Spatiotemporal associations between PM_2.5, greenspace, physical activity, and stress: a geographic ecological...

Urban environmental health studies often use static residence-based exposures, overlooking how mobility, cumulative exposure, and activity-travel contexts shape environmental perceptions, behavior and mental health. We examined the temporal- and context-specific associations among PM_2.5, eye-level greenspace, physical activity, environmental perceptions, and perceived stress in Hong Kong. A two-day geographic ecological momentary assessment (GEMA) of 800 adults integrated 1.7 million PM_2.5 measurements, 2.1 million accelerometer records, 4,800 GEMA assessments, and eye-level greenspace from 4.3 million street-view images. Exposures and physical activity were summarized from 5- to 600-min windows and analyzed across eight contexts using a directed acyclic graph-guided multi-relation double machine learning (DAG-MR-DML) framework. Associations varied substantially across contexts and exposure windows. Higher PM_2.5 was associated with more light-intensity physical activity but less moderate-to-vigorous physical activity overall, whereas context-specific associations differed in direction. Objective greenspace showed limited associations with physical activity and was associated with higher stress overall but lower stress during mixed activities and active travel. By contrast, better perceived greenspace was associated with more physical activity and lower stress in selected contexts. Objective greenspace was not associated with better perceived greenspace overall and showed opposing context-specific estimates. Higher PM_2.5 was associated with poorer perceived air quality, whereas perceived air pollution was associated with higher stress but not with physical activity. Cumulative PM_2.5 and physical activity showed no sustained overall associations with stress, although context-specific associations ranged from beneficial to adverse. These findings demonstrate that mobility contexts and cumulative windows substantially influence environmental-health associations, highlighting the need for context-aware urban health interventions.