Thirdhand Smoke Exposes Children Living with Smokers and Nonsmokers to Tobacco Smoke Toxicants: Racial, Income, and Housing Disparities in Hand Nicotine and Saliva Cotinine Levels.

Background: Children may be involuntarily exposed to chemical residue from thirdhand tobacco smoke (THS) in environments where tobacco has previously been used. Factors contributing to this exposure among children of nonsmokers with no known secondhand smoke (SHS) exposure are currently unknown.

Objective: To understand the contribution of THS residue to the overall tobacco smoke exposure (TSE) among children of nonsmokers and smokers.

Methods: We conducted a prospective observational cohort study of parents and their children (≤11 years; N=1013) at a children's hospital in the US. Hand nicotine (HN) and saliva cotinine (SC) were markers of THS pollution and TSE, respectively. Children were considered protected from tobacco smoke if there were no household smokers or vapers, strict household smoking bans, and no SHS exposure during the past week. Cluster-adjusted regression models were used to test hypotheses about HN and SC levels.

Results: Overall, 94.5% and 98.9% of protected and unprotected children had detectable HN, respectively. Children who were Black and protected (GeoM= 6.0 ng/wipe; 95%CI=[3.8, 9.1]) or unprotected (GeoM= 43.8 ng/wipe; 95%CI=[33.6; 57.1]) had approximately three times higher HN levels than children who were White and protected (GeoM= 1.8 ng/wipe; 95%CI=1.6; 2.0]) or unprotected (GeoM= 15.0 ng/wipe; 95%CI=[11.2; 19.8]). Unprotected children whose parents had ≤high school education had HN levels that were almost 12 times higher (87.8 ng/wipe) than unprotected children of parents who graduated college (7.5 ng/wipe). Unprotected children with ≤$15,000 household income had HN levels >25 times higher (73.3 ng/wipe) than unprotected children with >$90,000 income (≤ 2.8 ng/wipe). Household smoking bans were unsuccessful in lowering HN levels in Black children, but they were successful for White children (χ²(3)=12.7, p<0.01). HN was associated with SC in children with no SHS exposure (βˆ= 0.10), with stronger associations in the presence of 1 (βˆ= 0.28) and 2+ active smokers (βˆ= 0.32; all p<0.01).

Discussion: THS is a pervasive source of exposure to tobacco smoke pollutants. Stark disparities were noted, with higher exposure in Black and lower income children. Findings provide the strongest evidence to date on the contribution of THS as a significant source of overall TSE in children. https://doi.org/10.1289/EHP16332.