Jan Beyea
With bipartisan support, the US House Science, Space, and Technology Committee recently passed a bill to revitalize low-dose radiation research. The bill, which would authorize an estimated $96 million in funding, has also garnered support from researchers and groups with opposing views on the seriousness of effects of ionizing radiation in the low-dose region, defined as being below 100 millisieverts—roughly the amount of radiation from 10 CT scans.
Studies of excess cancers among survivors of the Hiroshima and Nagasaki bombings have estimated a 1 percent increase in long-term cancer risk for adults receiving a dose of 100 millisieverts (the risk is higher for children), with the risk below that level declining in proportion to the dose. However, stakeholders and researchers with different hypotheses continue to debate whether or not downward extrapolation by dose magnitude—the “linear no-threshold” model deemed most reasonable by a National Research Council committee of experts—is the best way to estimate risk. Some experts argue that the cancer risk drops off more quickly than estimated at lower doses, while others say that the risk at very low doses might actually be underestimated by the linear no-threshold model. A small group of researchers insists that radiation has beneficial effects at low doses, with some claiming that conspiracies dating back to the 1950s held back the truth of this theory of radiation “hormesis.” (For a detailed rebuttal of such charges, see my April 2017 article in Environmental Research.)
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The hope of many supporters of the proposed legislation, voiced by Rep. Roger Marshall, a Kansas Republican, is that it may assist “the development of nuclear energy opportunities,” in part by reducing the size of nuclear plant evacuation zones. The bill’s supporters presume that the finding of a threshold or hormesis region would demonstrate that the existing linear no-threshold model is an over-protection that, as Northwestern University radiation biologist Gayle E. Woloschak wrote in a letter of support for the bill, “may be wastefully expensive and deplete funds that could be used for other strategic goals for the nation.”
Past research by the Energy Department to upend the linear model has failed to fulfill that dream, finding health effects below 100 millisieverts from even protracted exposures. There is so much existing epidemiological data from exposed workers, patients receiving medical diagnostics, and residents living around the Soviet nuclear complex—as well as the Japanese atomic bombing survivors—that new research, whatever it shows, will need to be interpreted in the light of all the evidence. That will likely leave stakeholders and experts debating for a long time, and the public confused.
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Inherent uncertainty. New radiation research is likely to carry uncertainties, which means government policy must be conservative in its choice of the best dose-response model to use. Why is it difficult to tease out risks at low doses? Individual risks from medical diagnostics and from the (fortunately) limited releases of radioactivity at Fukushima are generally low under the linear extrapolation model. They are small compared with background disease rates, challenging epidemiological methods. The difficulty of finding effects among background cancers is actually good news for exposed individuals. However, the social risk is sufficiently large to justify keeping doses as low as reasonably achievable and balancing risks against benefits.
My colleagues and I call radiological events “reverse lotteries”: The individual risk of drawing a cancer-causing “ticket” from an event such as the Fukushima meltdowns is small, but because so many people are part of the lottery, real people do get impacted when they draw losing tickets.
Prospective risks and retrospective risks are perceived differently. If I learned that my family and I had already been exposed to a 1-in-1,000 cancer risk, I would be angry, but I would realize that the odds were highly in our favor; none of us would likely be injured. However, if you asked me to relocate to contaminated land where my children would be exposed to a 1-in-1,000 chance of cancer, I would want to stay away unless there were major benefits associated with the move, or if I thought I couldn’t afford to do otherwise. Risk tradeoffs are personal, and families can be painfully split on the best decision, as happened at Fukushima.
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