Is greener always healthier? Examining the nonlinear relationships between urban green spaces and mental health in Wuhan, China

Though much evidence demonstrates the benefits of urban green spaces (UGS) for mental health, it remains uncertain if a greener living environment necessarily leads to better mental health. This study makes up this gap by exploring the potential non-linear effects of UGS provision (availability, accessibility, visibility, quality) and utilization (frequency, duration) on mental health, focusing on their nonlinear patterns and thresholds. Using geospatial and social survey data from Wuhan, China, and controlling for socioeconomic, built, and social environmental factors, we find that: (1) Both UGS provision and utilization have significant nonlinear effects on mental health. (2) Inverted-U-shaped relationships exist between mental health and both UGS availability (measured by Normalized Difference Vegetation Index, NDVI) and accessibility (distance to nearest park), with peak benefits observed at an NDVI of 0.25 or a distance of 0.24 km. A similar L-shaped relationship is observed for UGS visibility, suggesting that higher visibility does not necessarily translate to improved mental health. The positive health effects of UGS quality exist, yet offer marginal benefits. (3) UGS utilization plays a crucial mediating role, explaining up to 47 % of the relationship between visibility and mental health. This highlights the importance of active engagement with UGS for realizing mental health benefits, supporting the ‘environmental provision→individual behavior→mental health’ pathway. These findings provide urban planners with valuable dosage references for UGS allocation, emphasizing the need to consider both provision and factors promoting utilization to maximize mental health benefits within urban environments.