Background: With the unprecedented urbanization light pollution has emerged as a ubiquitous problem, and there has been accumulating evidence on the links between exposure to light at night (LAN) and breast cancer risk. We conducted a systematic review and meta-analysis of published studies on the associations between LAN exposure and breast cancer risk.
Methods: We included all observational human studies wherein the exposure variable was LAN measured in indoor and outdoor environments, and the outcome was breast cancer. We employed summary relative risks (SRR) for breast cancer by comparing highest versus lowest categories of LAN exposure within a random-effects model. The National Toxicology Program's (NTP) Office of Health Assessment and Translation (OHAT) risk of bias rating tool was adopted to assess the risk of bias in individual studies and the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guideline was employed to assess confidence in the body of evidence.
Results: A total 14 studies comprising four cohorts (13,155 cases among 372,802 exposed subjects), nine case-control and one case-referent studies of female subjects (39,462 cases and 20,739 controls) across seven countries and published between 2001 and 20 were included for review. Participants in the highest LAN exposure category were associated with higher risk of breast cancer in reference to those in the lowest (SRR: 1.12; 95% CI: 1.06-1.18; I2 = 39% for outdoor LAN, and SRR: 1.13; 95%CI: 1.05-1.21; I2 = 19% for indoor LAN). Pooled evidence identified relatively pronounced association of outdoor LAN exposure and breast cancer among women with estrogen receptor positive (ER+) tumor (SRR: 1.21; 95% CI: 1.04-1.40) and premenopausal status (SRR: 1.21; 95% CI: 1.06-1.37). The final rate of confidence in the body of evidence generated was graded as 'moderate' based on GRADE guideline.
Discussion: LAN exposure was consistently associated with higher breast cancer risk corroborating NTP's recommendations which anticipates excessive LAN as human carcinogen.
Keywords: Breast cancer; Light at night; Light pollution; Melatonin; Meta-analysis.
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