Understanding how green space accessibility and utilisation relate to common mental health difficulties: A cross-sectional study in North West England.

Green space exposure is associated with better mental health, but the independent contributions of accessibility and utilisation, and whether utilisation mediates the effects of accessibility and mental health, remain unclear. The objectives of this study were to (i) quantify the independent associations of green space accessibility and utilisation with common mental health difficulties (CMD) prevalence; (ii) test whether utilisation mediates the effect of accessibility; and (iii) investigate whether accessibility and utilisation moderate one another's associations with CMD. We integrated novel small-area anonymised mobile phone data, high-resolution environmental metrics, and primary care records for 2.6 million people across Cheshire and Merseyside, England to quantify these pathways in 2024. Results showed that accessibility and utilisation were independently associated with lower CMD prevalence. Residential tree exposure showed the most consistent association among accessibility measures (RR = 0.936), while walking time in green space showed the strongest association among utilisation measures (RR = 0.900). Mediation analyses provided no evidence that utilisation mediated the association between accessibility and CMD. However, moderation models identified a compensatory interaction, whereby associations between active green space engagement and lower CMD prevalence were strongest in areas with poorer environmental access. Our findings demonstrate that accessibility and utilisation have distinct but interacting associations with population mental health, supporting urban policies that promote both locally accessible green environments and opportunities for their active use.