For decades, scientists have worked to pin down what causes mass die-offs of coho salmon in the Pacific Northwest, in which 40–90% of these iconic silvery fish migrating through streams in urban areas to go belly up before spawning. Now researchers report that they have pinpointed the culprit—a chemical that leaches out of tires to contaminate streams. (Science, 2020, DOI: 10.1126/science.abd6951).
Researchers determined years ago that something toxic in urban stormwater was killing the fish. More recently, they homed in on runoff from roadways—which includes tiny particles from vehicle tires that are shed via friction when rubber meets the road. But it took some chemical sleuthing to identify 6PPD-quinone. “The aha moment of this paper is that they actually nailed down the chemical, which is not an easy task,” says Chelsea Rochman, an ecotoxicologist at the University of Toronto who was not involved in the study.
Environmental engineer Edward Kolodziej at the University of Washington and his colleagues began by sampling water during die-off events, rushing over with coolers and sampling equipment when field scientists phoned in with reports of fish swimming in circles, rolling on their backs, or appearing disoriented. What they found was a stew of tire-derived chemicals, supporting the earlier association between the die-offs and roadway runoff. Meanwhile, they conducted toxicity testing, grinding up bits of tire tread and putting them into tanks of fish at room temperature. “Those solutions always killed the coho—and that was the only thing that killed the coho,” Kolodziej says.
To find the chemical killer, the researchers iteratively separated fractions of the tire leachate using high-performance liquid chromatography, tested the toxicity of the fractions on the coho to narrow in on the most toxic one, then identified the compound that was most toxic using high-resolution mass spectrometry. Their analysis pointed to a compound that didn’t come up in published characterizations of rubber or in databases of environmental chemicals.
Scouring US Environmental Protection Agency documents led them to a closely related compound used widely in tires to prevent their degradation by ozone exposure. A decades-old paper on the compound’s reactions with ozone identified 6PPD-quinone as a product. So the researchers synthesized 6PPD-quinone and tested it in fish. “It turned out to be toxic and structurally identical to what we purified from the tire leachate,” Kolodziej says. They also tested old samples of roadway run-off, some tied to die-off events, which turned out to be chock-full of this chemical.
This protracted process was necessary because tire ingredients are proprietary. This secrecy makes it enormously challenging to determine whether components are toxic, says Rochman. “We often don’t know we have an issue until the damage is done.”
It is likely that other organisms are affected by 6PPD-quinone in areas where roadway pollution is a problem, says toxicologist Jenifer McIntyre at Washington State University, who also worked on the study. Now that this chemical’s identity is known, researchers can investigate its effects around the world, she says.
Landscape features like raingardens—also called bioretention cells—filter stormwater through soils containing organic matter and can trap chemicals like 6PPD—quinone before they do harm. “More of these green stormwater infrastructures should be built to treat stormwater before it enters receiving waters” such as streams and estuaries, McIntyre says. “But additionally, we should be exploring whether there is a safer alternative than 6PPD that could be used in tires to protect them from ozone.”
In a statement, the US Tire Manufacturers Association calls the new findings “preliminary” and explains that 6PPD’s role in the prevention of degradation and cracking is important for passenger safety. “We are in contact with the University of Washington researchers and will continue to collaborate to advance understanding of these initial findings,” the association says in the statement.