4. Discussion
This is the first nationwide population-based study to investigate the potential effect of green space on schizophrenia risk. We demonstrated a dose-response association between the magnitude of greenspace during childhood and the risk of later developing schizophrenia. This finding was invariant to adjustment for urbanization and sex. Our analysis shows that exposure to more green space during childhood is negatively associated with the risk of schizophrenia, whereas green space heterogeneity, measured as the standard deviation, had no consistent association with the risk of schizophrenia. We found a tendency towards a stronger protective association of green space within the closest distance to a person's residence and results that might indicate that exposure during the earliest years are most strongly associated with the risk of schizophrenia.
We found the effect of green space to be largely independent of urbanization and also robust to controlling for a range of potential confounding factors. This may indicate that green space is a novel environmental risk-reducing factor for schizophrenia development. The potential mechanism behind this association remains unknown, but results from other studies points towards several plausible hypotheses. Air pollution has previously been linked to schizophrenia risk (Attademo et al., 2017; Oudin et al., 2016; Pedersen et al., 2004) and could be moderated by green space. Air pollution is often higher in urban areas, but can be reduced by trees and shrubs (Nowak et al., 2006), which may improve air quality under the right conditions (Escobedo et al., 2011), hence possibly decrease schizophrenia risk. Contact with nature is also thought to enhance immune functioning (Kuo, 2015), which may decrease infections during pregnancy, another proposed risk factor for schizophrenia (Mortensen, 2000).
