Biomonitoring evidence of smoke-free policy effectiveness in Korea: success against secondhand smoke but emerging residual exposure challenges

BACKGROUND: People who do not smoke can be involuntarily exposed to tobacco smoke through direct exposure, known as secondhand smoke (SHS), and indirect exposure to residual tobacco-derived pollutants that persist in indoor environments, known as thirdhand smoke (THS). To protect people who do not smoke from the adverse health effects of passive smoking, the Korean government has implemented and progressively expanded smoke-free zones since 1995. METHODS: This study assessed the effectiveness of these policies in Korea by analysing temporal trends of urinary cotinine concentrations among 16 193 adults who do not smoke from the Korean National Environmental Health Survey Cycle I-IV (2009-2020). To explain the recent cotinine trend change, further analysis was conducted by stratifying people who do not smoke based on passive smoke status: (1) non-exposure group, no SHS exposure and no person who smokes at home; (2) SHS-exposure group, with SHS exposure; (3) cohabiting-with-people who smoke group, no SHS exposure but living with person/people who smoke(s) at home. RESULTS: Sex and age-adjusted urinary cotinine concentrations in adults who do not smoke declined markedly from 3.05 μg/L in 2009 to 0.80 μg/L in 2014, followed by a slight increase thereafter to 1.97 μg/L in 2020. Between 2015 and 2020, the cohabiting-with-people-who-smoke group presented higher urinary cotinine concentrations compared with the non-exposure and SHS-exposure groups, suggesting that persistent nicotine uptake of people who do not smoke may be due to the growing contribution of THS and electronic cigarettes (e-cigarettes), which might explain the post-2014 increase. CONCLUSION: Our findings provide biomonitoring evidence that Korea's smoke-free policies effectively reduced SHS exposure, but the effects have plateaued in recent years, revealing emerging challenges to mitigate THS exposure and indoor e-cigarette use. Evidence-based smoke-free strategies reflecting real-world smoking behaviours are needed to ensure equitable protection of people who do not smoke across different exposure settings and to sustain a smoke-free environment.