Interactions between ambient air particles and greenness on cause-specific mortality in seven Korean metropolitan cities, 2008-2016

This study aims to investigate the association of particulate matter with an aerodynamic diameter smaller than 10 µm (PM₁₀) and greenness with cause-specific mortality and their interactions in seven Korean metropolitan cities. We obtained the annual standardized cause-specific mortality rates, annual mean concentration of PM₁₀, and annual Normalized Difference Vegetation Index (NDVI) for 73 districts for the period 2008-2016. We used negative binomial regression with city-specific random effects to estimate the association of PM₁₀ and greenness with mortality. The models were adjusted for potential confounders and spatial autocorrelation. We also conducted stratified analyses to investigate whether the association between PM₁₀ and mortality differs by the level of greenness. Our findings suggest an increased risk of all causes examined, except respiratory disease mortality, with high levels of PM₁₀ and decreased risk of cardiovascular-related mortality with a high level of greenness. In the stratified analyses, we found interactions between PM₁₀ and greenness, but these interactions in the opposite direction depend on the cause of death. The effects of PM₁₀ on cardiovascular-related mortality were attenuated in greener areas, whereas the effects of PM₁₀ on non-accidental mortality were attenuated in less green areas. Further studies are needed to explore the underlying mechanisms.