The Tyre Poison Nobody Talks About

Salmon die within hours of storms. New science shows the same dust drifts into cities and lodges in human lungs.

When rain hits city streets, salmon die within hours. Scientists have traced the killer to a chemical hidden in every car tyre, one that turns stormwater into poison. New research shows this dust doesn’t just wipe out fish, it rides the air, lodges in our lungs, and is already reshaping ecosystems worldwide.

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In Seattle’s Longfellow Creek, volunteers once counted the return of salmon each autumn like a ritual. The sight of silver flashes pushing upstream through an urban stream felt like a miracle, a reminder that wildness still pulsed in a city of glass and concrete.

Then came the rain. Heavy drops slammed the streets, flushing water off tarmac into drains, then into creeks. Hours later, the salmon that had made it home from the Pacific began to twitch, spiral and float belly-up. Some with eggs spilling from their bodies. Whole runs gone overnight.

For decades, no one knew why. The water wasn’t poisoned by oil or sewage. Metals were present but not at lethal levels. Yet the salmon kept dying. It felt like watching a ghost, fish appearing strong and healthy, only to collapse as if the rain itself had turned toxic.

It had.

A coho salmon gapes for air, one of the symptoms of "urban runoff mortality syndrome" (Credit: University of Washington)

A Secret Inside Every Tyre

Every car tyre is a black box of chemistry, more than 400 ingredients blended in proprietary recipes. Carbon black. Vulcanising agents. Heavy metals. Antioxidants that keep rubber from cracking.

One additive, 6PPD, turned out to be the killer. Tyre makers use it to stop rubber cracking in sunlight. In the air there’s a gas called ozone (you may have heard of the ozone layer that protects us from UV rays). Ozone is a form of oxygen, but with three atoms instead of two, and it’s so reactive it attacks rubber. 6PPD reacts with it first, sacrificing itself to protect the tyre. The problem is that this reaction creates a new chemical, 6PPD-quinone. When rain washes that into streams, it becomes deadly to salmon. Until recently, nobody had studied it at all.

In 2020, Tian and Edward Kolodziej’s team at the University of Washington identified it as the invisible assassin. When coho salmon were exposed to stormwater containing this compound, fish died in less than three hours.

The mystery was solved, but the story doesn’t end with salmon. It’s about to reach your car, your street, and your lungs.

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