TACOMA, July 7 — A single chemical, first identified in a University of Washington Tacoma lab six years ago, has reshaped how regulators and industry think about the rubber under their cars. Dr. Zhenyu Tian, a research scientist at the university’s Center for Urban Waters, led the 2020 study that pinned 6PPD-quinone — 6PPD-Q — as the agent responsible for mass die-offs of coho salmon in urban streams. The work, published in Science, found the compound was toxic to coho salmon at concentrations as low as 0.8 micrograms per litre.
Tian’s team used high-resolution mass spectrometry to trace the chemical from tire wear particles — the fine black dust shed as tyres abrade against asphalt — into stream water. That technical reality, the detail that matters, was a forensic chain of evidence linking a ubiquitous source to a specific biological wreck.
Before Tian’s work, the cause of the urban-runoff mortality events that had puzzled fisheries biologists for decades was simply unknown. The chemical itself, 6PPD, is added to tyres as an antioxidant and antiozonant; it keeps rubber from cracking. Its transformation product, 6PPD-Q, turned out to be the killer.
The discovery landed with unusual force. Regulatory agencies including the EPA increased scrutiny of 6PPD and its transformation products.
Subsequent studies, citing Tian’s original paper, began looking at 6PPD-Q’s effects on other species — including humans. The finding opened a new front in the long-running argument about what, exactly, comes off a vehicle beyond tailpipe emissions.
The chemical detective
Tian earned his PhD in environmental chemistry from the University of California, Riverside. He has published over 30 peer-reviewed papers on emerging contaminants — the class of pollutants that slip through conventional treatment and monitoring because nobody thought to look for them. His work on 6PPD-Q is the most cited of his career, and it continues.
He is studying the environmental fate of the compound: where it goes, how long it persists, what breaks it down. The 0.8 micrograms per litre threshold is, under the hood, an extraordinarily low concentration.
It means that a tiny amount of the chemical in water — roughly a few drops in an Olympic swimming pool — is enough to kill coho salmon within hours. The mechanism appears to involve the chemical disrupting the fish’s ability to regulate oxygen transfer across the gills, though the precise biochemical pathway remains under investigation. The 2020 study in Science was not the end of the story.
It was the beginning. Tian’s team had identified the agent; the question of what to do about it remains open. 6PPD is added to virtually all passenger-vehicle tyres sold in the United States.
Finding a replacement that performs the same function without generating a toxic transformation product is not straightforward.
Alternatives and regulatory pressure
Tian continues to study potential alternatives to 6PPD. The technical challenge is that the antioxidant properties of 6PPD are well-understood and effective; any substitute must match that performance without creating a new environmental problem. Some tyre manufacturers have begun testing alternative chemistries, but no drop-in replacement has yet been validated at scale.
The EPA’s increased scrutiny of 6PPD and its transformation products has taken several forms. The agency has added the compound to its list of chemicals of concern and has begun the process of evaluating whether to regulate it under the Toxic Substances Control Act.
That process is slow, technical, and contested. Industry groups have argued that the benefits of 6PPD in tyre durability and safety must be weighed against the environmental costs. For Tian, the work continues at the Center for Urban Waters, a research facility on the Tacoma waterfront that specialises in the chemistry of urban runoff.
The centre’s location, on the edge of Commencement Bay, gives researchers direct access to the kind of urban streams where coho salmon have been dying for years. The question now is whether the science that identified the problem can also point the way to a solution.
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