Secondhand Nicotine Absorption From E-Cigarette Vapor vs Tobacco Smoke in Children.

Importance: With the prevalence of e-cigarette use (vaping) increasing worldwide, there are concerns about children's exposure to secondhand vapor. Objective: To compare nicotine absorption among children who are (1) exposed to secondhand tobacco smoke only or (2) exposed to secondhand vapor only with (3) those exposed to neither. Design, Setting, and Participants: The US Continuous National Health and Nutrition Examination Survey (NHANES) is a repeat cross-sectional survey. Participants are interviewed in their homes and, several days after, visit a mobile examination center to provide biological specimens. This study uses data from a nationally representative sample of US households from 2017 to 2020. Participants were children aged 3 to 11 years with serum cotinine levels incompatible with current firsthand nicotine use (ie, <15 μg/L). The final analysis was conducted on January 9, 2024. Exposures: Reported exposure to secondhand smoke or vapor indoors in the past 7 days (only secondhand smoke, only secondhand vapor, or neither). Covariates included age, sex, ethnicity, family income, body weight, and height. Main Outcomes and Measures: The primary outcome was serum cotinine concentration, an objective biomarker of nicotine absorption. Geometric mean cotinine levels and 95% CIs were calculated using log-normal tobit regression, accounting for the complex survey design and weights. Results: The mean (SD) age of the 1777 children surveyed was 7.4 (2.6) years, 882 (49.6%) were female, and 531 (29.9%) had family incomes below the poverty level. Nicotine absorption, as indexed by serum cotinine level, was highest among children only exposed to secondhand smoke (0.494 μg/L μg/L; 95% CI, 0.386-0.633 μg/L), followed by those exposed only to secondhand vapor (0.081 μg/L; 95% CI, 0.048-0.137 μg/L), equating to 83.6% (95% CI, 71.5%-90.5%; P < .001) lower nicotine absorption. Among children with no reported secondhand exposure, the geometric mean cotinine level was 0.016 μg/L (95% CI, 0.013-0.021 μg/L), or 96.7% (95% CI, 95.6%-97.6%; P < .001) lower than for those with exposure to secondhand smoke. Results were similar after covariate adjustment. Conclusions and Relevance: In this cross-sectional study of US children, nicotine absorption was much lower in children who were exposed to secondhand vapor vs secondhand smoke, but higher than in those exposed to neither. These findings suggest that switching from smoking to vaping indoors may substantially reduce, but not eliminate, children's secondhand exposure to nicotine and other noxious substances.