The joint impact of greenspace and air pollution on mortality: a nationwide study in Finland

Greenspace has been linked to reduced mortality while air pollution has been associated with increased mortality. We examined whether the joint association of greenspace and air pollution on mortality varies across ages, sexes, educational levels, urban and rural environments, and differential levels of both exposures in a nationwide population of Finland. This study utilized routinely collected individual-level register data from Statistics Finland encompassing all citizens aged 30 years or older in 2000 (N = 3 003 519), followed until 2019. Greenspace was measured using the Enhanced Vegetation Index (EVI), and air pollution by the concentration of fine particulate matter ≤2.5 µm (PM2.5). Cox proportional hazards models were used to assess linear and non-linear associations between EVI, PM2.5, and mortality. Age was modeled flexibly with a natural cubic spline with four knots. Analyses were stratified by sex, educational level, and urbanicity. Both EVI and PM2.5 were independently associated with mortality (EVI: HR 0.99; 95% CI: 0.98-0.99 per IQR; PM2.5: HR 1.02; 95% CI: 1.01-1.03), with evidence of an interaction between the two (EVI × PM2.5: HR 0.99; 95% CI: 0.99-0.99). The joint association varied across age, sex, education, urbanicity, and the level of both exposures. PM2.5 was primarily linearly associated with increased mortality, accounting for EVI. In contrast, the relationship between EVI and mortality was less linear and more variable. Expanding greenspace may be most beneficial in areas where it is limited and where air pollution levels are high.