How objective and subjective greenspace, combined with air and noise pollution, impacts mental health through the mediation of physical activity

Global mental health is facing challenges. Environmental factors, such as enhanced greenspace and reduced air and noise pollution, alongside physical activity, are assumed to be significant promoters of mental health. However, previous studies have not thoroughly investigated the combined effects of greenspace, air pollution, noise pollution, and physical activity regarding their impact on mental health. Moreover, there is a scant consideration of the subjective versus objective assessments of greenspace exposure. Therefore, this study aims to bridge these gaps by systematically exploring how objective and subjective greenspace, combined with air and noise pollution, impacts mental health through the mediation of physical activity. Data were gathered from 683 participants in Hong Kong between November 19, 2021 and April 6, 2023, supplemented by NDVI data from Sentinel-2. Structural equation modeling and mediation analyses were employed. The findings indicated that (1) Objective greenspace does not necessarily affect perceived and real usage of greenspace; rather, the quality of greenspace plays a more critical role; (2) Physical activity significantly mediates the relationships between greenspace, air pollution, and noise pollution and mental health, with a more pronounced effect for greenspace; (3) Combined effects showed that greenspace has the most substantial total effect on mental health, followed by air pollution and noise pollution. Our study enriches the existing literature and suggests the necessity of integrating urban greenspace planning with environmental governance, focusing particularly on greenspace quality.