Confounding effects of socioeconomic status on the association between long-term PM_2.5 exposure and mortality in Korea

Background: This study assesses the national distribution of ambient fine particulate matter (PM_2.5) exposure across socioeconomic status (SES) and its confounding on long-term PM_2.5 mortality in Korea, aiming to minimize SES influence. Methods: A nationwide cohort of 5% of Koreans, aged 30 or older, from 2007 to 2019, from the National Health Information Database, was analysed. PM_2.5 exposure levels were estimated at the city level using the Community Multiscale Air Quality system. Mortality data were obtained from Statistics Korea. The study examined annual PM_2.5 exposure by SES indicators and its confounding on mortality risks associated with PM_2.5, using time-varying Cox proportional hazards models. Results: The study followed 1 453 036 individuals from 2007 to 2019, totalling 17 760 227 person-years (PYs). The non-accidental (A00-R99), cardiovascular (I00-I99) and respiratory (J00-J99) mortality rates per 1000 PY were 7.6, 1.9 and 0.8, respectively. We observed a trend of decreasing PM_2.5 exposure levels but increased mortality among medical aid beneficiaries, those with lower household incomes and those residing in neighbourhoods with a higher area deprivation index. When adjusting for these SES covariates, the long-term mortality effects of PM_2.5 shifted in the direction of increased risk [hazard ratio (HR) for cardiovascular mortality in the unadjusted model = 0.968 (95% CI: 0.909–0.959); HR in the fully adjusted model = 1.053 (95% CI: 1.004–1.105)]. Conclusion: In regions where SES and PM_2.5 concentrations are positively correlated, as in Korea, it is crucial to rigorously control for SES confounding to avoid underestimating the mortality effects associated with PM_2.5