17 November 2010

The statement by IARC that “Shiftwork that involves circadian disruption is probably carcinogenic to humans” [1] 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 [2]. 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 [3] 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 [4],[5],[6] and included a study recently published with relevant exposure information [7]. 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

References

1.      International Agency for Research on Cancer. IARC Monographs on the evaluation of carcinogenic risks to humans. Painting, firefighting, and shiftwork. Lyon: International Agency for Research on Cancer; 2010. 563-766.

2.      Fritschi L. Shift work and cancer. bmj 2009;339:b2653.

3.      Kjaer TK, Hansen J. Cancer incidence among large cohort of female Danish registered nurses. Scand J Work Environ Health 2009;35(6):446-53.

4.      Tynes T, Hannevik M, Andersen A et al. Incidence of breast cancer in Norwegian female radio and telegraph operators. Cancer Causes Control 1996;7(2):197-204.

5.      Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiol 2001;12(1):74-7.

6.      Lie JA, Roessink J, Kjaerheim K. Breast cancer and night work among Norwegian nurses. Cancer Causes Control 2006;17(1):39-44.

7.      Pronk A, Ji BT, Shu XO et al. Night-shift work and breast cancer risk in a cohort of Chinese women. Am J Epidemiol 2010;171(9):953-9.