Potential health risk of human exposure to tobacco-specific nitrosamines in second-hand and third-hand smoke

Environmental tobacco smoke (ETS), including both second-hand (SHS) and third-hand smoke (THS), seriously impacts human health. However, the potential health risk of non-smokers' exposure to ETS (especially THS) has not been fully characterized. The present study conducted smoking events in a 100 m3 room. Gaseous and sizefractioned particulate samples from SHS and THS along with wipe samples on desk and floor surfaces were collected and analyzed for tobacco-specific nitrosamines (TSNAs). The concentration, composition, and size distribution of indoor TSNAs varied between SHS and THS due to variability in particle deposition, air exchange, chemical reaction, and partitioning. Children had higher per body weight daily intake of TSNAs and nicotine through all exposure pathways than other population groups. The contributions of different exposure pathways to the intake of pollutants were influenced by their octanol-air partition coefficients. Incremental lifetime cancer risk (ILCR) for different population groups from exposure to residential TSNAs in ETS exceeded 1.0 x 10-6, even with exposure to THS only. The ILCR from e-cigarettes was lower than that from cigarettes, but it still posed potential carcinogenic risk, particularly through air-to-skin transdermal exposure to THS. Reducing health risk for exposure to both SHS and THS requires rigorous enforcement of smoking-free policies.