Trends in the socioeconomic inequalities related to second-hand smoke exposure as verified by urine cotinine levels among nonsmoking adults: Korea National Health and Nutrition Examination Survey 2008 - 2018

The expansion of smoke-free policies has reduced prevalence of second-hand smoke (SHS) exposure; however, declines differ according to socioeconomic positions (SEPs). We evaluated the trends in socioeconomic inequalities related to SHS exposure in non-smoking Korean adults from 2008 to 2018. We analyzed 30,027 non-smoking adults from the Korea National Health and Nutrition Examination Survey 2008 to 2018. We evaluated trends in urine cotinine levels, self-reported prevalence of SHS exposure at workplaces and homes, and people exhibiting non-measurable urine cotinine levels between 2008 and 2018. To evaluate the yearly decline differences of urine cotinine levels according to SEPs, we calculated the interaction effects of year and education, household incomes, and occupation from linear regression analysis. In the last 11 years, the geometric means of urine cotinine levels decreased from 3.53 (95% CI 2.96-4.19) ng/mL to 0.60 (0.57-0.64) ng/mL in males, and from 2.36 (2.03-2.73) ng/mL to 0.53 (0.51-0.56) ng/mL in females. Prevalence of SHS exposure at workplaces and homes also declined. In the multivariate model, the interaction effects between education and years were significant; therefore, indicating a substantial yearly decline of urine cotinine levels in higher educated individuals. The interaction effects between household incomes and years were only significant among males; the interaction effects between occupations and years were not significant. SHS exposure in non-smoking Korean adults has consistently decreased; however, socioeconomic inequalities related to SHS exposure by education level has widened. Policies targeting socioeconomically disadvantaged populations should be implemented to decrease the disparities of SHS exposure. Along with tobacco control policies, the prevalences of self-reported and urinary cotinine verified SHS exposure have decreased in the last 11 years. In contrast, the socioeconomic inequalities related to SHS exposure by education level has increased over time. This study emphasizes the need for implementing tobacco control policies to reduce disparities of SHS exposure.