshold relationship." This mouthful assumes that if high doses of radiation impose high risks, low doses must bring their own, albeit lower, risks. On the other hand, some nuclear experts believe that low doses are harmless, and there is indeed a "threshold" below which radiation exposure can be ignored. By analogy, they might say, smoking three packs a day is bad news, but smoking one cigarette a month won't kill you.
Despite the lack of data on radiation exposure, most regulatory bodies conservatively adopt the no-threshold approach. That is why, for example, the Nuclear Regulatory Commission * caps occupational exposure at 0.05 sieverts per year. Such fears have also driven considerable hand-wringing among doctors, who point to the four-fold increase in CT scans since the 1990s and claim that 2 percent of all cancers now are actually caused by the scans. But that's only true if there's a linear relationship between radiation exposure and cancer risk. In other words, it's assumed without any evidence that exposing a million people to 0.00001 sieverts is just as bad as exposing one person to 10 sieverts.
Is there any downside to being so conservative? Consider what happens when people think they've crossed over a "safe" limit. According to a review from the National Institutes of Health, inchoate fears and misinformation in the wake of the Chernobyl disaster led to roughly 100,000 additional abortions among pregnant women and over 1,250 suicides. Meanwhile, outsize fears of radiation risks may lead patients and physicians to avoid necessary X-rays. Last month, the International Commission on Radiation Protection recommended that Japan temporarily raise the annual radiation limit from 0.001 sieverts to 0.02 sieverts per person, and the Tokyo Electric Power Company suddenly raised radiation worker limits to 0.15 sieverts per year. Though the changes are scientifically defensible—since no data exist showing that 0.15 sieverts are dangerous—those who believe in the no-threshold model may assume the safety of citizens and workers has been sacrificed for convenience.
Ultimately, the debate over the presence or absence of a safe threshold is the most basic divide in our society's approach to environmental regulations. Whether we argue over the safety of BPA in infant bottles, lead in old houses, radiation from nuclear accidents, or trace amounts of radon in homes, we're talking about the same thing. Everyone knows lots of lead or radiation or radon is bad; we'll just never know for sure whether the relationship holds at lower doses.
Back in 1972, the nuclear physicist Alvin Weinberg presciently wrote that the dilemmas of low-dose radiation or toxic chemical exposures are "trans-scientific." To decide whether low dose radiation is dangerous would require a research project of monumental, impossible scope. Such questions, he wrote, "can be asked of science and yet cannot be answered by science. [This] elementary point has been lost in much of the public discussion of environmental hazards." The only solution was for policymakers and the public to learn to think in probabilities instead of absolutes. Instead of imposing a black-white dichotomy on suspected toxins, a graded approach—perhaps a gradient with shades of gray?—would make more sense.
Taken another way, Weinberg encouraged debates over public health regulations to acknowledge, and even embrace, uncertainty. He identified the one substance that certainly can be effective even in the smallest possible concentration: humility.
Correction, April 15, 2011:The article originally referred to the Nuclear Regulatory Commission as the Nuclear Regulatory Agency. (Return to the corrected sentence.)