'Environment' in cancer causation and etiological fraction: limitations and ambiguities

Dear Sir, ’We derived our pooled risk estimate of mesothelioma from nonoccupational exposure to asbestos from a previous publication authored by some of us, in which the methods for the meta-analysis were described in detail (1). In addition to the summary relative risk, our estimate of the number of non-occupational mesothelioma cases was based on frequencies in the populations experiencing exposures comparable with those in which the relevant epidemiological studies have been carried out. This parameter is subject to great uncertainty, and as a result, we selected it in our commentary as an example of the possible lack of temporal correspondence between risk and exposure data. Mirabelli and Merler suggest a different approach to estimate the number of mesothelioma cases attributable to non-occupational exposure to asbestos, based on the reasonable assumption that all cases with a recorded exposure are attributable to it. Their results of the Italian mesothelioma registry are of interest, and a recent analysis of the same data reported that 4.2% of cases in the registry had environmental asbestos exposure (2). It should be kept in mind that, although all reasonable efforts should be made to reduce and eliminate environmental asbestos exposure, this would have a limited effect on the overall cancer burden. Mesothelioma accounts each year for 1000 cases out of 300 000 total cancer cases (0.3%) in a country such as Italy, and hence 10% of mesothelioma cases represent 0.03% of all cancer cases.