Wednesday, May 2, 2012

Low-level radiation risks | Bulletin of the Atomic Scientists Special issue

Bulletin of the Atomic Scientists May/June 2012
Table of Contents

Jan Beyea
Special issue on the risks of exposure to low-level radiation

-- Full text

Every time a release of radioactivity occurs, questions arise—not only about the true exposures, but also about the health risk at low doses. Predictably, debates unfold in the news media and galvanize social media networks. Sometimes these conversations enlighten the public, but often times they only exacerbate the confusion and fear about the significance and reality of exposure. Fukushima is the latest example of this warped communications strategy.

This special issue of the Bulletin examines what is new about the debate over radiation risk, specifically focusing on areas of agreement and disagreement, including quantitative estimates of cancer risk as a function of dose. In this issue, we don’t pretend to put the questions about the scientific jigsaw puzzle to rest, but we do hope to provide a sophisticated update for you, presented by people whose work has increased understanding within the field. For example, social scientist Paul Slovic updates his classic work on perception of radiation risk. Roger Kasperson, another social scientist, writes on the intriguing framework that he and colleagues developed about the social amplification of risk, which helps to explain public reactions to events like Fukushima and Chernobyl. By implication, Kasperson’s analysis raises a challenge for those who communicate risk information, whether professionally or informally. To provide information needed in a democracy, these communicators may amplify risks to the point where needless fear is generated, or they may attenuate the risks to a degree that desirable responses are avoided.

Today, the scientific and medical establishment of most countries (with the exception of France, where the public strongly supports nuclear power) accepts a default hypothesis on the effects of radiation at doses below the range where epidemiologic data are conclusive. This is the so-called linear non-threshold theory (LNT), which the review committee of the US Institute of Medicine and the National Academy of Scientists refers to in these words:

“A comprehensive review of the biology data led the committee to conclude that the risk would continue in a linear fashion at lower doses without a threshold and that the smallest dose has the potential to cause a small increase in risk to humans. (National Research Council, 2006)”

Radiation protection organizations, such as the International Commission on Radiation Protection, also use the LNT to justify minimizing future exposures; however, they have a tendency to focus on the uncertainties of the hypothesis and oppose its use to estimate consequences from releases such as Fukushima and Chernobyl—no doubt out of concern that such estimates may amplify the perception of risk. Whether or not avoiding predictions of low-dose consequences really attenuates risk perception or, in fact, amplifies it by increasing public suspicion about a cover-up is an interesting question. Technical and policy analyst Gordon Thompson, in his contribution to this issue of the Bulletin, discusses some aspects of this dilemma from the perspective of a scientist who often works with community groups.

Whatever the use of the LNT, the data from the one-time exposures of the Japanese atomic-bomb survivors provide most of the quantitative data on the linear slope, meaning the magnitude of the dose response. Epidemiologist David Richardson, whose work with these data has provided much new information about risks of low-dose radiation, writes on the history of this most important studied population, discussing its strengths and limitations. Radiobiologist Colin Hill examines some of the new biological research, particularly, on genomic instability, bystander effects, and adaptive response—effects that may lead to a better understanding of responses at very low doses and may help quantify any deviations from the LNT. An important question is whether or not any of the epidemiologic evidence has been interpreted properly. Answering no to that question is biostatistician Sander Greenland, who writes that misleading interpretations of low-dose epidemiologic data result in an underestimate of the full health impacts, because of failure to account for diseases with accelerated onsets.

Quantitative perspectives on risk at low doses have changed dramatically over the last 40 years, back when I first engaged in public debates on the subject. Has it made any difference outside political campaigns and in the culture wars? In particular, how do quantitative risk estimates affect rules and regulations? Terry Brock and Sami Sherbini from the US Nuclear Regulatory Commission examine the role that risk estimates of health effects play in regulating nuclear power in the United States.

In my own contribution to this special issue, I survey data, arguments, and debates surrounding low-level radiation risks. Historically—in the absence of human epidemiologic data—biologic arguments and cell data, fiercely debated, were used to convert risk estimates derived from the atomic-bomb data to protracted exposures. My article explores the new, large-scale epidemiologic studies that are directly relevant—not to one-time exposures received at Hiroshima and Nagasaki, but to the protracted exposures that are received from continuous decay of radioactive isotopes associated with releases from Fukushima or from the Soviet and US weapons complexes.

I also analyze contrasting data that suggest that dose responses might be higher or lower than predicted by the LNT. Some researchers believe that the dose response is higher than the LNT at low doses (supralinear response), while others maintain the dose response drops rapidly below the range covered by epidemiologic data (quasi-threshold); both groups can find some support in recent epidemiologic studies demonstrating the complexity of the scientific jigsaw puzzle that researchers face. There are other researchers who believe that the dose response turns around at some point as dose is decreased, actually reducing the risk of cancer (hormesis theory); this evidence can be found in data collected from home radon measurements correlated to county lung-cancer rates—albeit in contradiction to more standard epidemiologic studies of the same association, which do show the expected dose response.

If our efforts in this issue of the Bulletin are successful, the reader will be ready to join the debate armed with a broad-based view of the epidemiologic evidence and its differing interpretations, along with an awareness of the stakeholder and researcher landscape.

Jan Beyea
The scientific jigsaw puzzle: Fitting the pieces of the low-level radiation debate

-- Abstract

Quantitative risk estimates from exposure to ionizing radiation are dominated by analysis of the one-time exposures received by the Japanese survivors at Hiroshima and Nagasaki. Three recent epidemiologic studies suggest that the risk from protracted exposure is no lower, and in fact may be higher, than from single exposures. There is near-universal acceptance that epidemiologic data demonstrates an excess risk of delayed cancer incidence above a dose of 0.1 sievert (Sv), which, for the average American, is equivalent to 40 years of unavoidable exposure from natural background radiation. Model fits, both parametric and nonparametric, to the atomic-bomb data support a linear no-threshold model, below 0.1 Sv. On the basis of biologic arguments, the scientific establishment in the United States and many other countries accepts this dose-model down to zero-dose, but there is spirited dissent. The dissent may be irrelevant for developed countries, given the increase in medical diagnostic radiation that has occurred in recent decades; a sizeable percentage of this population will receive cumulative doses from the medical profession in excess of 0.1 Sv, making talk of a threshold or other sublinear response below that dose moot for future releases from nuclear facilities or a dirty bomb. The risks from both medical diagnostic doses and nuclear accident doses can be computed using the linear dose-response model, with uncertainties assigned below 0.1 Sv in a way that captures alternative scientific hypotheses. Then, the important debate over low-level radiation exposures, namely planning for accident response and weighing benefits and risks of technologies, can proceed with less distraction. One of the biggest paradoxes in the low-level radiation debate is that an individual risk can be a minor concern, while the societal risk—the total delayed cancers in an exposed population—can be of major concern.

David Richardson
Lessons from Hiroshima and Nagasaki: The most exposed and most vulnerable

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Because of its scope, the Life Span Study of Japanese atomic-bomb survivors has become highly influential and widely accepted by regulators, policy makers, and courts of law. The usefulness of the study in regard to calculating excess cancer risk, however, should not disguise its limitations. The Life Span Study examines survivors, but not those who died in the years immediately following the Hiroshima and Nagasaki bombings. The study design therefore intentionally omitted frail people—including the very young and the very old—who may have been especially vulnerable to radiation. Regulators and risk assessors need to be aware of this shortcoming and to supplement Life Span Study results with information from studies that focus on those most vulnerable to radiation. A failure to do so may result in an underestimate of the harm caused by radiation, particularly at low doses.

Terry A. Brock and Sami S. Sherbini
Principles in practice: Radiation regulation and the NRC

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The US Nuclear Regulatory Commission uses quantitative radiogenic cancer risk information in a number of official areas. In this article, the authors describe two specific areas where quantitative cancer risk information is used: (1) the system of radiation protection for workers and the public, and (2) the performance of value-impact analysis (i.e., cost–benefit analysis) in the review of imposing new regulations on the industry. The authors write that two main factors have led to a change in the recommended occupational dose limit. First, the International Commission on Radiological Protection (ICRP) moved away from comparisons with safe industries, instead basing its assessment on cancer risk resulting from a lifetime of radiation exposure. The second factor is that additional epidemiological data have accumulated since 1977 and, combined with changes in the methods used to analyze this data, have resulted in a reassessment of the risk-per-unit radiation dose. The ICRP now recommends an annual occupational dose limit of .020 sievert. There was no corresponding change in the recommended dose limit for members of the public. Currently, the NRC is evaluating these changes and considering revising its regulations accordingly. Nonetheless, the authors write, dose limits play a very small role in modern radiation protection practices, the emphasis being on optimizing situations involving radiation exposure, with the result that most licensed facilities operate at annual doses to workers and members of the public that are well below any applicable limit.

Gordon Thompson
Unmasking the truth: The science and policy of low-dose ionizing radiation

-- Abstract

There is scientific consensus on a prevailing hypothesis that, down to near-zero levels, the occurrence of future cancer is proportional to the dose of radiation received. Some experts and professional bodies in the field, however, subscribe to this linear no-threshold (LNT) model in scientific discussions but object to the use of the model for policy-related purposes. Given the large economic interests that are affected by policy decisions, this article recommends that experts and professional bodies avoid the intermingling of scientific and policy debates and acknowledge a logical implication of the LNT hypothesis: Low-dose radiation will sicken and kill a number of people over time.

Colin K. Hill
The low-dose phenomenon: How bystander effects, genomic instability, and adaptive responses could transform cancer-risk models

-- Abstract

From the atomic bomb dropped over Japan to nuclear accidents at Three Mile Island, Chernobyl, and the Fukushima Daiichi Nuclear Power Station, there is strong public demand for information on the cancer risks from radiation exposure. In this article, the author explores some of the biological phenomenon that could alter or confirm current concepts of low-dose effects. Reviewing bystander effects, adaptive responses, and genomic instability, the author writes that these phenomena could revolutionize conventional understanding of how to approach cancer risk assessments in low-dose, possibly protracted, environments. Though current consensus supports a linear no-threshold model, evidence suggests that these biological responses just may overturn that thinking.

Roger E. Kasperson
The social amplification of risk and low-level radiation

-- Abstract

Some risk events, assessed as relatively minor by technical experts, can elicit strong public concerns and result in substantial impacts on society and the economy. This is especially true in cases involving low-level radiation exposure. Social amplification is a conceptual framework that seeks systematically to link technical assessments of health and safety impacts with assessments of individual and social risk perceptions and risk-related behaviors. Individuals and social groups can amplify (or in some cases, attenuate) risk as they process information about events, and events can produce secondary ripple effects that may spread far beyond the initial impact and may even affect unrelated technologies or institutions. Events that are highly dreaded, poorly understood, or both have high potential for these second-order effects. Social distrust of institutions and their managers plays an important part in amplifying risk. Understanding how trust is shaped, altered, lost, or rebuilt in the processing of risk by individuals and groups is a priority need in social amplification research. The social amplification of risk has become an essential part of an integrated assessment of risk. Recent changes in nuclear energy policy cannot be understood without serious examination of the social amplification and attenuation processes at work.

Paul Slovic
The perception gap: Radiation and risk

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People perceive different types of radiation risks in very different ways. Surveys of the general public in the United States and elsewhere have consistently shown that people perceive nuclear power and nuclear waste as having high risk, but perceive other sources of radioactivity—such as medical x-rays and naturally occurring radon gas—as posing much lower risk. The majority of radiation experts see things quite differently, rating nuclear power and nuclear waste as less risky than the general public does, and perceiving medical x-rays and radon as more risky than generally believed. This perception gap demonstrates that acceptance of risk is conditioned by a number of factors, such as trust in the managers of the technology and appreciation for the direct personal benefits of the technology. Risk-communication strategies that help people place the risks of nuclear power and nuclear waste in perspective by comparing them with other risks can help reduce fears of radiation. Education about radiation can also affect risk perceptions and attitudes. Although differences between the perceptions of laypersons and those of experts cannot be attributed in any simple way to degree of knowledge, it is clear that better information about radiation and its consequences is needed. There is a particularly urgent need to develop plans and materials for communicating with the public in the event of a radiological disaster. The fear, anger, and distrust following the accident at Fukushima shows that communication is still a major problem.

Sander Greenland
Underestimating effects: Why causation probabilities need to be replaced in regulation, policy, and the law

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Causation probabilities are often a component of decisions on awarding compensation for radiation exposure and descriptions of the number of cancers caused by radiation releases. In many instances, the use of epidemiologic data to calculate such probabilities may seriously underestimate the number of people harmed and the percentage of cancers induced or accelerated by the radiation exposure. Epidemiologic studies can more reliably underpin systems that award compensation using years of healthy life lost due to the exposure. Such a system has its own imprecisions but is more scientifically supportable than using causation probabilities to award compensation.

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