Artificial light at night and breast cancer risk: The findings from CECILE study (France)

BACKGROUND: Environmental exposure to artificial light at night (LAN) has become ubiquitous in modern societies. Experimental and epidemiological studies suggested that LAN exposure disrupts circadian rhythm, inhibits melatonin production, and increases the risk of breast cancer. OBJECTIVES: We aimed to assess the association of exposure to outdoor LAN with breast cancer risk, along with possible effect modifications of this association. METHODS: We used data from a population-based case-control study, including 1185 incident breast cancer cases and 1282 healthy controls enrolled in 2005-2007 from two French departments- Côte d’Or and Ille-et-Vilaine. Data for outdoor LAN exposure was obtained using radiance-calibrated images from Defense Meteorological Satellite Program (DMSP) for 1995-2006. Exposure was assessed for 10 years before inclusion in the study by cross-referencing the DMSP images and the geocoded locations of residences in ArcGIS. Logistic regression was used to obtain odds ratios (OR) and 95% confidence intervals (CI) for the association, adjusting for sociodemographic factors, urbanization, reproductive factors, lifestyle-related factors, night shift work, and air pollution. RESULTS: The overall OR for breast cancer per 1 interquartile range (IQR) increase in LAN exposure (159.9 nW/cm(2)/sr) was 0.98 (95% CI: 0.81-1.17). Subgroup analyses showed slightly higher ORs in post-menopausal women (OR: 1.15; 95% CI: 0.97-1.37) and a stronger association for HER2-positive breast tumors (OR: 1.55; 95% CI: 1.03-2.31). CONCLUSIONS: We did not find any overall association between outdoor LAN and breast cancer; however, possible associations in certain subgroups were indicated. The strength of this study was the use of calibrated satellite images for exposure assessment and the consideration of a large set of potential confounders. Further large-scale studies using improved exposure assessment methods, including blue light and indoor exposure measurement, are required. KEY MESSAGES: • Light pollution is growing as the newest form of environmental pollution in modern society, potentially affecting health. • More studies are required to explore and ascertain its public health impact.