© 2006 American Thoracic Society
Residential Proximity to Naturally Occurring Asbestos and Mesothelioma Risks: Further Consideration of Exposure Misclassification and Occupational ConfoundingFrom the Authors:We appreciate the thoughtful comments of Kelsh and colleagues on our recent article (1). We agree that such a study design has limitations, and we tried to be specific about some of them, including the variable content of asbestos in ultramafic rock, other sources of naturally occurring asbestos (NOA), the impact of meteorologic conditions, and potential confounding of occupational exposures. We would argue, however, that many of the potential design limitations pointed out would lead either to small changes, if any, in the estimated risk factor effect, or to an attenuation of the true underlying risk. Thus, our finding of a risk factor effect that was robust under different analyses of the same data at least should cause concern. We strongly encourage further research to address issues of actual exposure, including the effects of personal behaviors and meteorologic conditions, and the contribution of different sources of asbestos. A major component of these studies should be measurement of exposure including biological markers. Additional etiologic studies of mesothelioma are certainly important, including lifetime occupational and residence history, biological measures of exposure, and histologic confirmation of diagnoses. These studies will be very expensive, and will inevitably not convince some people who have an economic interest in denying the association of NOA and mesothelioma. The larger question raised by Kelsh and colleagues is whether there are sufficient existing scientific data to address potential exposure to NOA. This question requires us to acknowledge the ample global data showing an association of NOA and mesothelioma, and the known potent carcinogenicity of NOA fibers, such as tremolite. Do we really want to perform the ultimate test of whether environmental asbestos is associated with mesothelioma in California and elsewhere in the United States by waiting the very long latency for this untreatable malignancy, and counting the deaths? We would suggest that prudent public health policy says not. Studies of exposure are critical to understand who is at higher risk, and how to reduce their exposure. While we heartily agree with Kelsh and colleagues that more research is needed, this should not delay efforts to mitigate exposure by straightforward means: dust reduction during construction, reducing exposure from high-exposure activities, and focusing especially on reduced exposure for children (e.g., in school playgrounds) as they are presumably most vulnerable to long-term consequences.
University of California, Davis, Davis, California FOOTNOTES Current affiliation for Xue-lei Pan: Merck & Co., Inc., Whitehouse Station, New Jersey. Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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