Carcinogenicity of night shift work

In June, 2019, a Working Group of 27 scientists from 16 countries met at the International Agency for Research on Cancer (IARC) in Lyon, France, to finalise their evaluation of the carcinogenicity of night shift work. This assessment will be published in volume 124 of the IARC Monographs.
In 2007, shift work involving circadian disruption was classified as “probably carcinogenic to humans” (Group 2A), on the basis of sufficient evidence in experimental animals and limited evidence of breast cancer in humans. In this updated evaluation, the Working Group chose the name “night shift work” to better describe the exposure circumstances and to reflect the main evidence base for the human cancer studies. The re-evaluation was motivated by the large number of new, high-quality epidemiologic studies including additional cancer sites. However, the Working Group noted the considerable variability in the detail and quality of exposure information on night shift work reported in these studies. Exposure information was more detailed in case-control studies, including in those nested within cohorts, than in cohort studies. A number of occupational, individual, Carcinogenicity of night shift work lifestyle, and environmental factors might mediate, confound, or moderate potential cancer risk in night shift workers.
The Working Group concluded there was limited evidence that night shift work causes breast, prostate, and colorectal cancer. This evaluation was based on comprehensive searches of the literature, screening of the studies using established inclusion criteria, and evaluation of study quality, including a standardised review of exposure assessment. Greater weight was given to the most informative human cancer studies based on methodologic considerations, including study size, potential selection bias, night work assessment quality (most notably, potential for misclassification), and control for potential confounding factors. The largest number of informative studies examined breast cancer, several examined prostate and colorectal cancer, while fewer were done on other cancers.

Source: Carcinogenicity of night shift work. (2019). Lancet, 20(8), 1058-1059.

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