Source-specific risks assessment of size-resolved PM bound multiple toxicants: Variation of source-specific risks in respiratory tracts

Inhalation risks depend on the size distribution of ambient particulate matter (PM) bound toxicants, which are influenced by various sources. This study explored variations in source-specific risks across different respiratory tract regions and their association with size-resolved toxicants. Potentially toxic elements contribute significantly, accounting for over 93% of the inhalation risk, followed by polycyclic aromatic hydrocarbons at 0.3%-7%, and organophosphate esters at less than 0.01%. For adults, the contribution of non-cancer and cancer risks within respiratory tracts was as follows: upper respiratory tract (similar to 61%) > pulmonary (similar to 30%) region > tracheobronchial (similar to 8%) region. Differently, for children, the order was pulmonary (similar to 43%) > upper respiratory tract (similar to 41%) > tracheobronchial (similar to 17%). In the adult population, resuspended dust (RD) presented elevated non-cancer risks in the upper respiratory tract compared to the tracheobronchial and pulmonary regions, while coal combustion (CC), traffic emission (TE), and waste recycling or incineration (WI) contribute most to pulmonary risks. In contrast, for children, non-cancer risk associated with CC surpassed RD in the upper respiratory tract, whereas RD exceeded CC in the tracheobronchial region. This discrepancy arises because RD primarily consists of coarse PM with higher deposition fractions in the adults' upper respiratory tract and children's tracheobronchial region, while toxicants from CC, TE and WI, with PM < 0.43 mu m, are inclined to have a greater impact on the pulmonary region. Regarding source cancer risks, RD exhibited a consistent decrease across all three respiratory tract regions, with primary dominance observed in industrial emissions (IE), CC, and TE sources.