Imagine a quiet autumn stream in the middle of a big city. Cars rush past, while in the water, after a long journey from the ocean, red coho salmon swim upstream. They are returning home to spawn. But when it rains, the fish suddenly start circling, surfacing and gasping for air as if they can’t breathe. Within a few hours, many of them die. For more than 20 years, no one could figure out why. The answer turned out to be closer than anyone thought: it was right beneath car tires.
The Mystery of Longfellow Creek
Seattle has a small stream called Longfellow Creek, which flows through the western part of the city. Years ago, it was cleaned up and restored so salmon could live there again. And they came back! Every autumn, volunteers visited the creek, counted the fish and celebrated each new return.
But a strange phenomenon spoiled the joy. Scientists call it “urban runoff mortality syndrome.” It sounds complicated, but the idea is simple: after heavy rain, water runs off the streets into streams, and the fish suddenly die. In some urban streams in the region, most of the returning coho salmon died before they could spawn. Other fish, such as Chinook salmon and rainbow trout, handled the same water much better. That was another mystery.
Imagine you’re a detective. You have a crime scene (the creek), victims (the fish) and not a single clue. That’s how researchers from the U.S. National Oceanic and Atmospheric Administration (NOAA), including biologist Nate Scholz, and their colleagues at Washington state universities felt.
A Hundred Suspects and No Culprit
First, the scientists checked the most obvious possibilities. Could pesticides from lawns be to blame? Or heavy metals, such as copper washed off brake pads? Or oil and grease from roads? All of these are indeed found in urban water. But when fish were placed in water containing the same amounts of these substances, they didn’t die the way they did in the actual creek.
The researchers then came up with a clever next step. They collected rainwater from roads and tested it. The fish died in it, so there was definitely a toxin. The next task was to identify which one. This water contains thousands of different substances, and finding the one poisonous culprit was like looking for a needle in a huge haystack.
Scientists had already noticed one promising fact: if dirty rainwater was passed through a layer of ordinary soil with plants—in effect, a “rain garden”—the fish survived. That meant the toxin could be filtered out. This narrowed the search, but the culprit’s name was still unknown.
Tea Made from Car Tires
The answer came from an unexpected direction. Chemists, including Zhenyu Tian and Edward Kolodziej of the University of Washington Tacoma, along with Jennifer McIntyre of Washington State University in Puyallup, decided to investigate tires. Car tires gradually wear down, leaving tiny particles of black dust on the road. Rain washes them into ditches and from there into streams.
The scientists did something that sounds almost like a fairy tale. They “brewed” pieces of tire in water, like tea. The fish in this tire tea died with the same symptoms as those in urban streams!
Next, using highly precise instruments, the researchers identified the specific substance. It’s called 6PPD-quinone. Here’s how it forms:
- Manufacturers add a substance called 6PPD to tires. It works like “sunscreen” for rubber: it protects it from ozone in the air so the tire won’t crack and remains safe.
- When 6PPD reacts with ozone, it forms a new substance: 6PPD-quinone.
- Rain washes the substance off the road and into the water. Even tiny amounts are deadly to coho salmon.
To give you an idea of just how little it takes: the concentration dangerous to coho salmon is about 95 nanograms per liter of water. If you dissolved that much in an Olympic-sized swimming pool, it would weigh about as much as just ten grains of rice. That’s how little it takes to kill a fish.
Scientists reported this discovery in 2020 in the journal Science, in a paper titled “A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon.”
How to Help the Fish: Rain Gardens and New Ideas
The good news is that there’s already a partial solution to the problem. Remember how rainwater passed through soil turned out to be safe? That’s the principle behind “rain gardens,” known scientifically as bioretention systems. They are shallow pits filled with a special mix of soil, sand and plants along roads. Rainwater seeps through them like a sponge, leaving the toxin behind in the soil. Rain gardens are being built in Seattle and other cities. They do several useful things at once: clean the water, help prevent street flooding and make the city more beautiful.
But that’s only part of the answer. The best solution would be to keep the harmful substance off the road in the first place. That’s why scientists and engineers are looking for a replacement for 6PPD. It’s not an easy task: the new additive must protect tires reliably without harming the environment. Washington state authorities have put 6PPD on a list of substances that should be replaced as a priority. And in 2023, several Native American tribes, for whom salmon are central to their culture and way of life, asked the U.S. Environmental Protection Agency to restrict the substance. So the story of Seattle’s salmon has become part of a much bigger conversation taking place across the country.
Why This Story Still Matters
This story has striking parallels with issues people are talking about right now.
First, there’s the issue of microplastics and fine particles from wear and tear. We’re used to thinking about exhaust fumes, but tires also leave a mark, because they wear down a little with every trip. Experts note that heavier electric vehicles may wear out tires faster, making this issue even more important. Electric vehicles are better for air quality, but tires are still tires.
Second, think of the canaries miners used to take underground. The birds sensed dangerous gas before people did and sounded the alarm. Salmon in an urban creek played a similar role: the fish were the first to get sick, warning us that something was wrong with the water. Researchers have already found 6PPD-quinone not only in streams, but also in road dust and river water in different countries. So this is an issue that goes far beyond Seattle.
What You Can Do
What I like about this story is that the mystery wasn’t solved by magicians, but by attentive people: volunteers who visited the creek and counted fish, and scientists who weren’t afraid to “brew tea” from tires. Any observation can turn out to be important if we pay attention.
If you want to help nature, here are a few simple ideas:
- Tell your friends and family that tire dust also gets into the water, so they know about it too.
- If you’re driving
