SEATTLE — Dr. Zhenyu Tian, a research scientist at the University of Washington, led the study that cracked one of the most persistent puzzles in environmental toxicology: why coho salmon returning to urban streams were dying in large numbers shortly after rainstorms. The answer, published in the journal Science in December 2020, was a chemical called 6PPD-quinone, or 6PPD-Q, a transformation product of a common antioxidant used in tires. It was a breakthrough that rewrote the understanding of urban runoff mortality syndrome.
The work was precise and technical. Tian and his colleagues, including Dr. Edward Kolodziej at the University of Washington Tacoma, used high-resolution mass spectrometry to trace the source of the toxicity.
They did not guess at the culprit; they followed the chemical trail from dead salmon back through stormwater samples and finally to tire wear particles on the road. 6PPD itself is added to tires to prevent degradation from ozone. Under the hood of environmental chemistry, the detail that mattered was that this protective compound, when exposed to air and water, transforms into something acutely lethal to coho salmon at extremely low concentrations. The discovery solved a long-standing mystery.
For years, biologists and environmental managers had observed that adult coho returning to spawn in urban creeks like those in the Puget Sound region would die within hours of a rain event, even when water quality tests for conventional pollutants came back normal. The technical reality was that the toxin was not on the standard screening lists.
Tian’s team identified 6PPD-Q as the agent, providing the first mechanistic explanation for what had been an intractable problem. Since that initial paper, Tian’s research has expanded significantly. He is now studying the fate and transport of 6PPD-Q in the environment — how it moves from roads into streams, how long it persists, and what happens to it as it travels.
The work is not limited to salmon. His group is investigating potential effects on other species. As ever, the detail that matters is whether the chemical’s toxicity is specific to coho or whether it poses a broader threat to aquatic ecosystems.
Regulatory Response and Agency Action
The findings have moved beyond the academic literature and into the regulatory sphere. The Environmental Protection Agency has taken note of Tian’s work, as have state agencies in California and Washington. These bodies are now using the data to inform efforts to manage and mitigate the impact of tire-derived contaminants.
The question of how to regulate a chemical that is both ubiquitous — every car sheds tire particles — and highly toxic to a specific species is a complicated one. The technical reality is that there is no easy substitute for 6PPD in tire manufacturing, and the particles themselves are difficult to capture at scale.
Tian holds a Ph.D. in environmental chemistry from the University of California, Riverside. His background in analytical chemistry is evident in the rigor of the 2020 study. The identification of 6PPD-Q required not just suspecting a tire chemical, but being able to isolate and confirm the exact molecular transformation product that was causing the mortality.
That level of precision is what separates a hypothesis from a confirmed environmental hazard. The implications of his work extend beyond salmon.
Urban runoff is a complex mixture of chemicals from vehicles, roads, and infrastructure. The discovery of 6PPD-Q as a primary toxic agent provides a target for remediation efforts. If you know what you are looking for, you can begin to measure it, model its transport, and design interventions.
That is the detail that matters for regulators and water quality managers.
What to Watch Next
Looking forward, the key questions are about scale and mitigation. How widespread is 6PPD-Q contamination across urban watersheds? Can stormwater treatment systems be designed to remove it effectively?
And what are the long-term ecological consequences for species beyond coho salmon? Tian’s ongoing research is expected to provide some of those answers.
The regulatory landscape is also evolving, with state and federal agencies considering whether to set water quality criteria for the compound. For now, the work of Tian and his collaborators remains the foundational reference point for anyone trying to understand the intersection of tire chemistry and salmon mortality.
