Tires Are a Hidden Environmental Nuisance

The pollution that cars emit goes beyond what spews from their tailpipe.

Alec Soth / Magnum

The storm unleashed one evening in late November 2018. The first splashes of rain wet the streets of Oakland, California. Then, a crescendo of water pounded roofs, drops glancing off gutters with metallic pings. As the stormwater 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, or 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 nearly 70 liters from the stream of stormwater 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.”

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 vehicles’ environmental impacts to include tires that shed particles near bodies of water.

“Stormwater 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 more than 51 million waste tires generated each year in California, waste managers are finding ways to reuse them, even though researchers are only beginning to grapple with tires’ impacts in stormwater and recycling. Tire pollution, it turns out, may be farther reaching than anyone imagined.


Tires have one problem that’s unlikely to be fixed anytime soon: They shed. The friction of rubber on abrasive surfaces is what lets a heavy vehicle grip roads and stop when needed, sloughing off tiny bits and pieces of the tire. A 2017 scientific-literature search of 13 industrialized and industrializing countries found that an average car loses between half a pound and more than four pounds of tire fragments annually. In the car-happy United States, the amount jumps to nearly 11 pounds.

Once, tires were made entirely from natural rubber. Today, they contain a mix of natural rubber and synthetic rubber, made from plastic polymers; the synthetic material accounts for 20 to 60 percent of each tire. The ingredients and proportions tend to be proprietary, depending on the brand, but tires usually 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 the tires structure. The finished product isn’t considered toxic, but some individual ingredients are, including heavy metals like cadmium and lead, as well as 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 stormwater 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 lets the researchers 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 U.S. 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, Sutton says, for tire particles found “in the wild” to look different.  Exposed to the elements, the fragments may degrade in ways that lab work doesn’t show.


The threat that 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. 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 were 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 shade of 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 shores. 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 that they eat other microplastics. Studies show that fish pass more than 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 at Tacoma and their colleagues suspect that tire fragments may be harming coho salmon in streams around Seattle. Autumn rains wash the city’s streets clean just as the salmon are swimming up their home creeks to spawn. Scientists have known for decades that stormwater is killing coho—but because 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. But 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.

“Coho are enormous and brightly colored, so people can readily see them suffering,” says Sutton, who notes that salmon’s troubles could signal other systemic problems. “A smaller fish,” she says, “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’s 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.

Mixing old tires into new roads is a 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. Experts hoped that this would also lessen air pollution, as tire wear contributes to airborne particulate matter—increasing it by up to 30 percent in some 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,” she says, “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. 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, dozens 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 it reached the unsatisfying conclusion that other sources of zinc oxide—including tire fragments—could also be at play, so it wasn’t possible to definitively pin the blame on recycled tires.

Amid this remaining uncertainty, one immediate low-tech alternative is installing rain gardens at road-runoff zones. Stormwater pools in these gardens, seeping into the earth; this filters out harmful particles before they can reach natural bodies of water. 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. Another long-term path might be tackling the source of pollution itself. Lighter cars and lower speed limits, for example, would help reduce tire-shedding. These changes would require the cooperation of car manufacturers, regulators, and consumers. But perhaps there’s no better place to start than in California, where the coastline and car culture are intertwined, for better or for 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 encounter it in other neighborhoods. Escondido is trying its best to be environmentally friendly, he says, as he drives past new bike lanes and parks the city’s hybrid car at the town hall. If the risks turn out to be too high, he says, he’d push for a better fix, if he could find one. We shake hands and say goodbye.


This post appears courtesy of Hakai Magazine,