Ingestion of Surface Residues Dominates Quaternary Ammonium Compounds (QACs) Exposure in Chinese Urban Homes: Evidence from Silicone Wristband Passive Sampling and Urinary Biomonitoring

The quantitative characterization of multiple exposure routes to quaternary ammonium compounds (QACs) remains underexplored. In this study, paired samples of indoor dust, bulk air, hand wipes, silicone wristbands, and urine were collected from 109 adults residing in urban homes from South China in 2023. First, seven urinary biomarkers, including hydroxylated and carboxylated metabolites of C10–C14 benzylalkyldimethylammonium compounds (BACs), were identified using a combined in silico and in vitro workflow. Then, 23 QACs, including 6 C8–C18 BACs, 6 dialkyldimethylammonium compounds (C8–C18 DADMACs), 6 alkyltrimethylammonium compounds (C8–C18 ATMACs), and 5 emerging QACs, were ubiquitously detected in various environmental matrices, including dust (median ∑QAC concentrations of 39.6 μg/g), bulk air (130 pg/m³), hand wipes (1420 ng for two hands), and silicone wristbands (225 ng/g), respectively. A significantly positive correlation was observed between the logarithmically transformed masses of QACs detected in silicone wristbands and those from dust, bulk air, and hand wipes (r: 0.564, p < 0.01). Moreover, urinary hydroxylated and carboxylated C10- and C12-BACs were significantly correlated with corresponding parent compounds in wristbands (r: 0.481–0.607, p < 0.01). Finally, back calculation from urinary exposure biomarkers revealed that ingestion of surface residues was the dominant exposure route for C10-, C12-, and C14-BACs, accounting for 3.7%, 49.6%, and 18% of total exposure, respectively. The findings from this study propose suitable urinary exposure biomarkers and silicone wristbands as useful indicators for accurate internal and external exposure assessment, respectively, and highlight the importance of ingestion of surface residues as a major exposure route.