Association between Secondhand Smoke Exposure and Heavy Metal Concentrations in Young Children: Based on Data from The Korean National Environmental Health Survey Cycle 3 Ministry of Environment (2015-2017)

Objectives: This study aimed to investigate the association between secondhand smoke (SHS) exposure and urinary concentrations of mercury (Hg) and cadmium (Cd) in Korean children aged 3 to 5 years. Methods: Data were analyzed from 871 young children who participated in the third cycle of the Korean National Environmental Health Survey (KoNEHS) conducted from 2015 to 2017. SHS exposure was assessed using caregiver-reported questionnaires, and participants were categorized into exposed and non-exposed groups. Urinary Hg and Cd levels were compared between groups using multivariable-adjusted regression models. To overcome the limitations of subjective exposure assessments, urinary cotinine levels were analyzed as an objective biomarker of SHS exposure in relation to heavy metal concentrations. Results: No significant differences were observed in urinary Hg and Cd concentrations between the questionnaire-based SHS exposure groups. However, elevated urinary cotinine levels were significantly associated with increased urinary Hg concentrations (β = 0.251, 95% confidence interval [CI]: 0.015-0.487, <i>p</i> = 0.038) and Cd concentrations (β = 1.123, 95% CI: 0.698-1.547, <i>p</i> < 0.001) after adjustment for covariates. These findings suggest that low-level SHS exposure, even when unreported, may contribute to the internal accumulation of toxic metals in early childhood. Conclusions: Our findings highlight secondhand smoke as a potential unrecognized source of heavy metal exposure in young children. While caregiver-reported SHS exposure may underestimate true exposure levels, cotinine-based analyses reveal significant associations with urinary mercury and cadmium concentrations. These results underscore the need for enhanced strategies for early detection and prevention of environmental toxicant exposure in young children.