17 Nov, 10 | by lelliott
17 November 2010
The statement by IARC that “Shiftwork that involves circadian disruption is probably carcinogenic to humans”  raises the issue of evidence-based measures for prevention, which was the focus of our letter. Assuming that nightshift work is indeed causally related to breast cancer one option for prevention would be to set limits for the individual cumulated amount of night work . Such an approach would, however, only prevent cancer cases if there is a multiplicative exposure-response relation or a threshold. If this is not the case, only setting an upper limit for years of nightshifts for the individual workers would distribute the risk among more workers but not reduce the total number of cases.
Therefore, we analyzed the 9 epidemiological studies in order to examine the exposure-response relation. We agree with Hansen and Stevens that the study by Kjaer et al  lacks individual information on night-shift work but only proxy measures. We therefore reanalysed the data excluding this study and 3 other studies relying on proxy measures of nightshift work ,, and included a study recently published with relevant exposure information . We obtained a meta-odds ratio of 1.004, 95% CI 1.001-1.008 by year of nightshift work. There was no indication of neither a threshold nor a meaningful exposure-response relation and thus the available limited epidemiological data do not provide evidence to warn against long-term nightshift work for the individual worker. On the other hand, there are plenty arguments for reducing the total population burden of nightshift work, among others the suspicion raised about increased breast cancer risk.
Henrik A Kolstad & Jens Peter Bonde
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