Occurrence, Transformation, and Toxicity of Tire-Derived Chemicals 6PPD and 6PPD-q in the Environment

6PPD (N¹-(4-methylpentan-2-yl)-N⁴-phenylbenzene-1,4-diamine), a widely used rubber antioxidant in tire manufacturing, has garnered increasing global attention, following the discovery that its ozone-oxidation product, 6PPD-q, is the primary toxicant responsible for urban runoff mortality syndrome (URMS) in salmon. This work provides a comprehensive review of 6PPD and 6PPD-q in the environment, focusing on four key aspects: source, occurrence, transformation, and toxicity. Key findings include the following: (1) Tire wear particles are the main source of 6PPD and 6PPD-q in the environment, with their release directly linked to traffic density. (2) Ozone levels, temperature, climate conditions (e.g., snowmelt and rainy seasons), and environmental settings (e.g., roadways, tunnels, and parking lots) can greatly affect exposure levels. (3) In addition to atmospheric ozonation, oxidation in the atmosphere, free radical oxidation, and photocatalytic oxidation also play key roles in transforming 6PPD to 6PPD-q within aquatic systems and soils. (4) URMS is caused by 6PPD-q attacking the organs of fish, leading to blood-brain barrier and vascular dysfunction, with observed interspecies differences in sensitivity; these differences may be linked to variations in metabolic capacity. (5) Both 6PPD and 6PPD-q can enter the human body through the food chain, but their metabolic mechanisms and pathological changes are still unclear. Given the significant research gaps, this review concludes with proposed future research directions to deepen understanding of 6PPD's and 6PPD-q's environmental impacts.