Unpacking the effects of urban green areas' amount, access and quality on mental health: A meta-analysis

Although the positive effects of urban green areas on mental health are widely recognized, most studies have focused primarily on the amount of greenness. Our research offers a novel perspective by evaluating not only the amount but also the quality and accessibility of green areas, distinguishing between objective and subjective exposure metrics. We performed a meta-analysis to quantitatively evaluate the current level of evidence on how three types of green exposure – amount, access, and quality of green areas - affect both general mental health and mental disorders. Our findings revealed that, while the three types of exposure to green areas positively influence mental health, the ecological quality of green areas (related to species diversity, variety of vegetation types) appears to have a pronounced effect. However, a targeted sensitivity analysis revealed a key limitation: while the benefits of amount and access remain robust across objective metrics, the impact of green space quality relies on subjective perception. The results highlight that quality benefits mental health primarily through perceived biodiversity or habitat structural heterogeneity, shaped by visible structural cues rather than objective biological complexity alone. They also point to a critical shift from just increasing the amount of greenness to enhancing ecological quality and ensuring better access. Finally, our findings underline that the interrelated nature of these exposure characteristics must be addressed in an integrated way to maximize the mental health benefits of urban green areas. By focusing on the quality of existing green areas, urban planners can create effective interventions to enhance residents' mental well-being, even without increasing the size of green areas. Investments in promoting access and enhancing ecological quality can provide significant mental health benefits, complementing the traditional expansion of green areas.