A toxicological comparison of tire and road wear particles emitted on rural and urban roads, alone or with the co-exposed pollutant benzo[a]pyrene: Size and origin as key factors in controlling the in vitro proinflammatory response

Tire and road wear particles (TRWP) make up a significant and increasing proportion of non-exhaust emissions and contribute to air pollution. This study examines how the size and origin of TRWP affect their toxicological profile. For this purpose, macrophages were incubated for 24 h with fine (0.39-0.69 µm) or coarse (4.2-10.2 µm) TRWP collected on-road either in urban or rural areas. Incubations were carried out in the presence or absence of benzo[a]pyrene (B[a]P, an archetypal road traffic gaseous pollutant) to investigate possible combined toxic effects. The in vitro cell response was evaluated in terms of cytotoxicity (LDH release), proinflammatory response (TNF-α production) and oxidative stress (reactive oxygen species (ROS) production). No cytotoxicity and no oxidative stress were induced by TRWP, whatever their size and origin. However, all types of TRWP (urban or rural and in both sizes) were capable of triggering a dose-dependent proinflammatory response. A similar toxicity profile was observed with or without B[a]P. For fine emissions, urban TRWP elicited more proinflammatory effects than rural TRWP, while for coarse emissions, rural TRWP were more proinflammatory at low dose. These results, coupled with the study of the TRWP's chemical composition, reveal that: i) both the size and the origin of TRWP are key factors in their toxicity, and ii) the origin-driven toxicity stems from at least two sources. One source is the abrasion of tires and road materials, and the other is the particulate road traffic emissions, ambient pollution and road surface contaminants incorporated into TRWP.