© 2006 American Thoracic Society
Residential Proximity to Naturally Occurring Asbestos: Health Risk or Ecologic Fallacy?To the Editor:In their study entitled "Residential Proximity to Naturally Occurring Asbestos and Mesothelioma Risk in California," Pan and colleagues have used an innovative method of digital geologic mapping to assess environmental asbestos exposure (1). There are two important limitations to the study design that bear comment, as they do not allow determination of a causal association between sources of naturally occurring asbestos (NOA) and mesothelioma. First, the study design does not account for the important principle of latency—mesothelioma typically manifests clinically many years following initial asbestos exposure. For instance, among 4,060 asbestos-exposed participants in the CARET cohort—with 23 cases of mesothelioma documented at time of chemoprevention trial discontinuation—the mean period since first asbestos exposure was 35 yr (2). Use of home address at time of diagnosis does not consider residential location during the relevant latency period, decades preceding diagnosis. This is likely to result in exposure misclassification, given the very strong associations the authors observed between mesothelioma risk and occupation (i.e., the shipyard worker who retires to a residence in proximity to a NOA source). Second, while the measurement of distance from residence to NOA source provides data at an individual level, it does not provide a direct measure of asbestos exposure. By deriving "exposure" data from a geographic area, the design requires an aggregate analysis of exposure (i.e., geologic map), rather than individual measures of exposure (e.g., fiber/cc), and thus suffers the limitations of an ecologic study (3, 4). This is another important source of exposure misclassification, as ambient environmental exposures are typically quite low—orders of magnitude below those in occupational settings (5). Further, for NOA to cause mesothelioma, asbestos fibers must be bioavailable via inhalation in ambient air. In settings of NOA in other regions of the world, this has typically been associated with use of asbestos-containing soil in construction materials (e.g., use of "white soil" in eastern Turkey as a plastering agent) (6). The mere presence of asbestos fibers in geologic strata does not imply airborne concentrations substantially above background levels; such a relationship should not be assumed, and direct measurements documenting such a correlation are not provided by the authors. Given these limitations, the findings of Pan and associates should be considered hypothesis generating. The authors are to be complimented for using sophisticated mapping techniques to raise an important public health issue, which now requires confirmation by studies specifically characterizing temporal (longitudinal) and bioavailable asbestos exposure. Until such follow-up studies are conducted, conclusions regarding an association between residential proximity to ultramafic rocks and mesothelioma risk at the individual level—based on ecological data—must be interpreted with caution, to avoid an ecologic fallacy. On the other hand, the findings are of sufficient concern that caution should also be applied to the large-scale development of homes in NOA source areas in California. To wait until the many years of potential latency have passed for recent immigrants to NOA source areas to have developed mesothelioma before taking public health protective action would be equally fallacious.
University of Washington, Seattle, Washington
University of California, San Francisco, California
Yale University School of Medicine, New Haven, Connecticut FOOTNOTES Conflict of Interest Statement: C.A.B. has served as an expert witness for individuals with asbestos-related disease involved in workers' compensation and litigation represented by various attorneys/legal firms. He has had no contractual-financial relationship with these individuals or their legal representatives, with all work performed on an hourly "fee for service" basis. J.R.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. C.A.R. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.R.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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