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Readers are invited to submit letters for publication in this department. Submit letters online at http://joem.edmgr.com. Choose “Submit New Manuscript.” A signed copyright assignment and financial disclosure form must be submitted with the letter. Form available at http://www.joem.org under Author and Reviewer information. To the Editor: Pira and colleagues have published an updated analysis of the mortality experience of a cohort of talc miners and millers in Northern Italy. The authors concluded “We confirmed the lack of association between exposure to asbestos-free talc, lung cancer, and mesothelioma.” With respect to mesothelioma, the authors found no cases among their cohort of 1722 miners and millers. They reported that 2.0 cases would have been expected based upon national and regional mesothelioma mortality rates. Unfortunately, the authors’ conclusion about a lack of association between asbestos-free talc and mesothelioma is blunted by their failure to discuss the power of their study to detect an excess risk. There is some controversy about whether the talc mined in Val Chisone was truly asbestos-free. The American geologist and mineralogist Sean Fitzgerald analyzed a sample of Grade 1615 American Ground Italian (AGI) talc from Val Chisone on behalf of an American law firm.1 He reported that “Anthophyllite, tremolite, and chrysotile asbestos were all identified in a TEM preparation of this sample. Chrysotile fibers exhibited their characteristic ‘soda straw’ morphology, highly characteristic electron diffraction pattern (SAED), and a chemistry commensurate with serpentine. Anthophyllite fibers were found with characteristic asbestiform morphology, amphibole crystalline structure by SAED, and appropriate ratio of silica to magnesium as determined by electron diffraction spectroscopy. Tremolite fibrous structures demonstrated pronounced asbestiform morphology as well, and also had distinct amphibole structure demonstrated by diffraction.” It is thus possible that the Val Chisone ore contained low-level asbestos contamination. Pira and colleagues used national and regional mesothelioma mortality rates as a comparison for the Val Chisone miners. National and regional rates would be dominated by the mesothelioma risk of workers such as manufacturers, ship yard, and construction workers with heavy occupational exposure to asbestos. Ideally, one would want to compare mesothelioma risk in the talc miners to a population with no exposures to asbestos other than ambient exposures. National and regional rates might thus not be suitable as a comparison for a mining population with no, or low-level, exposure to asbestos. Therefore, consider the following “thought experiment” in which the Val Chisone miners are compared with the chrysotile miners of Quebec, Canada. McDonald et al2 reported on mesothelioma mortality in Quebec miners and millers. They found 33 deaths from mesothelioma in a cohort of 9072 men. The mesothelioma mortality rates were 33.7 per 100,000 person-years among miners and millers in the Thetford Region and were 13.2 per 100,000 in the Asbestos Region. The average across the two regions was 25 per 100,000 person-years. The 1722 miners and millers at Val Chisone were observed for 59,000 person-years. Now let us make the two assumptions that the asbestos dust concentrations were as much as (1) 10% (very high), or, (2) 1% of the asbestos dust concentrations experienced by the Quebec miners and millers. How many mesothelioma deaths would be expected in the Val Chisone cohort under these exposure conditions? In situation (1), we would expect the rate to be 10% of the Quebec rate of 25 per 100,000. In situation (2), we would expect the rate to be 1% of 25 per 100,000. Given that there were 59,000 person-years in the Val Chisone cohort, we would then expect to see 1.5 cases of mesothelioma for situation (1) in which asbestos exposure levels were 10% of those in Quebec, and to see 0.15 cases in situation (2) where exposure levels were 1% of those in Quebec. Now, Pira and all found no cases of mesothelioma in the Val Chisone cohort. According to the Poisson distribution, used by Pira et al to compute standardized mortality ratios for their data: there is a 22% chance of observing no cases when 1.5 were expected (situation 1); and, there is an 86% probability of observing no cases when 0.15 were expected (situation 2). The exposure at which there is a 50/50 chance of observing no case, or, of observing one or more cases of mesothelioma corresponds to an exposure of 4% of that among miners and millers in Quebec. I conclude that, given the size of the Val Chisone cohort, even at risk levels corresponding to asbestos exposures as high as 4% of those of the Quebec miners and millers, one is as likely to observe no cases of mesothelioma as one in likely to see one or more cases. It is thus overly optimistic to conclude that occupational exposure to Val Chisone talc is not associated with an increased risk of mesothelioma, given that the ore at Val Chisone likely had low-level asbestos contamination producing worker exposures substantially lower than those among the Quebec miners and millers. Observation of a much larger cohort would be required to have confidence in a conclusion that there is no risk associated with these exposures. A more appropriate conclusion might thus be that, rather than confirming the lack of an association, the study of Pira and colleagues did not have the statistical power to detect an association, if one did exist, at the low levels of asbestos exposure that may have occurred in their cohort.

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