Why Are So Many Coho Salmon Dying? The Answer Might Be in Your Tires

For years, scientists near Seattle, Washington, watched Coho salmon die in a very strange way. The fish would swim in circles, act dizzy, and die within hours of entering urban streams. Nobody could figure out why — until a team of researchers followed the clues all the way to a chemical inside car tires.

Researcher Zhenyu Tian was one of the scientists trying to crack the case. He worked at the Center for Urban Waters at the University of Washington, Tacoma. The salmon were dying before they could spawn, or lay their eggs, which meant fewer new fish were being born each year. Tian worried that if the problem wasn't solved, the Coho salmon could disappear completely in just a few decades.

Scientists first ruled out the usual suspects. They checked water temperature, oxygen levels, and common toxins — but none of those were to blame. Then researchers from NOAA and Washington State University spotted an important clue in the data: the salmon deaths were connected to traffic. The more cars that drove near a stream, the more fish died. That pointed scientists toward urban runoff — the rainwater that washes pollution off roads and into waterways.

Zhenyu's team began testing the chemicals found in road runoff. They looked at things like particles from concrete, antifreeze, and tiny bits worn off car tires. When they compared those chemicals to the water that was killing the salmon, one stood out: the liquid that leached, or seeped out, from tire particles. That was their smoking gun. But tires contain hundreds of chemicals, so they still had to figure out exactly which one was deadly.

To narrow it down, Tian's team tested the chemicals in batches — like slicing a loaf of bread to find one bad piece. They cut the list from hundreds of chemicals down to tens, and then down to just one. But there was a twist: the deadly chemical didn't match any chemical that tire makers add on purpose. The team was stuck for months, unsure where to look next.

Then, in 2020, during the COVID-19 pandemic, Tian had a breakthrough — in the shower after a morning run. He realized that tire additives can react with other things in the air, like ozone, and turn into new chemicals called transformation products. He searched for a tire additive that had the same basic chemical structure as the mystery killer. He found it: a chemical called 6PPD, which is added to tires to make them last longer. When 6PPD reacts with ozone in the air, it turns into 6PPD-quinone — and that is what was killing the Coho salmon.

This discovery changed the way many people think about pollution from cars. Nick Molden, who works for a company in the United Kingdom called Emissions Analytics, said that most people used to focus only on exhaust coming out of tailpipes. But thanks to the salmon research, scientists started looking much more closely at tire particles — the tiny bits of rubber that wear off tires as cars drive. They found that a typical car sheds about 100 milligrams of rubber per kilometer, while exhaust rules only allow 5 milligrams of particles from a tailpipe.

The problem may actually get worse as electric vehicles, or EVs, become more popular. EVs are heavier than most gasoline-powered cars, and heavier cars wear down tires faster — like pressing harder on a rubber eraser. However, EVs also use regenerative braking, which slows the car using the motor instead of the brakes, and that can reduce tire wear. So the relationship between EVs and tire pollution is complicated.

Scientists and engineers are working on several solutions. One idea is to change the chemicals inside tires so they are less toxic. Researchers in South Korea are even building a device that works like a vacuum cleaner on a car, sucking up tire particles before they enter the air or streams. Filtering street runoff and regular street sweeping can also help protect fish like the Coho salmon.

The Coho salmon mystery shows that pollution can come from unexpected places. As cars produce less exhaust thanks to stricter rules and new technology, tire particles are becoming one of the biggest pollution problems left to solve on our roads. Scientists say that understanding where pollution comes from is always the first step toward fixing it.