Association between changes in secondhand smoke exposure and incident metabolic syndrome among Korean former smokers

Background and aims: Although secondhand smoke (SHS) exposure has been linked with metabolic syndrome (MetS) in never-smokers, its effects among individuals who have quit smoking remain unclear. This study investigated the relationship between changes in SHS exposure and incident MetS in a large cohort of Korean former smokers. Methods and results: We analyzed 17,269 Korean former smokers without MetS at baseline from a longitudinal cohort, with a median follow-up of three years. SHS exposure was assessed through structured questionnaires at both baseline and follow-up, categorizing participants into four groups: no exposure, new exposure, former exposure, and sustained exposure. MetS incidence was higher in the new and sustained SHS exposure groups than in the no exposure group (34.1, 42.9, 37.4, and 44.9 per 1000 person-years for no, new, former, and sustained exposure groups, respectively). Multivariable Cox proportional hazards models demonstrated that the new exposure and sustained exposure groups had significantly increased MetS risks compared to the no exposure group (HR [95 % CI], 1.32[1.09–1.60] and 1.43[1.23–1.66], respectively). A dose-response relationship was observed with increasing daily and cumulative SHS exposure. The former exposure group showed no reduction in MetS risk (HR 0.99[0.88–1.11]) but showed favorable trends in individual MetS components including fasting glucose, waist circumference, and blood pressure. Conclusion: Continued or newly acquired SHS exposure increases the risk of MetS in former smokers, suggesting the need for complete SHS avoidance even after smoking cessation.