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Tom Sorahan has raised a number of points about our recent paper on mortality amongst workers at the former Capper Pass tin smelter on Humberside, to which we would like to respond.First, there was an error in the drafting of the definition of the cohort, which should read ‘... 1462 males who had been employed both for at least twelve months and between the dates 1/11/1967 and 28/7/1995 . ...’ The description of person-years accumulation in the subjects and methods section and subsequent evaluation of standardized mortality ratios (SMRs) is correct.
Our data are not consistent with the operation of a ‘super selection effect’ in relation to the lung cancer mortality of the workers first employed before 1955. Such an effect, if it existed at all, could not be specific for lung cancer and we find no evidence for it in mortality from all causes or from non-malignant conditions. Given the relatively small number of lung cancer deaths in each of the four dates of first employment categories, the role of chance in creating the observed distribution of mortality cannot be entirely discounted. Nonetheless, we have suggested that exposure to an occupational carcinogen, the effect of which diminishes with time since exposure, could explain all the features of lung cancer mortality seen in Table 7 of our paper.
Finally, thanks to continued funding from Rio Tinto, work on exposure estimation has been proceeding and we hope it may be possible to offer a further manuscript on the relationship between exposure and mortality for publication in due course.
On behalf of: K. Binks, R. Doll, M. Gillies, C. Holroyd, S. R. Jones, D. McGeoghegan, L. Scott, R. Wakeford, P. Walker.
e-mail: steve.jones@westlakes.ac.uk
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