Tire particles flood freshwater with toxic microplastics

A new review pulls together decades of research on tire wear particles (TWPs) – the tiny bits that break off tires and get washed away. It turns out wind and rain can sweep huge amounts of these fragments off highways and straight into nearby streams and ponds.

In heavy-traffic zones, TWPs can make up 50 to 90 percent of all microplastics that run off roads during rainfall. When soils are included, nearly 45 percent of microplastics found on land or in freshwater environments can be traced to tire abrasion.

Concentrations recorded in field samples vary wildly – from just 0.00001 mg per liter to an astonishing 10,000 mg per liter. These extremes highlight the presence of localized hotspots.

Tires release a toxic mix

When it comes to tires, speed and grip matter more than ecological impact. The review lists 2,456 different chemicals in tire rubber – and lab tests show that at least 144 of them can leach out.

Prominent among them are hexamethoxymethyl-melamine, dibutyl phthalate, and the antioxidant 6-PPD, which transforms into its highly toxic quinone derivative.

Manufacturers embed heavy metals – like zinc, manganese, cadmium, and lead – into the polymer matrix as vulcanizing agents and fillers.

Hans-Peter Grossart of the Leibniz Institute of Freshwater Ecology and Inland Fisheries is a co-author of the study.

“During the leaching process, tire abrasion releases more chemicals than thermoplastics such as PE. We also assume that more substances are leached out than we already know,” he said.

Tire dust disrupts ecosystems

Laboratory exposure studies reveal that tire particles and their leachates prompt oxidative stress, DNA damage, immune suppression, and developmental defects in a wide array of aquatic organisms. Affected species show reduced feeding, stunted growth, and lower reproductive success.

Scaling up, the review links these individual ailments to shifts in community composition, which in turn disrupt nutrient cycles and diminish biodiversity.

Changes in microbial and algal communities could also disrupt how carbon and nitrogen cycle through the environment – processes that are key to oxygen production and clean water.

Warming worsens toxic effects

Rising temperatures and freshwater acidification are likely to make the ecological impact of tire particles even worse.

Higher temperatures accelerate chemical reactions, increasing the rate at which additives leach from particles, while lower pH can heighten metal solubility and bioavailability.

Grossart cautioned that these environmental shifts will likely exacerbate the effects of tire wear and its leaching by altering its toxicity and its interactive effects on microbial activity, nutrient cycling, and the resilience of ecosystems.

Tire dust sinks near roads

Studies indicate that wind can spread tire wear particles over a wide area. However, most fragments fall near their release points and accumulate along road verges, drainage ditches, and sediment beds.

The researchers estimate that only about two percent of tire particles carried by rivers reach the sea. That means capturing them earlier – at the watershed level – could have a big impact.

The review calls for physical barriers and improved drainage systems to trap particles before they reach natural waterways.

Drivers play a role too

The researchers suggest that tire manufacturers should explore alternative formulations with fewer hazardous additives.

They also urge urban planners to improve the separation between road surfaces and water bodies. Yet drivers also hold part of the solution.

“Effective reduction strategies involve the development of alternative tire manufacturers, and a better demarcation of roads and wastewater from natural areas,” Grossart said. “Ultimately, everyone can make their own contribution with a prudent driving style.”

Calls for tire pollution limits

Despite the mounting evidence, major knowledge gaps persist. Standardized field protocols are rare, hindering cross-study comparisons, and most of the thousands of tire-derived chemicals remain untested for toxicity.

The review urges cross-disciplinary collaborations to develop real-world monitoring networks and predictive models. These tools would help regulators set science-based limits for TWP emissions.

The review highlights TWPs as a major but overlooked source of freshwater pollution, based on volume and chemical complexity.

Without concerted action – from product redesign to traffic management – the dust generated by every mile driven will continue to seep into rivers and lakes, carrying with it a suite of toxins that ecosystems must somehow absorb.

The review is published in the Journal of Environmental Management.

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