Long-term Effects of Cumulative Average PM2.5 Exposure on the Risk of Hemorrhagic Stroke

Background: Epidemiological studies have revealed associations between the fine particle (PM2.5; aerodynamic diameter <2.5 mu m) exposure and cardiovascular disease. Researchers have also recently begun investigating the association between PM2.5 exposure and hemorrhagic stroke (HS) and identifying subpopulations vulnerable to PM2.5 exposure. Long-term cumulative average PM2.5 exposure may affect the risk of HS, and these effects may be modified by risk factors. Methods: This retrospective study evaluated the effects of PM2.5 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 PM2.5 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-mu g/m(3) increment in cumulative average PM2.5 exposure was noticeably associated with HS (hazard ratio [HR] = 1.43; 95% confidence interval [CI]: 1.09-1.88). The adverse effects of PM2.5 exposure were modified by >= 65 years of age (HR = 2.00; 95% CI = 1.32, 3.02) and obesity (body mass index >= 25 kg/m(2); HR = 1.91; 95% CI = 1.28, 2.84). Conclusions: Cumulative average PM2.5 exposure might increase the risk of HS. Elderly (>= 65 years) and obese individuals may he more vulnerable to the effects of PM2.5 exposure.