Bidirectional effect modification of greenness exposure and air pollution on ischemic stroke mortality across...

Ischemic stroke (IS) is a leading cause of death and disability. While greenness exposure has health benefits in the general population, its role in reducing mortality in IS patients and its interaction with air pollution are not well understood.

Based on a cohort of 249,537 IS patients from Shandong Province, China (2013-2019), multilevel Cox proportional hazards models were applied to assess associations between cumulative and contemporaneous NDVI/EVI exposure and mortality outcomes. Air pollutant data were derived from a combination of satellite remote sensing and ground-level monitoring at high spatial resolution. Restricted cubic spline models were employed to characterize exposure-response relationships. Subgroup analyses identified vulnerable populations, and stratified analyses evaluated bidirectional effect modifications between air pollutants and greenness exposure.

Each interquartile range (IQR) increase in contemporaneous NDVI was associated with decreased mortality risk by 16.9% for all-cause, 17.3% for cardiovascular, and 17.6% for stroke mortality. Cumulative NDVI exhibited stronger protective associations, corresponding to risk reductions of 17.1%, 18.3%, and 14.9%, respectively. Exposure-response curves indicated linear inverse associations between cumulative NDVI and mortality, whereas contemporaneous NDVI exhibited U-shaped associations with all-cause mortality (P-nonlinearity < 0.05). Subgroup analyses revealed amplified protective effects among individuals aged ≥ 60 years, urban residents, and males, particularly pronounced in high air pollution contexts. Notably, air pollution-related mortality risks were significantly mitigated by higher levels of greenness exposure.

Greenness exposure across different temporal scales significantly reduced mortality risk among IS patients, demonstrating notably enhanced protective effects in regions with elevated air pollution. Among air pollutants, NO₂ significantly modified this relationship. Despite limitations such as spatial resolution variability in exposure assessments, these findings offer compelling evidence for greening interventions in regions experiencing high air pollution.