Greenspace exposure and its dual role as mediator and moderator in the relationship between urban density and mental health

The relationship between urban density, urban green space (UGS) exposure, and mental health remains complex and understudied, particularly in rapidly urbanizing cities notably characterized by high-density development. Based on a survey of 824 respondents in Guangzhou, China, this study advanced prior research by systematically unraveling the dual roles of UGS exposure: UGS visitation played as a mediator and UGS availability served as a moderator in the urban density–mental health nexus—a critical gap in existing literature. Our key findings indicated that: (1) Urban density exhibited context-dependent effects: its impacts on three mental health indicators (depression, life satisfaction and sense of worthwhilenss) hinged on UGS availability. (2) Visiting community gardens, municipal parks, and waterfront UGS was particularly effective in reducing depression risk, enhancing life satisfaction, and fostering a sense of worthwhileness, respectively. Engaging with communal gardens and municipal parks was positively associated with all three mental health outcomes. The effects of UGS visitation on mental health surpassed those of UGS availability and urban density. (3) Crucially, we revealed a dual mechanism: UGS visitation fully mediated the adverse effects of high density on mental health, while UGS availability moderated this relationship, mitigating negative impacts of high urban density. These findings provide novel empirical evidence for optimizing UGS planning: increasing UGS and its visitation can reduce the negative mental health impacts of high urban density and prioritizing communal gardens and municipal parks is advisable, given their positive effects on three mental health outcomes.