< Why are so many Coho salmon dying? The answer might be in your tires

ANNOUNCER 2: You're listening to Short Wave from NPR.

CAMILLA DOMONOSKE: Hey, Short Wavers. Camilla Domonoske here in the host chair with a mystery, a decades-long puzzle centering on a string of inexplicable deaths. The victims were Coho salmon, showing up dead in urban streams in Puget Sound around Seattle in very suspicious circumstances.

ZHENYU TIAN: They show weird symptom, like they are drunk. So they cannot swim in a straight line. They're, like, swimming in circles. And usually, when they show that symptom, they will be dead in, like, a few hours.

DOMONOSKE: That's Zhenyu Tian, whom you might call one of the lead detectives on this case. He was a post-doc research scientist at the Center for Urban Waters at the University of Washington, Tacoma when he was called in to the case. These fish spawn in freshwater, strike off for the open seas, trek back to the place of their birth, but then they die before they can spawn. And this keeps happening for years.

TIAN: If this trend continues, they might be, like, extinct in a few decades. So that's the problem we want to solve.

DOMONOSKE: And we was a lot of people, biologists, modelers, community scientists, environmental chemists. Some researchers rule out all the obvious culprits, temperature, oxygen, known toxins. It's none of those. They need a lead.

TIAN: So I think the first big step is our collaborators, NOAA and the Washington State University--

DOMONOSKE: They look through the data about where fish were dying and started running models.

TIAN: They are getting this important clue that the mortality risk of Coho salmon is related to traffic.

DOMONOSKE: Traffic, it's something to do with the roads, the urban runoff. So after years of work, the list of suspects is narrowing. But what part of the runoff? Enter another team of scientists from the University of Washington, including Zhenyu. They're testing chemicals in runoff.

TIAN: For example, things from the concrete, things from the antifreeze, like tire particle leachate, and compare the chemistry of those things versus the water that kills Coho salmon. Based on that comparison, we see that, OK, tire particle leachate is very similar to that.

DOMONOSKE: Smoking gun. It's coming from tire particles, but tires have tons of chemicals in them. And they need to find which one is the culprit. So today on the show, how a team of researchers finally cracked the case of the Coho salmon and how their discovery is having ripple effects. I'm Camilla Domonoske, and you're listening to Short Wave, the science podcast from NPR.

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DOMONOSKE: So knowing that tires have tons of chemicals in them that would need to be narrowed down, Zhenyu and his team step in and start testing batches.

TIAN: You can think of this as cutting a loaf of bread. Like, you cut them into thin slices. You have something abnormal in the bread, but you don't know where it is. So by cutting into slices, you can test each slice individually so that you can narrow down the scope from, like, hundreds to tens to ones. That's the logic.

DOMONOSKE: And one-by-one, they narrowed down the options until they found the deadly chemical. But that chemical, whatever was killing the salmon, it wasn't a tire additive. It didn't match anything put into tires. They still had no idea what it was. So Zhenyu and his team were stuck. They'd zeroed in on this chemical killing the salmon. They knew it came from the tires somehow, but it just didn't match any tire additive. Zhenyu says, yeah, that's the scientific process for you.

TIAN: You are trying different possibilities, trying different assumptions. And the assumption for additive just didn't work.

DOMONOSKE: OK, new assumption, if this wasn't a chemical added to tires, maybe it's a chemical that's produced when a tire additive reacts with something. Chemists call that a transformation product. But how do you find that? They were kind of stuck for months until Zhenyu had an idea. Now, at this point, it was 2020. The pandemic had started.

TIAN: We are not allowed to work a long time in the lab. So I was, like, running a little bit more and taking a shower, and that's kind of, like, where you got that, yeah.

DOMONOSKE: What-- you literally had the idea in the shower?

TIAN: Yeah, I probably should start that morning run again, but I don't have time for now.

DOMONOSKE: Zhenyu's boss, if you're listening, give this dude some more time to take runs. But anyway, this was his Eureka moment. During his post-run shower, the location of so many great ideas, he realized how to find the additive.

TIAN: Typically, for transformation, you will have oxygen and hydrogen changed. But the carbon structure will be most likely stay, and also, the nitrogen may be not changed.

DOMONOSKE: They knew their mysterious, murderous chemical, the product after a transformation, was made of very specific amounts of nitrogen, hydrogen, and oxygen. So he searches for a tire additive with the same carbon and nitrogen makeup as their murderous chemical. And they found it, a tire additive called 6PPD that helps tires last longer because it's an antioxidant. It reacts with ozone to make 6PPD-quinone that has more oxygen bonds, and it kills the Coho. This breakthrough got a lot of attention, because while everyone knew, yeah, probably not great that all these tire particles are full of all these chemicals--

TIAN: Those things can be bad, but they are generally in a-- the mode of action is, like, chronic, like, a long-term kind of effect. But this thing is, like, killing big fish in hours. So that is why it's kind of, like, getting more attention and, like, changing the way people think.

DOMONOSKE: Changing the way people like Nick Molden think. He's with a company called Emissions Analytics in the UK. They used to study tailpipe emissions. These days, they do a lot of work on tire emissions, all the particles that go into the air and waterways as your tires wear away.

NICK MOLDEN: People have only really started looking at this since 2020, and they did that for a specific reason because of a link to the death of the Coho salmon. That was what made people start looking. Now that people have started looking, we're realizing the problem all along has been much bigger than expected.

DOMONOSKE: Now, to be clear, some researchers into air and water quality have always been worried about tire emissions. But the general public, regulators, the auto industry were much more focused on tailpipes. And regulations to cut tailpipe emissions have, in fact, been super effective. They've come down a lot. So much that Nick's company, as well as peer-reviewed research, have found that tire emissions are now, on some metrics, a bigger problem than tailpipe emissions.

MOLDEN: To put it in context, the typical emissions from a vehicle, round numbers, are about 100 milligrams of rubber material per kilometer driven, roughly speaking. And the maximum limit you're allowed on particles out of an exhaust pipe, tailpipe is 5 milligrams.

DOMONOSKE: 5 versus 100. And to add a wrinkle, this realization is coming right as Electric Vehicles are becoming more popular, and they're heavier. Heavier vehicles mean more tire emissions as you drive. One researcher told me, just picture using a rubber eraser. The harder you press, the more you wear it down.

MOLDEN: So you've got two things going on simultaneously. We're realizing how big the problem was originally, and the evolution of technology is making that existing problem worse.

TIAN: Now, he is not saying that EVs are worse than gas cars overall. Tailpipes produce different kinds of pollution. We're just talking about one kind here, particulates. And the biggest benefit of EVs is that they reduce carbon emissions to reduce climate change. That benefit is clear, even if you factor in making batteries and charging vehicles. And when it to tire emissions specifically, it's complicated. EVs have pros and cons. While EVs are heavier, which makes these emissions worse, they also have regenerative braking.

MOLDEN: So that's the way you use the motor as a dynamo to slow the vehicle down, instead of the friction brakes. That has the potential-- if you use that a lot, if you drive really conservatively and use that regenerative braking, you can actually get tire emissions lower than an old internal combustion engine.

DOMONOSKE: So like I said, it's complicated. But suffice to say, tire wear pollution is a problem for all road vehicles, including EVs. So what's the solution? There are lots of ways to help, like start with the tires. We can change the chemicals in them to be less toxic. We can get fewer particles in the first place, tweak the way roads are designed, or encourage people to drive less aggressively. That makes a huge difference, actually. Or of course, drive less. Or after we've made particles, street sweeping and filtering runoff can help animals like the salmon. And then there are ideas like the one they're working on at the Korea Institute of Machinery and Materials. I reached out to ask about some testing that they've done. And Seokhwan Lee told me they're also working on collection systems to suck up and store road dust and tire particles a car would otherwise send into the air. He said, it's a two-part system.

SEOKHWAN LEE: The first part is ingesting system. So the second part is the collector.

DOMONOSKE: Is it like taping a vacuum cleaner on the back?

LEE: Yeah, it's similar. The basic principle is same for the vacuum cleaner. Yeah, that's right.

DOMONOSKE: We'll see where that goes. In the meantime, the world makes billions of tires every year. Producing them, using them, and disposing of them are all sources of pollution. And as tailpipes get cleaner and electric vehicles get more popular, this is kind of the new frontier for reducing emissions on roads, cutting down on pollution that doesn't tailpipes. Before we head out, if you liked this episode, make sure to follow us if you haven't already. That way, you never miss a new episode. And if you liked this episode, check out on satellites monitoring emissions and how air pollution could create superbugs. We'll link to them in our show notes. This episode was produced by Jessica Yung and edited by our showrunner, Rebecca Ramirez. It was fact checked by Tyler Jones, and Kwesi Lee was the audio engineer. Beth Donovan is our senior director, and Collin Campbell is our senior vice president of podcasting strategy. I'm Camilla Domonoske. Thanks for listening to Short Wave from NPR.

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