The Other Emissions Coming from Cars

By now, it’s hard to overstate the link between cars and their emissions from fossil fuel combustion. But beyond the exhaust coming out of vehicle tailpipes, there’s another source from these machines that pollutes the environment every time we drive: miniscule particles from brakes and tires.

“Each time you hit the brake pedal, you remove tiny amounts of the car’s braking system because of the friction between the wheels and the brake pads. We need to do that to stop the car, but every time we do, we lose materials,” says Reto Gieré, a professor in the Department of Earth and Environmental Science. “The same is true for tires.”

Gieré, a geochemist, has studied road dust for the past eight years. He recently began focusing on the microplastics from tires, which are “virtually everywhere,” he says, especially prominent at stoplights or other places cars frequently stop and accelerate, as well as around curves. Once the particles mix with the overall road dust, they typically don’t stay in one place, he adds. “They are flushed off the road surface in a big rainstorm, for example, and from there, they are distributed into the environment. Once they’re in the environment, there’s practically nothing we can do to remove them.”

With this research as the backdrop, the United Nations Environment Programme (UNEP) asked Gieré and colleagues—a team that included Penn doctoral candidate Jaydee Edwards and collaborators from the German Meteorological Service, the Czech Republic’s Tomas Bata University in Zlín, and Dresden University of Technology—to write a brief about vehicle tire particles in the environment. 

Among many findings, the researchers showed that typically, these tire-wear particles are no larger than 0.15 centimeters, have an elongated shape, and are covered by even smaller particles; that heavier vehicles discharge considerably more of these microplastics; and that chemical components added to tires to increase durability and maximize performance are leached into the environment.

Unlike most academic papers, this publication was geared toward a general audience, with the aim of drawing broad attention to a relatively new area of concern for the UN. “People don’t think about the tire side of vehicles, generally. Most people go straight to the exhaust emissions, which makes a lot of sense,” says Edwards, who is in the fifth year of her PhD work. “This paper tries to highlight not just the particles themselves but where they are going, how they’re generated, their toxicity, and the amounts that are generated, which are quite high. We’re hoping to help people understand why they should actually care about this.”

Taking Edwards’s point further requires digging into some data. For one, scientists estimate that these particles are one of the most abundant types of primary microplastics released into the environment. In numbers, that equates to 28 percent—or 400,000 metric tons—of the total microplastics that seep into the oceans annually. Once in the water, they get consumed by fish and other sea-dwelling creatures, some of which people subsequently eat.

Gieré brings up the example of salmon in the Pacific Northwest. “In Washington State, salmon are dying in huge numbers, and research has shown it’s due to the runoff from roads, in particular one chemical that can be traced back to tires,” he says. “That’s why many toxicologists are working with tires to find out their effects.” Shrimp, he adds, have also been shown to ingest tire-wear particles, though in that case, “the particles move through the body relatively fast.”

Once in the environment, these particulates interact with light, air, ozone, soil, sediment, and more. And metals like zinc, often used in the formation of tires, can be toxic to aquatic wildlife in high concentrations. Even though what’s known about the subject continues to expand, Gieré says, “we know very little about this still.”

When we put on clothing or we use plasticware, we have this interaction with them that brings the microplastic front of mind. Tires are part of something we use every day but we’re not actively engaging with the tire itself. People associate cars with fossil fuels, not with other emissions.

In fact, educating the public about these microplastics is the main intent of the UNEP brief itself, and continuing such outreach is one of several suggestions the researchers offer to mitigate this emissions problem in the future. “When we put on clothing or we use plasticware, we have this interaction with them that brings the microplastic front of mind,” Edwards says. “Tires are part of something we use every day but we’re not actively engaging with the tire itself. People associate cars with fossil fuels, not with other emissions.”

The researchers say such outreach would be most effective if it came from a range of stakeholders, from tire manufacturers and the auto industry to research institutions, environmental organizations, educators, and the media. Tire manufacturers could go a step further, too, Gieré says. “Two of our collaborators are tire engineers. They’re working on tires that, when they get damaged, will heal again; it’s called self-healing. This will prolong the life of a tire and emit fewer particles.”

In urban areas, the researchers say, road drainage systems could be connected to wastewater-treatment facilities that would remove tire particles from runoff. Then there’s the option to ask humans to change their behavior, like not hitting the gas pedal hard as soon as a traffic light turns green—but Gieré says that’s likely the most difficult aspect to alter.

Regardless, the UNEP brief is an important step to making people more aware of this car-emissions issue, Edwards says. “I really hope that in this research, we can help people not only focus on fossil fuels, but also the other ways that the items we use pollute the environment. It’s been shocking to see the abundance of tire particles and their long toxicity effects.”