Secondhand smoke exposure among non-smoking adolescents in three Vietnamese cities in 2025: A cross-sectional study

INTRODUCTION: Secondhand smoke (SHS) remains common among adolescents, but evidence on setting-specific exposure in Vietnam is limited. This study estimated the prevalence of any and setting-specific SHS exposure from conventional cigarettes among non-current smoking students in three Vietnamese cities and identified associated factors. METHODS: We conducted a secondary analysis of self-reported questionnaire data from a school-based cross-sectional survey in Hanoi, Da Nang, and Ho Chi Minh City in 2025. The analysis included non-current smoking students aged 13 to under 18 years. The primary outcome was any SHS exposure in the past 7 days, defined as exposure at home, in indoor public places, or in outdoor public places. Secondary outcomes were setting-specific exposures. Associations were examined using Poisson regression with robust standard errors clustered at the school level, reported as adjusted prevalence ratios (APRs) and 95% confidence intervals (CIs). RESULTS: Among 4437 non-current smoking students, 67.2% reported any SHS exposure, 22.0% reported home exposure, 50.4% reported indoor public-place exposure, and 57.5% reported outdoor public-place exposure. Indoor smoking at home was the strongest factor associated with any SHS exposure (APR=1.62; 95% CI: 1.55-1.70). Higher prevalence was also observed among male students (APR=1.05; 95% CI:1.01-1.10), Grade 12 students (APR=1.15; 95% CI:1.06-1.25), students in Ho Chi Minh City (APR=1.08; 95% CI: 1.01-1.15), and selected weekly allowance groups, which may reflect greater mobility and spending autonomy. Setting-specific analyses showed lower home exposure but higher public-place exposure among older students. CONCLUSIONS: SHS exposure was highly prevalent among non-current smoking students in three major Vietnamese cities and varied by household and public-place contexts. Further longitudinal and implementation studies are needed to clarify causal pathways and strengthen the evidence base for strategies to reduce adolescent SHS exposure in home and public environments.