Environmental Occurrence and Toxicity of 6PPD Quinone, an Emerging Tire Rubber-Derived Chemical: A Review

N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) is a chemical added to tires to prevent their oxidative degradation. 6PPD is highly reactive with ozone and oxygen, leading to the formation of transformation products such as 6PPD quinone (6PPDQ) on the tire surfaces and, subsequently, in tire and road wear particles. 6PPDQ is a toxicant that has been found in roadway runoff and receiving water systems. Its presence in municipal stormwater has led to the acute mortality of coho salmon during their migration to urban creeks to reproduce, generating global interest in studying its occurrence and toxicity in the environment. This review aims to provide a critical overview of the current state of knowledge of 6PPDQ, assisting researchers and policymakers in understanding the potential impacts of this emerging chemical on the environment and human health. As there are many unanswered questions surrounding 6PPDQ, further research is needed. This review highlights the importance of including transformation products in regulations for 6PPD, as well as all emerging synthetic chemicals of concern.