Dose-response relation of artificial light at night and breast cancer: a public health perspective

Abstract Background Artificial light at night (ALAN) is a widespread environmental pollutant in urban settings, known to disrupt the natural light-dark cycle and circadian rhythm, with potential adverse effects on human health. Breast cancer is the most common incident cancer in women and the second leading cause of cancer-related death worldwide. Evidence between ALAN exposure and increased breast cancer risk is growing, thus we aimed to perform a systematic review and update the epidemiological literature on this topic (PROSPERO registration no. CRD420251000897). Materials and methods Adhering to PRISMA guidelines, we conducted systematic searches across EMBASE, PubMed and Web of Science databases. We included studies if they reported quantitative measures of ALAN exposure and breast cancer risk. We performed a dose-response meta-analysis using restricted cubic splines to model the shape of the association at increasing outdoor ALAN exposure levels. We performed subgroup analyses to explore potential effect modifiers, including source of ALAN and population characteristics. Results We included 21 studies, involving over 740,000 participants. ALAN showed to increase risk of breast cancer in both indoor and outdoor settings. The dose-response curve followed an almost linear pattern for outdoor ALAN when overall women were considered. Subgroup analyses indicated a notably stronger association in premenopausal women, with a risk ratio of 1.17 (95% confidence interval: 1.02-1.34) at 70 nW/cm2/sr and above. Conclusions Overall, our results provide strong evidence of a positive association between ALAN exposure and breast cancer risk. This may have significant public health implications, e.g., the need to implement mitigation strategies to lower ALAN exposure, particularly in densely populated areas and in more susceptible populations like premenopausal women. Key messages • Exposure to artificial light at night is associated with increased risk of breast cancer in women. • The association between ALAN and breast cancer risk is particularly pronounced in premenopausal women, highlighting this group as potentially more susceptible.