Elsevier

European Journal of Cancer

Volume 41, Issue 13, September 2005, Pages 2023-2032
European Journal of Cancer

Night work and breast cancer risk: A systematic review and meta-analysis

https://doi.org/10.1016/j.ejca.2005.05.010Get rights and content

Abstract

The association between occupations that involve night shift work (a surrogate for exposure to light at night with subsequent melatonin suppression) and breast cancer risk is uncertain. We therefore conducted a systematic review and meta-analysis of observational studies to assess the effects of night work on breast cancer risk.

Data sources were MEDLINE from January 1960 to January 2005, experts in the field, bibliographies, and abstracts. Search terms included night work terms, flight personnel terms, cancer terms, and risk terms. Independent data extraction by two authors using standardised forms was performed. The method of DerSimonian and Laird was used to derive combined estimates and Egger’s; and Begg and Mazumdar’s tests for publication bias were conducted.

Based on 13 studies, including seven studies of airline cabin crew and six studies of other night shift workers, the aggregate estimate for all studies combined was 1.48 (95% CI, 1.36–1.61), with a similar significant elevation of breast cancer risk among female airline cabin crew (standardised incidence ratio (SIR), 1.44; 95% CI, 1.26–1.65), and female night workers (relative risk (RR), 1.51; 95% CI, 1.36–1.68) separately. We found some evidence suggesting confounding due to incomplete adjustment for breast cancer risk factors, with smaller effects in the studies that more completely adjusted for reproductive history and other confounding factors. Egger’s and Begg and Mazumdar’s tests for publication bias showed no significant asymmetry (P > 0.05).

Studies on night shift work and breast cancer risk collectively show an increased breast cancer risk among women. Publication bias is unlikely to have influenced the results.

Introduction

Indirect evidence from observational studies 1, 2, 3, 4, 5, 6 suggests an association between melatonin suppression and breast cancer risk, using surrogates for light exposure. Over the past few years, studies of occupational exposure to light at night and the risk of breast cancer have been accruing rapidly. The original rationale for studies of flight attendants had been that their occupational exposure to cosmic radiation causes an excess cancer risk. It was reasoned subsequently that the observed increase in breast cancer risk could as well be due to a melatonin deficiency resulting from work-associated exposure to light at night [7]. As a result of a combined effort from members of the European Community, almost ten additional studies have been published since 2000 to explore cancer incidence and mortality in airline cabin crew members [8]. Strikingly, they consistently support the contention that female cabin crew members are at increased risk for breast cancer [9], although the incomplete assessment of possible confounding factors remains an important limitation of these retrospective cohort studies. To date, more than a dozen retrospective and two prospective studies have reported on a relationship between occupational exposure to light at night and breast cancer risk, many of which have been compromised by small case numbers or incomplete adjustment for confounding, particularly the reproductive histories of women 1, 2, 3, 4, 5, 6, 9.

To overcome some of these limitations, we systematically summarised data from all published studies to evaluate current evidence for an association between occupational light exposure at night and breast cancer risk in women.

Section snippets

Search strategy and data extraction

We conducted a systematic review of all English and non-English articles using MEDLINE from January 1960 to January 2005. We additionally contacted experts in the field and searched bibliographies and abstracts.

Medical subject heading terms included night work terms: [circadian rhythm or work schedule tolerance or personnel staffing] and [scheduling or night shift or shift work] and flight personnel terms: [flight attendant or flight attendants or cabin crew or cabin crews or flight personnel

Studies included

We identified 13 relative risk estimates for breast cancer from studies that met the inclusion criteria, seven for female airline cabin crew and six for female night shift workers. The characteristics of these studies are presented in Table 1, Table 2.

Description of studies

Previous studies consistently demonstrated an elevated risk of breast cancer with engagement in any form of night work. The two prospective cohort studies to date of night shift work and breast cancer risk utilized data from the Nurses’ Health

Conclusion

This meta-analysis included 13 observational studies that examined breast cancer risk among female shift workers and cabin crew. In pooled results, we found a statistically significant 48% increase in the risk of breast cancer among shift workers. Separate analyses of both shift workers excluding flight attendants and flight attendants yielded virtually identical results.

Exposure to artificial light at night, when the production of melatonin is at its physiological height, sharply reduces

Conflict of interest statement

None declared.

Acknowledgement

The authors would like to thank the librarians from Countway library for their help with literature search.

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