The Risks of Ignorance in Chemical and Radiation Regulation
The Nuclear Regulatory Commission sought comments last June on whether it should switch its default “dose-response model” for ionizing radiation from a linear no threshold model to a hormesis model. This highly technical debate may sound like it has nothing to do with the average American, but the Nuclear Regulatory Commission’s (NRC) decision on the matter could set the stage for a dramatic shift in the way health and environmental standards are set in the United States, with implications for everyone.
Regulators use dose-response models to explain how human health responds to exposure to environmental stressors like chemicals or radiation. These models are typically used to fill gaps where data is limited or non-existent. For example, analysts might have evidence about health effects in rodents that were exposed to very high doses of a chemical, but if they want to know what happens to humans at much lower exposure levels, there might not be much available information, for both practical and ethical reasons.
The linear no threshold (LNT) model has a tendency to overestimate risk because it assumes there’s no safe dose — or “threshold” — for an environmental stressor. (We discuss the LNT model in our new Mercatus Center research, “Regulating Under Uncertainty: Use of the Linear No Threshold Model in Chemical and Radiation Exposure.”) The response (cancer, in most cases) is assumed to be proportional to the dose at any level, even when exposure is just a single molecule. LNT is popular with regulators in part because of its conservative nature. When setting standards, the logic goes, better to be safe than sorry. That is, it’s better to assume that there is no threshold and be wrong than to assume a safe dose exists when one does not.
But does the use of the LNT model really produce the “conservative” results its proponents claim? There are very good reasons to doubt it.
The first is that there are no absolute choices; there are only tradeoffs. Regulations that address risk induce behavioral responses among the regulated. These responses carry risks of their own. For example, if a chemical is banned by a regulator, companies usually substitute another chemical in place of the banned one. Both the banned chemical and the substitute carry risks, but if risks are exaggerated by an unknown amount, then we remain ignorant of the safer option. And because LNT detects — by design — low-dose health risks in any substance where there is evidence of toxicity at high doses, businesses are led to use newer, not-yet-assessed chemicals.
Economic costs borne from complying with regulations also produce “risk tradeoffs.” Since compliance costs are ultimately passed on to individuals, lost income from regulations means less money to spend addressing risks privately. When their incomes fall, people forgo buying things such as home security systems, gym memberships, healthier food, new smoke detectors, or safer vehicles. And when regulators inflate publicly addressed risks but leave private risks unanalyzed, it becomes impossible to weigh the pros and cons of public versus private risk mitigation.
But the most compelling reason to doubt that LNT is a “conservative” standard is simply that it’s likely to be wrong in so many cases. The assumption that “any exposure” causes harm is contradicted not only by common sense, but by a growing body of research. In the decades since LNT was first adopted by regulatory agencies, more and more evidence supporting a threshold — or even a “hormetic” — model of dose response has been found.
Hormesis occurs when low doses of exposure actually cause beneficial health outcomes, and, coincidentally, the scientific evidence for hormesis appears strongest in the area where the LNT was first adopted before its use spread to other areas: radiation. For example, low-doses of radiation exposure have been shown to have protective effects against kidney damage in diabetic patients, and low doses of X-rays have been associated with an anti-inflammatory response to treat pneumonia. There is now evidence of hormesis in hundreds of experiments, but the LNT rules out — by assumption — the possibility of these kinds of beneficial health responses.
Unfortunately, the way regulators typically respond to these problems is simply by ignoring them. Hence a better moniker for the use of the LNT model might be “Ignorance Is Bliss.” So long as regulators ignore the inconvenient truths posed by the possibilities of hormesis and risk tradeoffs, they can continue going to work every day maintaining the belief they are protecting public health. But the uncertainty in their risk assessments is so great that, in fact, regulators often have no idea whether they’re improving public health or doing just the opposite.
A reconsideration of the LNT is long overdue. At the very least, risk analysts should characterize uncertainty using multiple-dose response models — including a threshold model or a hormetic model — when no model has the overwhelming support of the scientific evidence. And analyzing risk tradeoffs should be a routine part of rulemaking.
The NRC should be commended for acknowledging the doubts about the LNT. When the time comes for the agency’s decision, let’s hope they choose knowledge over ignorance.
James Broughel is a research fellow with the Mercatus Center at George Mason University. Dima Yazji Shamoun is the associate director of research at the Center for Politics and Governance and a lecturer at the economics department at the University of Texas at Austin. They are coauthors of new Mercatus Center research on “Regulating Under Uncertainty: Use of the Linear No Threshold Model in Chemical and Radiation Exposure.”
