Every time you commute to work or go to the grocery store, your car tires wear down, causing small pieces of rubber to flake off the tire’s surface. That microscopic debris can be washed into rivers, streams, and estuaries when it rains.
“Driving a car or riding a bus is like dragging an eraser across the earth, except the pieces are microplastics. Toxic microplastics.”
“Driving a car or riding a bus is like dragging an eraser across the Earth, except the pieces are microplastics, which are toxic microplastics,” said Britta Beichler, ocean plastics research director at the Ocean Conservancy.
When you think of microplastics, products like packaging and microbeads may come to mind long before tires. But rubber fallen on roads is actually a major source of ocean plastic pollution, with some studies showing that tire wear particles account for almost half of the microplastics in terrestrial and aquatic systems. And tire debris has only recently been classified as nano- and microplastic particles in various environmental studies, meaning the presence of these toxic tiny particles may be underreported.
Tires contain a mixture of natural and synthetic rubber, other chemicals, additives, and metals. If a tire breaks and enters the environment, the small pieces can become food for fish and other marine life. Many studies tend to focus on particles and chemicals from unused tires rather than actual road-worn tire debris.
To better understand how tire particles affect aquatic ecosystems, researchers exposed a pair of estuarine species to a mixture of both weathered and pristine tire particles. The researchers aimed to understand the ecological risks of tire pollution under more realistic conditions by assessing how the fish and shrimp they studied consumed tire particles, and how both those particles and the chemicals they released into the water affected the species’ growth and behavior. They presented their findings as follows: environmental pollution.
tire taste
In the environment, the tire particles that creatures come into contact with naturally vary in size. The smallest ones are often released directly into the air the moment they are produced. “We’re going to see more contamination (from tire particles), not just in our waterways, but also on roadsides, for example,” says Suzanne Brander, an ecotoxicologist and professor in the Department of Civility at Oregon State University and one of the study’s authors.
“These tire particles are small and can move, some in the air and some in the water. That’s why they’re so prevalent.”
Rain washes those particles off the road surface and into storm drains, where they can connect to fresh water sources. “The cycle starts from there. These tire particles are small and can move, some in the air and some in the water. And that’s why they’re so widespread,” said Bechler, who was not involved in the study.
Tires are made of complex materials and contain thousands of potential toxins. 6PPD is one such ingredient. It is used to prevent rubber from cracking, but even low concentrations can be highly toxic to salmon.
The researchers used a combination of standard tire types found on U.S. roads. 14% are light trucks, 41% are cars, and 45% are trucks and buses. The tire particles were weathered by suspending them in water with organic matter, mechanically treating them with glass beads, shaking, and autoclaving. This broke them down into microparticles with a diameter of 1 to 20 micrometers and nanoparticles with a diameter of less than 1 micrometer. Another portion of the sample was further processed to isolate the compounds released from the tire particles that corresponded to the leachate.
Researchers exposed inland silverside fish (Menidia berylina) and Ami-ebi (American Mysis Bahia) At an early stage, adjust different tire particle and leachate concentrations to mimic different levels of environmental pollution.
“We observed significantly higher ingestion rates for both species when exposed to weathered tire particles compared to pristine particles,” said lead author Clarissa Raguso, a marine scientist and postdoctoral fellow at Portland State University.
Britta Baechler, Ocean Conservancy’s ocean plastics research director, collects tire debris from a road in Portland, Oregon. Credit: Britta Baechler
Although no significant mortality was observed for either fish or shrimp, weathered tire particles reduced the growth of both species, and it took a lower amount of tire particles for shrimp to be affected. Shrimp also ingested more tire particles overall, likely due to their bottom feeding style.
“We were surprised by the species-specific responses,” Raguso said. “We expected weathered tire particles to consistently have the strongest effect across both species and all endpoints.”
Tire particles affected the behavior of both species, but fish were more affected by pristine particles and shrimp were more affected by weathered particles.
“Although stronger effects on growth and uptake were observed in both species, increased behavioral changes with weathering were observed only in Ami shrimp, suggesting that vulnerability to tire contamination varies by species,” Raguso said.
Stress-related behavioral effects altered the animals’ neurological function, with some exposures causing hyperactivity and decreasing stress responses, while other exposures resulted in decreased activity. In the wild, these behavioral changes can make animals more susceptible to predators or disrupt reproduction and feeding. This change can have cascading effects on the food chain.
“Sweet shrimp are a very important food source for endangered species,” Brander said. “Gray whales, for example, eat millions of these types of creatures a day. The big fish that we catch for seafood eat mysids. Even though we’re looking at these tiny larval fish and shrimp that humans don’t eat, they’re a pathway to what we’re eating.”
Reduce tire pollution
Tires are a major source of microplastic pollution, but potential solutions are being considered to ensure fewer particles end up in waterways. One possibility is to change the chemical composition of tires so that they emit fewer harmful particles during use. Another study at Portland State University is installing traps to capture tire particles in stormwater runoff before they enter the marine environment. Another project aims to equip vehicles with devices that capture tire dust before it hits the road.
“This work is important because it brings the study of microplastics closer to real-world situations, the types of particles that living organisms are actually exposed to in nature,” Bechler said. “And understanding how these particles behave after weathering is critical to assessing ecological risk and informing future prevention and mitigation strategies.”
—Rebecca Owen (@beccapox.bsky.social), science writer
