In 2014 biologist John Weinstein and his graduate students went looking for microplastics—the small bits of degraded plastic that researchers have discovered are spread throughout the environment.
The team was based at The Citadel military college in Charleston, South Carolina, where Weinstein is a professor. Working in a coastal city, they expected to find at least some evidence of microplastics, which are swept into the ocean. And sure enough, samples kept turning up.
Much of what they collected came from anticipated, identifiable sources, such as broken-down plastic bags. But more than half of the pieces were black, tubular, and microscopic, with no obvious origins.
“They’re elongated, almost like cigars,” says Weinstein. “It was a mystery.”
Weinstein and his students looked around the Charleston harbour at common black plastic items—such as fishing nets—searching for a comparison. But there weren’t any matches. The breakthrough came when they found very similar cigar-shaped plastics in a waterway right off of a main road. Then it dawned on them what they were dealing with: tiny bits of car tyres.
“It was a surprise,” says Weinstein. “Usually you don't find what you're not looking for.”
The find may not have been as shocking as it first seemed, however. Tyres are actually among the most common plastic polluters on earth. A 2017 study by Pieter Jan Kole at The Open University of The Netherlands, published in the International Journal of Environmental Research and Public Health, estimated that tyres account for as much as 10 per cent of overall microplastic waste in the world’s oceans. A 2017 report by the International Union for Conservation of Nature put that number at 28 per cent.
“Tyre wear and tear is a stealthy source of microplastics in the environment,” wrote Kole and his co-authors. “But awareness is low and currently there is no alternative for tyres.”
What are tyres made of?
For thousands of years, wheels were made of stone or wood—no covering needed. Leather was eventually added on top to soften the ride, followed later by solid–rubber iterations. Cars were invented in the late 1800s, and pneumatic—or air filled—tyres followed not long after.
At the time, tyre rubber came primarily from rubber trees—the cultivation of which has contributed to mass deforestation across the globe. But as the 20th century dawned and cars became less expensive and increasingly common, the world needed more rubber than was readily available. In 1909 German chemist Fritz Hofmann, working for the German chemical company Bayer, invented the first commercial synthetic rubber. Within a year the material was in car tyres. By 1931 the U.S. chemical company DuPont had industrialised the manufacturing of synthetic rubber.
Today tyres consist of about 19 per cent natural rubber and 24 per cent synthetic rubber, which is a plastic polymer. The rest is made up of metal and other compounds. Producing tyres still has monumental environmental impacts, ranging from continued deforestation to the climate-harming fossil fuels used to make synthetic rubbers to the assembly process. Modern car tyres require about 26 litres of oil to make, while truck tyres take 83 litres.
As tyres roll away the kilometres, they abrade, throwing off small pieces of synthetic plastic—essentially plastic—that then are washed off roads and into streams, eventually finding their way to the oceans.
IMAGE BY HANNAH WHITAKER, NATIONAL GEOGRAPHIC
But what’s also becoming increasingly clear is that, as the rubber wears, tyres throw off tiny plastic polymers that often end up as pollutants in oceans and waterways.
“Tyres,” says Joao Sousa, who studies marine plastics at the International Union for Conservation of Nature, “rank really high in terms of contribution” to the microplastics problem.
Tyre manufacturers Goodyear, Michelin, and Bridgestone all referred comment to The Tyre Industry Project, an industry-backed research group that counts 11 major tyre makers as members.
“There is no globally accepted definition of microplastics,” writes Tyre Industry Project representative Gavin Whitmore in an email. Their studies, he adds, “have found [that tyre and road wear particles] are unlikely to negatively impact human health and the environment.”
How do they break down?
Tyre tread patterns help determine a vehicle's grip on the road, as well as handling, manoeuvring, and breaking. But better grip can also mean more friction. And as we drive, abrasion causes bits of our tyres to break off.
A 2013 report by Tyre Steward Manitoba, in Canada, found that passenger light truck tyres lost nearly 1,134 grams of rubber during their service life (average of 6.33 years). The Kohl study found that Americans produce the most tyre wear per capita and estimates that, overall, tyres in the U.S. alone produce about 1.6 million tonnes of microplastics each year.
Exactly how much of that waste ends up in waterways depends on many factors, says Sousa, ranging from where the road is located to the weather; rain, for example, can cause more particles to flow into the environment. Research into the topic is relatively new, he notes, so estimates will improve as more work is done. But with millions of vehicles travelling the streets every day, he says, “you start to have a grim idea about the amount of tyre (particles) released.”
Once tyre particles have made it into rivers or oceans, they can have noticeable effects on marine life. John Weinstein, at the Citadel, exposed shrimp to tyre particles in lab settings and found that the animals ate the particles, which also got stuck in their gills. Once ingested, particles balled up in the shrimp’s guts.
“It doesn't die immediately,“ he says. ”There are these chronic long-term effects that really haven't been studied.”
End of the road
Better understood is what happens to tyres once they’ve run their course and need to be disposed of—“end of life,” as the tyre manufacturing industry calls it.
The trajectory for used tyres is, in many ways, positive. For example, the recycling of tyre scraps into products such as playgrounds, sports fields, and building materials has increased dramatically over the years. The U.S. Tyre Manufacturers Association (USTMA) says the re-use of tyres has gone from 11 per cent in 1990 to 81 per cent in 2017.
But that number comes with a major caveat: It includes what is called “tyre derived fuels” (TDF) —the burning of tyres for energy.
According to Reto Gieré, an environmental scientist at the University of Pennsylvania, if tyres are burned in facilities specifically designed for the task, it can be done fairly cleanly and is a decent way to recapture energy. But tyres, he says, also contain high levels of potential pollutants such as zinc and chlorine, so if they’re burned in mixed-fuel facilities or without proper safeguards, he says, “we have a big mess.”
Tyres that aren’t recycled or burned mostly end up in landfills—some 16 per cent, according to a 2018 USTMA report. The amount of landfill-disposed tyres per year nearly doubled between 2013 and 2017. John Sheerin of USTMA told Recycling Today magazine that, with demand for tyre derived fuels in a decline, even more tyres could start heading toward landfills.
Can we do better?
The tyre hasn’t seen a major redesign in decades, but there has recently been a bigger push to develop more sustainable options. In 2017, for instance, researchers led by the University of Minnesota found a way to produce isoprene, a key ingredient in synthetic rubber, out of natural sources such as grass, trees and corn instead of fossil fuels. Last year Goodyear unveiled a concept tyre made from recycled rubber that has moss in the middle, which is designed to soak up carbon dioxide as it travels.
Still, bits of these new tyres could end up in the environment as well. Reducing tyre wear, the Kole study says, would likely come at the cost of other performance metrics, such as rolling resistance, a tradeoff that may prove difficult for manufacturers to accept.
“I'm not aware of any new technology to address tyre or road wear,” says Weinstein.
But he does see other, less direct, ways of combating the problem. Road surfaces, he suggests, could be made less abrasive or more porous to either reduce or help collect tyre wear particles. He also believes that there’s room for better technology to capture tyre particle runoff from roads. It’s a route that he’s currently exploring with a town near Charleston.
Overall, though, what he sees as most urgent is further research and increased scientific and public awareness.
“There needs to be more studies done,” he says. “I don't know if this is on a lot of people's radar right now.”