Long-term Effects of Cumulative Average PM_2.5 Exposure on the Risk of Hemorrhagic Stroke

Background: Epidemiological studies have revealed associations between the fine particle (PM_2.5; aerodynamic diameter <2.5 μm) exposure and cardiovascular disease. Researchers have also recently begun investigating the association between PM_2.5 exposure and hemorrhagic stroke (HS) and identifying subpopulations vulnerable to PM_2.5 exposure. Long-term cumulative average PM_2.5 exposure may affect the risk of HS, and these effects may be modified by risk factors. Methods: This retrospective study evaluated the effects of PM_2.5 on the time-to-first-diagnosis of HS among 62,676 Seoul metropolitan city residents with 670,431 total person-years of follow-up; this cohort is a subset from a nationally representative cohort of 1,025,340 individuals from the Korean National Health Insurance Service database (2002–2013). A time-dependent Cox proportional hazards model was used to adjust for age, sex, household income, insurance type, body mass index, smoking status, medical history, and family history. The annual mean PM_2.5 concentrations for 25 districts were used as the time-dependent variable. Subgroup analyses of the traditional risk factors of HS were performed to evaluate potential effect modifications. Results: Each 10-μg/m³ increment in cumulative average PM_2.5 exposure was noticeably associated with HS (hazard ratio [HR] = 1.43; 95% confidence interval [CI]: 1.09–1.88). The adverse effects of PM_2.5 exposure were modified by ≥65 years of age (HR = 2.00; 95% CI = 1.32, 3.02) and obesity (body mass index ≥25 kg/m²; HR = 1.91; 95% CI = 1.28, 2.84). Conclusions: Cumulative average PM_2.5 exposure might increase the risk of HS. Elderly (≥65 years) and obese individuals may be more vulnerable to the effects of PM_2.5 exposure.