The pathways linking objectively-measured greenspace exposure and mental health: A systematic review of observational studies

Background: With an increasing volume of research exploring the pathways linking greenspace exposure to mental health, there is a need to synthesise the strength of evidence on the mediation mechanisms. Synthesising the strength of evidence on the pathways can help us better understand how greenspace exposure affects mental health and guide future green space interventions to improve mental health. Purpose: The purpose of this study was to synthesise current evidence on the pathways linking objectivelymeasured greenspace exposure and mental health using a systematic review approach. Methods: Literature searches were conducted in March 2020 from five common databases (i.e., MEDLINE, Embase, PsycINFO, SPORTDiscus, and Web of Science) using search terms of green spaces and mental health. English language and human population were imposed as search limits. The Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guideline was followed to synthesise and report results. Results: The initial systematic searches yielded 16,415 records, of which 26 studies met the inclusion criteria. Mixed or non-significant evidence was revealed for the mediating effects of air quality, physical activity, perceived stress, perceived restorativeness, residential noise, social cohesion, and social support on the pathways from greenspace exposure to mental illness. The effect of greenspace exposure on mental well-being was mediated by air quality, perceived stress, and physical activity. In addition, mixed evidence was revealed for the mediating role of social cohesion on the path from greenspace exposure to mental well-being. Conclusions: This systematic review highlighted the key pathways linking green spaces to mental well-being via harmful mitigation, psychological restoration, and encouragement of health-related behaviours. Evidence for the pathway between greenspace exposure and mental illness is equivocal at present. Future research is recommended to adopt well-designed longitudinal and experimental studies to establish causal inferences on mental health benefits of greenspace exposure.