The poison, it turned out, was never on the ingredient list. Six years on from the identification of 6PPD-quinone as the killer of coho salmon in urban streams, the technical reality still has a cruel neatness: the chemical that saves tyres from rot was quietly making a second, deadlier chemical on every busy road. The paper, led from the University of Washington Tacoma campus, ran in Science on Dec. 3, 2020.
The scale of the die-off was what made the hunt urgent. Coho spend their adult lives at sea, and only about 0.1% of a generation returns to its home streams to spawn. Every autumn, more than half of the fish coming back to Puget Sound’s urban creeks were dying before they could lay their eggs; in some streams, every last one of them died. Strange behaviour and deaths were first noticed in creeks around Bellingham and Seattle in the 1980s, and by 2018 researchers had settled on vehicle tyres as the likely source. Heat, low dissolved oxygen and known contaminants such as zinc had already been ruled out.
Then came the fractionation, and the fractionation was the story. A single tyre’s tread is a hodgepodge of hundreds of substances; Zhenyu Tian, then a research scientist at UW Tacoma’s Center for Urban Waters, and his colleagues spent two years brewing a laboratory tea from tyre crumbs and splitting it into ever-smaller portions, testing each for its effect on fish, until one unidentified molecule dominated what remained. Edward Kolodziej, the study’s co-senior author: “Here we started with a mix of 2,000 chemicals and were able to get all the way down to this one highly toxic chemical, something that kills large fish quickly and we think is probably found on every single busy road in the world.” The breakthrough clue was a molecular formula: 18 carbons, 22 hydrogens, two nitrogens, two oxygens. Tian remembered the moment plainly: “Then one day in December, it was just like bing! in my mind. The killer chemical might not be a chemical directly added to the tire, but something related.” The match, give or take a few atoms, was 6PPD.
The chemistry under the hood, quite literally, is elegant in a grim way. Ground-level ozone, cooked out of traffic pollution by sunlight, breaks the bonds that hold a tyre together; 6PPD is dosed into rubber to react with the ozone first, sparing the tyre. The reaction transforms it, and one product is 6PPD-quinone: purplish-pink when concentrated, invisible when dissolved, absent from the chemical databases and made by no one, because it goes into nothing. “You put this chemical, this transformation product, into a fish tank, and coho die, like, really fast,” Tian said. Nearly all the world’s tyres contain 6PPD, and the team found its offspring in Los Angeles roadway runoff and in urban creeks near San Francisco. This was never going to stay a Puget Sound story.
The industry’s reply was measured to the millimetre. “It’s used to prevent degradation and cracking of the rubber compounds, which is critical for tire safety,” said Sarah Amick, an attorney with the U.S. Tire Manufacturers Association, who in one phone interview reached repeatedly for the words “preliminary,” “initial” and “early.” “We feel it’s premature to be talking about alternatives when, again, the study is preliminary,” she said, calling instead for street sweeping and for rain gardens and bioswales to filter runoff before it reaches a creek. Tian noted that lining every road with bioswales would be difficult, though they could cut the toll at key locations. The university side of the argument came from Kolodziej: “We’ve spent a lot of time and effort making sure tires are safe for people,” he said. “They last a long time. They keep people safe.” Making them safe for salmon as well, he added, should now be a priority.
The stakes are not small. Dutch researchers estimated in 2017 that tyre wear on American roads releases 1.5 million tons of microplastics into the environment a year, roughly 10 pounds per person and more than double the per-capita figure of any of the 12 other countries they studied. In Puget Sound, this runoff is the leading source of toxic pollution, and fish in urban bays often carry tumours and lesions. The Science paper credited 27 chemists, biologists and engineers as coauthors; outside experts such as Tom Young of the University of California, Davis, who called the detective work “really excellent,” are now asking the obvious next questions: why chum salmon shrug off what kills coho, and what the quinone does to other species, humans included.
As ever, the detail that matters is the quiet one: the killer was never manufactured, stocked or sold. It was conjured on the road itself, one ozone molecule at a time, and it took a two-year brew of tyre crumbs to give it a name.
Sources
- Tire-related chemical is largely responsible for adult coho salmon deaths in urban streams | UW News
- Scientists point to chemical in car tires that’s been killing coho salmon – OPB
