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RISK ASSESSMENT: Predicting Asbestos's Fallout
Jonathan M. Samet*
Forecasting Product Liability Claims
Epidemiology and Modeling in the Manville Asbestos Case
by Eric Stallard, Kenneth G. Manton, and Joel E. Cohen
Springer, Berlin, 2005. 424 pp. $84.95, £69, €89.95. ISBN 0-387-94987-9. Statistics for Biology and Health.
A century ago, asbestos seemed a material ideally matched to the needs of increasingly industrialized and motorized Western societies. It has the useful properties of heat-, fire-, and chemical resistance along with strength and flexibility (1). Consequently, it became widely used in building materials, friction products, and fire-retarding fabrics. But after peaking in the 1970s, asbestos consumption fell quickly as recognition of the risks it posed to health led to bans and substitution with other materials. Although sentinel cases of asbestosis (the scarring disorder of the lungs caused by inhaling asbestos fibers) were reported as early as 1900, asbestos was not widely recognized as causing cancer until the 1950s and 1960s, when epidemiological and clinical studies linked asbestos exposure to mesothelioma (cancer of the lining of the lung and abdominal cavity) and other lung cancers. The identification of asbestos as a carcinogen was delayed because the increased risks for these cancers only become apparent decades after first exposure.
Millions of workers in the United States and other countries have now been exposed to asbestos. Many have developed asbestos-caused diseases, and millions of current and former workers are still at risk. Thousands of lawsuits on behalf of affected workers have been filed against companies that processed asbestos and made asbestos-containing products. The costs of compensating the claims led to bankruptcy for many companies, including the Johns-Manville Corporation, which in 1982 filed for Chapter 11 bankruptcy on the basis of the numbers of claims already filed or anticipated. The numbers of lawsuits and the costs to industry and insurers prompted calls in the United States for a federal legislative remedy, but attempts to pass such legislation have been unsuccessful to date.
Predicting the future course of litigation and planning legal remedies require estimates of the future numbers of claims for asbestos-caused disease. How can we estimate the burden of disease caused in a population by asbestos (or any other avoidable risk factor)? In a thoughtful 1953 paper, the epidemiologist Morton Levin proposed a formula for this calculation that is now widely referred to as the attributable risk (2). In his formula, the proportion of disease attributable to a factor increases as the frequency of the exposure increases and as the disease risk associated with exposure increases. Even with the data available in 1953, the formula provided an estimate indicating that the majority of lung cancer cases in males could be attributed to smoking.
The concept of attributable risk has been extended in contemporary quantitative risk assessment, a formalism codified in a 1983 report of the U.S. National Research Council (3). Such risk assessments first involve the determination that an exposure poses a risk, "hazard identification," and, if appropriate, a characterization of population risk ("risk characterization"). Consistent with Levin's formula, estimating the risk requires information on the exposure of the population ("exposure assessment") and on the effect at each level of exposure ("dose-response"). In most instances, risk assessment requires assumptions, but these can be systematically tabulated and consideration given to the consequences of the resulting uncertainties. Quantitative risk assessment is a tool widely used in decision-making and legally required for some classes of environmental agents.
Mesothelioma cells.
CREDIT: CNRI/PHOTO RESEARCHERS, INC.
In Forecasting Product Liability Claims, demographers Eric Stallard (Duke University), Kenneth Manton (Duke University), and Joel Cohen (Rockefeller University) use a risk assessment framework to estimate the numbers of claims expected during the period between 1990 and 2049 for asbestos-related disease among men exposed to Johns-Mansville-produced asbestos and asbestos products. This work came from their membership on an expert panel appointed--under Rule 706 of the Federal Rules of Evidence--by Jack Weinstein, the U.S. District Court judge who was administering the Manville Personal Injury Settlement Trust. When he was assigned jurisdiction over the Trust in 1990, it was failing, tens of thousands of claims had been filed against it, and more were anticipated. To ensure that sufficient funds would be available for payouts long into the future, Weinstein needed to be able to anticipate the number and nature of claims that would be made against the Trust. This monograph sets out in full detail the quantitative forecasting model that the expert panel developed to help Weinstein resolve the Johns-Manville litigation. In its foreword, the judge offers sharp insights into asbestos litigation and the need for legislative remedy.
The authors' work was not the first risk assessment applied to workers exposed to asbestos. A 1982 report by Alexander Walker offered projections of future disease in support of the decision by Johns-Manville to file for Chapter 11 bankruptcy (4). Irving Selikoff, whose pioneering epidemiological studies and sustained advocacy brought the epidemic of asbestos-caused disease into the spotlight, had also provided forecasts.
Stallard, Manton, and Cohen faced the challenge of estimating the two components of Levin's formula: the number of exposed individuals and the risk of their becoming ill and making claims. The authors begin with a detailed and critical review of the earlier models. Not surprisingly, they found flaws, but some of the same underlying conceptual approaches are incorporated in their model as well. They estimated the exposed population by using national data on mesothelioma occurrence to project the size of the source population that yielded the observed cases, relying on a model for the relation between asbestos exposure and mesothelioma occurrence. The annual numbers of claims that would be made for the estimated exposed population were based on the experience of the Manville Trust over the interval between 1990 and 1992. In the text, the authors set out their assumptions and computations in detail and provide extensive sensitivity analyses. The authors' model yields a staggering estimate of cumulative claims through the middle of the next century--517,000--and thousands of claims are projected annually from now until 2049.
Because estimates of numbers of claims will be needed to develop a federal compensation system, the book's model could underlie the implementation of such a plan. Johns-Manville Corporation, although the largest asbestos company operating in the United States, did not have the majority of the asbestos market. The authors' calculations, which can be reasonably generalized to other companies, imply that hundreds of thousands of claims may still be filed in the United States before the epidemic of asbestos-caused disease comes to its end.
Forecasting Product Liability Claims is notable for its illustration of the possibility of using epidemiologic and demographic methods to develop models for broad policy purposes. It also documents a successful instance of asking court-appointed experts to provide guidance on a highly adversarial issue. Nonetheless, only those interested in the details of the models will want to read the book from cover to cover. It is formula-rich and dense in its description of data sources and the machinery of the models, as it should be. Readers with interests in environmental or occupational health, product liability, or science and the law may prefer to scan the book to gain an appreciation of the approach.
References
R. Maines, Asbestos and Fire: Technological Trade-offs and the Body at Risk (Rutgers Univ. Press, New Brunswick, NJ, 2005).
M. L. Levin, Acta Unio Int. Contra Cancrum 9, 531 (1953).
National Research Council, Risk Assessment in the Federal Government: Managing the Process (National Academy Press, Washington, DC, 1983); www.nap.edu/books/0309033497/html.
A. M. Walker, Projections of Asbestos-Related Disease 1980-2009 (Epidemiology Resources, Chestnut Hill, MA, 1982).
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