Outdoor Exposure to Artificial Light at Night and Breast Cancer Risk: A Case-Control Study Nested in the...

Exposure to light at night (LAN), particularly blue light, is suspected to disrupt circadian rhythm, inhibit melatonin production, and eventually increase the risk of breast cancer.

We assessed the association between exposure to outdoor LAN and breast cancer risk in the E3N-Generations cohort, a large population-based cohort study of French women followed-up from 1990 to 2011.

We conducted a nested case-control study in the cohort, including 5,222 incident breast cancer cases and 5,222 matched controls. Outdoor LAN exposure at residential addresses was assessed using radiance-calibrated satellite images from the Defense Meteorological Satellite Program (DMSP). Logistic regression models were used to obtain odds ratios (OR) and 95% confidence intervals (CIs), adjusting for socio-demographic, reproductive, hormonal, and lifestyle-related factors, as well as exposure to air pollutants ($$, $$) evaluated from land use regression and chemistry-transport models, and proximity to greenspaces estimated from the Normalized Difference Vegetation Index (NDVI) in a buffer of $$.

Before adjustment for environmental covariates, the ORs associated for LAN exposure increased monotonically from the first to the fourth quartile. This increasing trend was less pronounced after adjustment for air pollutants ($$ and $$) and NDVI, but the fully adjusted OR per interquartile range (IQR) of LAN exposure ($$) remained slightly elevated [$$; 95% confidence interval (CI): 1.02, 1.20]. The adjusted ORs were slightly more elevated in postmenopausal ($$; 95% CI: 1.02, 1.18) than in premenopausal women and in women living in urban areas with low greenness.

The weak positive associations observed in this study that persist after adjustment for environmental covariates, support the hypothesis that outdoor LAN may increase breast cancer risk. Our results, suggesting that urban greenness could mitigate the role of LAN exposure in breast cancer risk, should be investigated further. Future studies on cancer risk in relation to outdoor LAN should assess exposure to indoor sources, including electronic devices, and characterize the light spectrum, particularly the blue light. https://doi.org/10.1289/EHP15105.