Importance: With the prevalence of e-cigarette use (vaping) increasing worldwide, there are concerns about children's exposure to secondhand vapor.

Question: How does children’s nicotine absorption, as indexed by serum cotinine level, differ among those (1) exposed to secondhand tobacco smoke only, (2) those exposed to secondhand e-cigarette vapor only, or (3) those exposed to neither?

Findings: This cross-sectional study of 1777 US children aged 3 to 11 years found that, compared with children exposed to secondhand smoke only, nicotine absorption was 83.6% lower in those exposed to secondhand vapor only and 96.7% lower in those exposed to neither.

Meaning: These findings suggest that children absorb much more nicotine from secondhand smoke than from secondhand vapor; switching from smoking to vaping indoors may substantially reduce children’s secondhand exposure to nicotine and other noxious substances, but both smoke and vapor increase children’s absorption vs no exposure.