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Car crossing the Bixby Bridge, Big Sur, California
Vehicle emissions are clearly a pollution problem, but new research puts the spotlight on the environmental effects of tire particles. Photo by Pgiam/iStock

When Rubber Hits the Road—and Washes Away

A stealthy source of pollution leaves the highway in astonishing amounts and heads to sea, toxic chemicals and all.

Authored by

by Laura Trethewey

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The storm unleashed one evening in late November 2018. The first splashes of rain wet the streets of Oakland, California, with a smell like damp stone. Then, a crescendo of water pounded roofs, drops glancing off gutters with metallic pings. As the storm water sluiced over sidewalks and streets, it erased the boundary between land and sea, carrying branches, plastic bottles, motor oil, and more into San Francisco Bay.

At 10:30 that night, an industrial slough near the Oakland Coliseum roared to life. The slough wasn’t particularly noticeable hidden behind chain-link fences. But the vast surrounding parking lot made it perfect for measuring the stuff scoured from the city streets by rain. All the water falling across five square kilometers of mostly impervious pavement ran through this choke point. Huddled in rain gear on an overpass, a research team from the San Francisco Estuary Institute (SFEI) was ready for the cascade. As a stream of cars carrying concertgoers rolled out of the coliseum parking lot, the researchers used sampling rods to sip more than 70 liters from the stream of storm water below.

Later, the team discovered a shocking amount of rubbery black fragments in their samples. Over three years, as they tested water at 12 stormwater outlets and sediment at 20 sites around the bay, they found much the same. Some 7.2 trillion synthetic particles are washing into San Francisco Bay each year, says Rebecca Sutton, a senior scientist at SFEI and the study lead. “Almost half those stormwater particles, so a really high percentage, were rubbery particles that we think are mostly coming from tires.”

researcher sampling storm water

In California, researchers from the San Francisco Estuary Institute sampled storm water over three years to determine if tire particles were washing into San Francisco Bay. Photo courtesy of Alicia Gilbreath

In California, where most commuters cling to their cars, conversations about the environmental impact of automobiles usually involve what spews from tailpipes. Electric vehicles are sold as the solution for car emissions. But SFEI’s work has expanded the debate about the environmental impacts of cars to include tires shedding particles near the sea.

“Storm water really hasn’t gotten a lot of attention from the scientific community when it comes to emerging contaminants,” Sutton says. But the rubbery fragments she’s turned up suggest millions of reasons that it should. Tire particles in the water may harm aquatic and marine organisms—just as other microplastics do—including through chemical exposure, movement inside an animal’s body, and bioaccumulation of toxins through the food chain.

With over 51 million waste tires created each year in California, waste managers are finding ways to reuse them, even though researchers are only beginning to grapple with their impacts in storm water and recycling. Tire pollution, it turns out, may be farther reaching than anyone imagined.


Tires have one engineering principle that’s unlikely to change: they shed. The friction of rubber on abrasive surfaces is what allows a heavy vehicle to grip roads and stop when needed, sloughing off bits and pieces of the tire. A 2017 scientific literature search of 13 industrialized and industrializing countries found that an average car loses between a quarter kilogram and two kilograms of tire fragments annually. In the car-happy United States, the amount jumps to nearly five kilograms—or about the weight of a cat.

Once, tires were made entirely from natural rubber. Today, they contain a mix of natural rubber and between 20 and 60 percent synthetic rubber, made from plastic polymers. The ingredients and proportions tend to be proprietary, but usually tires also include sulfur, used to vulcanize rubber; zinc oxide, to shorten vulcanization; reinforcing fillers like silica and carbon black; and oils that help processing. Steel wires and fabric are added to give tires structure. The finished product isn’t considered toxic, but some individual ingredients are, including heavy metals like cadmium and lead, and high aromatic oils (more commonly known as polycyclic aromatic hydrocarbons or PAHs), which are considered carcinogenic in some jurisdictions. The mix makes tires a difficult-to-recycle “monstrous hybrid”—a term coined by zero-waste writers Bill McDonough and Michael Braungart—leaving local officials struggling to find ways to keep them from clogging landfills.

Because studying tire fragments in storm water is relatively new, the field is riddled with inconsistencies. There’s no set protocol for measuring, collecting, or defining tire particles, and there’s no consensus on what to call them or what they look like. Researchers for the Tire Industry Project, supported by tire manufacturers, wear down individual tires on a road in a lab, suck up the particles shed in the process, and then identify particle shape and size with a scanning electron microscope and with pyrolysis, a heating method that allows the researchers to single out tire ingredients. “The particles that we find [a half-half mix of tire tread and road pavement] are generally very consistent,” says project manager Gavin Whitmore. “They’re cigar-shaped and 100 micrometers, about the thickness of an American dollar bill.”

In comparison, the fragments that the SFEI researchers found were variable in size and shape. Sarah Amick of the US Tire Manufacturers Association suggests that this might mean the fragments come from surfacing roads with coal tar sealant or chip seal. However, coal tar sealant isn’t used in California, and some chip seal contains recycled tires. It makes sense for tire particles found “in the wild” to look different, Sutton says. Exposed to the elements, the fragments may degrade in ways that lab work doesn’t show.


The threat these tire fragments pose globally is just beginning to come into focus. In 2017, the International Union for Conservation of Nature estimated that 28.3 percent of microplastics in the ocean come from tires, landing them in the top seven contributors. But the real number is likely higher. A study published in July suggests that vast quantities of tire fragments find their way into the ocean not just via rivers and waterways, but also through the air. Swept on the wind, they drift far from where they are shed. The study warned that so many tire particles are landing in the Arctic that they pose a climate change risk. By turning the snowy tundra a less reflective white, the polluted Arctic ice may absorb more light and melt even faster.

Because tire particles are denser than seawater, the SFEI team found that they tend to sink and accumulate in sediment near shore. Small fish, oysters, and other animals at the bottom of the food chain live in this rich environment. “It’s a pretty direct pathway for exposure,” Sutton says. Bottom feeders could be consuming fragments in the same unaware way they eat other microplastics. Studies show that fish pass over 90 percent of the microplastics they eat, but toxicity may still taint their tissues and travel up the food chain. Lab work suggests that marine animals affected by plastic pollution can experience respiratory and reproductive issues, cell damage, and even death.

Researchers at the University of Washington Tacoma and their colleagues have long suspected that tire fragments may be harming coho salmon in streams around Seattle. Autumn rains wash the city’s streets clean just as the salmon swim up their home creeks to spawn. Scientists have known for decades that storm water is killing urban coho—up to 90 percent in some streams—but since it can carry thousands of possible contaminants, it was difficult to figure out which ones were having lethal effects. The researchers relied on volunteers to call the lab when they spotted coho floundering in the stream, gasping for air at the surface before dying. Field observations directed the researchers to specific creeks where they tested the water and discovered high concentrations of chemicals that are present in tires and can leach into water.

This 2018 study found correlation but not causation in the wild. But a National Oceanic and Atmospheric Administration study in 2016 did confirm the connection with research on hatchery salmon. When exposed to stormwater collected from busy urban streets, coho die. And in December 2020, the University of Washington Tacoma researchers finally nailed the chemical so lethal to coho, a “globally ubiquitous tire antioxidant” called 6PPD-quinone.*

“Coho are enormous and brightly colored, so people can readily see them suffering,” says Sutton, but salmon’s troubles could signal broader, systemic problems. “A smaller fish could experience the same impacts, but you wouldn’t see it (if you were) walking through a rainy creek.”


One potential, and potentially problematic, solution for reducing tire shedding involves changing the texture of pavement. California’s concrete and asphalt highways act like cheese graters on tires. On a Thursday in February, shortly before the coronavirus lockdown, I join Matthew Souterre and Marissa Padilla to check out an alternative way to surface roads in Escondido, a bedroom community north of San Diego, where the pair work in the city engineering services department.

Souterre looks in his rearview mirror. “Marissa, where do you have us going to next?” he asks.

Padilla, in the back seat, shuffles some papers. “The Miller-Alexander area,” she says.

“The Miller area …” Souterre repeats absently until his memory jogs. He executes a silent U-turn, passing bungalows painted in neutral desert tones.

Less than a year earlier, Souterre and Padilla used a grant from the state’s recycling agency, CalRecycle, to divert 15,198 tires from landfills. The tires were processed into hot asphalt to form rubberized pavement, which reduces traffic noise and tire shedding, and speeds water drainage due to its porosity.

We arrive at a quiet residential street and I climb out to take a closer look at the road surface. It looks like … pavement. Souterre and Padilla point out its highlights: no weeds sprouting and no alligatoring—where pavement splits into slabs like reptile skin.

road being repaved with ground up tires

Some organizations, such as CalRecycle in California, are reusing old tires in roadways, grinding up the material and processing it into pavement. Photo courtesy of the City of Escondido

Mixing old tires into new roads is an ideal, full-circle solution that California, burdened with diverting tens of millions of junked tires from landfills annually, has embraced. In 2005, the California State Legislature mandated recycling waste tires in state pavement and aimed to rubberize 35 percent of new pavement projects beginning in 2013. It was hoped that would also lessen air pollution: tire wear contributes to airborne particulate matter—up to 30 percent in high-traffic areas—and the dust can inflame human lungs. But Sutton, of the SFEI, worries that paving streets with ground-up car tires may be unloading their heavy metals and chemicals into sensitive aquatic ecosystems.

“To be honest, the concerns we’re now having about tires are brand new concerns. I’m not sure those have been part of the strategy as CalRecycle was trying to come up with new uses for tires,” Sutton says. “We want to fix the issue. But reuse needs to be wise, or we’re just going to create a new problem.”

CalRecycle recently funded a study on whether zinc oxide in rubberized pavement was leaching into California waterways, 40 of which occasionally exceed the Clean Water Act’s standards for the heavy metal. The study found that rubberized pavement does indeed leach 40 percent more zinc oxide than non-rubberized pavement, but reached the unsatisfying conclusion that other sources of zinc oxide could be at play—including tire fragments—so it’s not possible to definitively pin the blame on recycled tires.

Amid remaining uncertainty, one immediate low-tech alternative is rain gardens built at road runoff zones. Stormwater pools there, seeping into the earth, which filters out harmful particles before they can reach natural waterbodies. The SFEI team is sampling several rain gardens around San Francisco. One early test site shows a promising 90 percent reduction in particles, including tire fragments. The NOAA scientists working on hatchery salmon also found encouraging results: simply filtering the run off through soil was enough to prevent coho deaths, and presumably prevent harm to other creatures that are more difficult to observe.* Another long-term path might be tackling the source of pollution itself. Lighter cars and lower speed limits, for example, would help reduce shedding. These changes would require the cooperation of car manufacturers, regulators, and consumers. But perhaps there is no better place to start than in California, where the coastline and car culture are intertwined, for better or worse.

At the end of my tour through Escondido, Souterre says he hasn’t heard of downsides to rubberized pavement. In his experience, people like it and often request it once they see it in other neighborhoods. Escondido is trying its best to be environmentally friendly, he says, as we drive past new bike lanes and he parks the city’s hybrid car at the town hall. If the risks turn out to be too high, he says he would push for a better fix, if he could find one. We shake hands and say goodbye.

A few weeks later, when the pandemic has taken hold and handshakes have become a thing of the past, I wonder if this might be a hopeful lesson. If we can change our behavior so rapidly to avoid getting sick with COVID-19, perhaps that better fix isn’t as out of reach as it seems.

* This story was updated to reflect new information released December 3, 2020.