Letters to the Editor |
Re: Mortality experience of male workers at a UK tin smelter
Dear Sir,The study of mortality in workers from the Capper Pass tin smelter published by Binks et al. [Occup Med (Lond) 2005;55:215–226] has the potential for making an important contribution to the identification and quantification of occupational cancer risks. I would, however, like to make some comments on the preliminary findings for lung cancer.
The authors note that following the acquisition of the smelter by Rio Tinto plc in 1967 ‘more attention was paid to occupational hygiene and environmental control’. No doubt this is true, although not enough attention, apparently, to prevent lung cancer risks for Capper Pass employees first employed in the recent past (1970–1995) being twice the national average [observed 15, standardized mortality ratio (SMR) 198] and similar to those experienced by workers hired in the period 1955–1969 (observed 32, SMR 220). No excess lung cancer risk was found, however, in ‘early entrants’ (workers hired during the period 1937–1954) (observed 15, SMR 92). This latter group of workers almost certainly received more exposure to the known lung carcinogen arsenic (and other potential carcinogens) than later entrants and the authors discuss the somewhat anomalous findings for the early entrants in some detail. The authors are not in a position, of course, to report on the experience of all those hired in the period 1937–1954, only the subgroup still employed in November 1968. The possibility needs to be considered that a ‘super selection’ effect is in operation. In other words, only the very fittest and strongest of the early entrants were still in employment in 1968. If this hypothesis is correct then we should also expect to see suspiciously low mortality from all causes in the early entrants under study. Hopefully, the authors will be able to shed some light on this issue.
At the same time, the authors might like to reconsider their description of the allocation of person-years at risk (pyr). According to the definition of the cohort (‘... at least 12 months employment between 1/11/1967 and 28/7/1995 ...’), nobody should contribute pyr before 1 November 1968, although the methods section states that some workers contribute pyr from the same day in 1967. Any inappropriate allocation here will have resulted in underestimating the overall SMRs.
Finally, the authors wished to identify further analyses that could usefully be carried out, although they do not discuss what these might be. Perhaps funds can be made available to examine disease risks in relation to estimated exposure histories, a requirement for claimants participating in the Capper Pass Claims Review Scheme, the very scheme that ‘provided the impetus’ for this first report. (I am providing expert evidence on lung cancer risks to the Capper Pass Claims Review Scheme on behalf of the claimants.)
Occupational Epidemiology, Institute of Occupational and Environmental Medicine, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK e-mail: t.m.sorahan@bham.ac.uk
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