A Retrospective Cohort Study of Shift Work and Risk of Incident Cancer among German Male Chemical Workers

By Yong, Mei; Blettner, Maria et al. | Scandinavian Journal of Work, Environment & Health, September 1, 2014 | Go to article overview

A Retrospective Cohort Study of Shift Work and Risk of Incident Cancer among German Male Chemical Workers


Yong, Mei, Blettner, Maria, Emrich, Katharina, Nasterlack, Michael, Oberlinner, Christoph, Hammer, Gaël P., Scandinavian Journal of Work, Environment & Health


By interfering both with biological and social circadian rhythms, shiftwork involving work at night poses several challenges to those who perform it. The resulting misalignment may be associated with generally increased health risks. Shiftwork involving exposure to light at night and the subsequent depression of endogenous melatonin was hypothesized to be an etiologic mechanistic pathway for developing breast cancer (1, 2). More generally, chronotype, sleep pattern, and circadian genes are supposed to act in the adaptation to shiftwork, and subsequently have influence on the direct or indirect pathways from shiftwork to the potential adverse health outcomes (3).

In 2007, the International Agency for the Research on Cancer (IARC) classified "shiftwork that involves circadian disruption" as probably carcinogenic (2A), based on sufficient evidence in experimental animals but limited evidence from epidemiological studies (4, 5). Since then, a body of literature has emerged from research on shiftwork and its impact on cancer risk for all sites (6, 7) and several site-specific cancers, such as prostate (6, 8), ovaries (9, 10), skin (11), non-Hodgkin lymphoma (12), pancreas (13), and lung (14) in addition to breast cancer (15-21). However, evidence from epidemiological studies still remains inconsistent. In recent reviews, findings for an adverse association between night work and breast cancer were considered to be suggestive but not conclusive, while limited and inconsistent for cancers at other sites and all cancers combined (22, 23, 24).

We have previously studied acute injury, chronic illness, and total mortality in two cohorts of male wage employees, respectively engaged in rotating shiftand day work, followed up from 1996-2006 (25, 26), and all-cause and cancer-specific mortality (27) and mortality due to ischemic heart disease (28), followed up from 2000-2009. Here we found no difference between ever and never shiftworkers, nor dose-response relations with duration of shiftwork. The present study aims to further investigate total and site-specific cancer incidence risks, in this large industry-based cohort with largely homogeneous occupational profiles.

Methods

Study design and population

This study was designed as a retrospective cohort study of male production workers in a German chemical company. The cohort was defined in a previous study that was described in a former publication (25). Briefly, the initial shiftcohort consisted of 14 037 male wage employees who completed ≥1 year of rotating shiftwork between 1995-2005. A cohort of 17 095 day workers was identified as the referent population using the same search strategy in the work histories but excluding all employees who ever performed shiftwork or whose job titles were indicative of office work. Most of the initial cohorts were residents of Rhineland-Palatinate in Germany, 12 609 (90%) shiftand 15 219 (89%) day workers. The present study that aims to compare the cancer incidence risks between the two sub-cohorts is based on this population. Incident cancer cases were identified through anonymous record linkage with the cancer registry of Rhineland- Palatinate. The algorithm uses pseudonyms of the personal information including names, sex, date of birth, and place of residence to match cases. Cancer incidence was recorded for the period from 2000-2009. The later onset of the follow-up period compared to our earlier studies was due to the fact that an acceptable completeness of the cancer registry data was only achieved since the year 2000, owing to obligatory notification in the State of Rhineland-Palatinate.

Cancer registry data

This study is based on cancer cases registered in the Rhineland-Palatinate cancer registry that covers a population of 4 million with about 18 000 cancer cases added each year. Since 1 January 2000, all Rhineland- Palatinate physicians and dentists are legally obliged to report incident cancer cases to the cancer registry. …

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