INTRODUCTIONLow parental education level and parental smoking are major risk factors for household secondhand smoke (SHS) exposure among adolescents. We investigated the trend in household SHS exposure according to sex, school, and parental education level to determine whether the decline in household SHS exposure over time depends on parental education level.METHODSWe used cross-sectional Korea Youth Risk Behavior datasets (2006–2020; 806829 subjects were eligible). We applied binary logistic regression to assess household SHS exposure trends and evaluated the interaction between period and parental education level.RESULTSHousehold SHS exposure over 15 years has declined. The difference (0.121) was the smallest for male middle school students with low-educated parents. The slope for the estimated probability of household SHS exposure among students with high-educated parents was steeper than that for those with low-educated parents, except for female high school students (difference=0.141). Students with low-educated parents were at higher risk of household SHS exposure (male middle school students, adjusted odds ratio, AOR=1.52; 95% CI: 1.47–1.56; male high school students, AOR=1.42; 95% CI: 1.38–1.47; female middle school students, AOR=1.62; 95% CI: 1.58–1.67; female high school students, AOR=1.62; 95% CI: 1.57–1.67). The interaction between parental education level and period was significant. We also found a significant interaction between parental education level and parental smoking (other × present interaction, AOR=0.64; 95% CI: 0.60–0.67; low–low × present interaction, AOR=0.89; 95% CI: 0.83–0.95).CONCLUSIONSChanges in parental education level over time mainly contributed to changes in adolescents’ household SHS exposure. Adolescents with low-educated parents were at higher risk of household SHS exposure, with a slower decline. These gaps must be considered when creating and implementing interventions. Campaigns and community programs to prevent household SHS need to be emphasized among vulnerable adolescents.