Is outdoor artificial light at night confounding studies on green space and health?

Abstract

BACKGROUND AND AIM: There is a growing body of evidence that exposure to green space, particularly biodiverse green space, is good for our health: spending time in green space can improve depressive symptoms, obesity, and sleep problems, and reduce the risk of breast and prostate cancer. We also know that exposure to light at night, particularly urban light pollution, is bad for you: it increases the risk of breast and prostate cancer, and can worsen depression, obesity and sleep problems. As both green space exposure and outdoor artificial light at night (O-ALAN) have been implicated in the same health outcomes, we aimed to establish if O-ALAN was itself correlated with green space, potentially confounding studies of green space and health outcomes (and vice versa). METHODS: We used Australian major cities as a case study, and investigated the correlation between green space diversity (Shannon diversity index of National Vegetation Information System’s vegetation classes within 3 km radius buffer zones), and night-time light data (NOAA/NCEI Nighttime Visible Infrared Imaging Radiometer Suite Day/Night Band 2015 Annual Composite product). RESULTS:Our results show a clear inverse correlation between O-ALAN and green space diversity for major Australian cities (P 0.001, Adjusted R-squared = 0.325, n = 139). The finding makes intuitive sense because more developed areas are likely to have fewer trees and more artificial light, which is consistent with our hypothesis that that O-ALAN could be confounding studies of the relationship between green space and health. CONCLUSIONS:Both green space (ideally including a measure of diversity) and O-ALAN should be included in spatial epidemiological models examining environmental determinants of health. Without considering the potential confounding effects of O-ALAN, public health-driven initiatives such as green space conservation and restoration in urban environments may not be optimised for health benefits. KEYWORDS: Light pollution, Green space, Built environment