Globally, people are increasing migration to cities, with more than 50% of the global population residing in cities. Urban life-style has improved health care, but has also been linked to increasing mental health problems. Natural environments and biodiversity have been proposed as important environmental factors influencing mental health. Focusing on schizophrenia, a serious and chronic mental illness that affects about 29 million people worldwide, we investigate the link between mental illness and exposure to natural environments measured as green space. Green space could mediate schizophrenia risk through noise and particle pollution removal, stress relief, or other unknown mechanisms. However, the effect of green space has not been disentangled from that of urbanization, and it is unclear whether the quantity or quality of green space is more important. We used Landsat satellite data to quantify green space for Denmark in 30×30m resolution for the years 1985-2013. The effect of quantity and heterogeneity of green space and urbanization at place of residence on schizophrenia risk was estimated using cox regression from a very large longitudinal population-based sample of the Danish population (943,027 persons). Schizophrenia risk was controlled for a range of individual and socioeconomic characteristics to avoid confounding with the effect of green space.
Higher mean green space around place of residence decreased schizophrenia risk in a dose-response relationship even when controlling for urbanization. In contrast, local spatial heterogeneity in green space availability had no clear effect on schizophrenia risk. We found green space to decrease schizophrenia risk independent of urbanization, pointing to green space as a new environmental risk factor for schizophrenia development. This study supports findings from other studies highlighting the natural environment as an important ecosystem service provider for mental health, and points to a new interdisciplinary methodological framework that combines epidemiological studies with big data approaches from ecology. This framework can be expanded to other mental health problems and used to examine the generality of the link between mental health and green space. Cities are typically green-space poor environments, but by including green space in urban development plans mental health might be cost efficiently improved. Further, derived effects such as increased nature appreciation and interest in biodiversity conservation could create positive feedbacks through increased use of both urban and rural green space.