N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is an antioxidant and antiozonant widely used in tires. When 6PPD reacts with ozone, it transforms into a breakdown product called 6PPD-quinone (6PPDQ). Both compounds are currently being studied because of their high aquatic toxicity to fish, particularly coho salmon (Oncorhynchus kisutch).

The Washington State Legislature directed Ecology to perform an Alternatives Assessment on 6PPD in motor vehicle tires. The goal is to identify and evaluate potential replacement compounds for these products. As part of this work, Ecology is studying how potential alternatives interact with species of concern and compare those effects with what is already known about 6PPD and 6PPDQ. One of those species of concern is freshwater mussels.

Freshwater mussels are an imperiled group of organisms with high cultural and ecological significance. However, there are major data gaps concerning how 6PPD affects aquatic species other than salmonids. Mussels are of particular concern due to their unique parasitic life cycle where a fish host is required to fully develop into a juvenile. If their fish host is sensitive to 6PPD, mussel populations could also be at risk. This project aims to determine lethal and sublethal toxicity of 6PPD, 6PPDQ, and any potential alternatives to freshwater mussels.

This objective will be accomplished by conducting acute toxicity tests with the compounds to determine median lethal concentrations (LC₅₀s) of the tested compounds to freshwater mussels. Sublethal toxicity will be studied by then determining if the presence of 6PPD, 6PPDQ, and potential alternatives impacts the mussel’s ability to use a fish host for the important parasitic stage of its life cycle.