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Zinc accumulation as a primary contributor to co-exposure toxicity with 6PPD-quinone in earthworms: Multi-level evidence
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Tire dust washes zinc and a toxic chemical called 6PPD-quinone into soil, and this study found that zinc buildup—not the chemical itself—was the main driver of harm to earthworms exposed to both, weakening their bodies' ability to handle stress. This matters because tire particles are a widespread source of pollution in soil and waterways, and understanding which ingredient does the most damage helps scientists better assess real-world risks from mixtures of chemicals, not just one substance at a time.
Tire and road wear particles introduce complex mixtures of metals and organic additives into soils, yet the primary drivers of their combined toxicity remain poorly resolved. Here, we investigated the individual and combined effects of zinc oxide (ZnO), a major inorganic tire additive, and 6PPD-quinone (6PPD-Q), a toxic transformation product of the tire antioxidant 6PPD, using a soil invertebrate exposure model. A gradient of ZnO concentrations was tested alone and in combination with an environmentally relevant level of 6PPD-Q to simulate co-contaminated soils. While single exposures induced oxidative stress, metabolic perturbation, and gut microbiome alteration, co-exposure under high Zn conditions resulted in increased mortality and growth inhibition relative to single treatments. Chemical analysis revealed enhanced Zn accumulation in organisms under co-exposure, whereas 6PPD-Q bioaccumulation declined at elevated Zn levels, indicating that internal Zn burden was more consistently associated with organism-level impairment than measured internal 6PPD-Q concentration under the tested conditions. Integrated biochemical, histological, and multi-omics analyses showed that co-exposure was associated with enhanced oxidative stress, altered energy metabolism, impaired neuroimmune function, and destabilized gut microbial structure and predicted metabolic potential. Complementary multi-omics analyses revealed coordinated alterations in antioxidant defense, detoxification capacity, and mitochondrial metabolism under high-Zn co-exposure, consistent with a reduced physiological tolerance to chemical stress. Together, these findings support the interpretation that zinc accumulation likely represents a primary contributor to co-exposure toxicity, while 6PPD-quinone may amplify adverse outcomes by constraining organismal stress-buffering capacity. These results highlight the need to consider metal-organic interactions and differential toxic contributions in mixture-based environmental risk assessment of tire-derived contaminants.
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Researchers compared the toxicity of airplane and automobile tire wear particles in earthworms, finding that airplane-derived particles caused more severe antioxidant enzyme suppression, intestinal tissue damage, and gut microbial diversity loss — with LC-MS toxicity profiling identifying 6PPD, RD, and DCH as the highest-risk compounds driving these effects.
Toxicity of two different size classes of tire particles from mixed end-of-life car tires to the springtail Sinella curviseta
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Researchers tested two size classes of tire particles on springtails, small soil-dwelling organisms, and found that while both sizes affected reproduction, the smaller particles caused more severe growth reductions. The tire particles released zinc into the soil, but the measured zinc levels alone did not fully explain the toxicity observed. The study suggests that tire wear particles harm soil organisms through a combination of chemical leaching and other particle-related effects.
Toxicity of tire wear particles and the leachates to microorganisms in marine sediments
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Researchers investigated the toxicity of tire wear particles and their chemical leachates on bacteria in marine sediments. The study found that aged tire wear particles were more toxic than pristine ones, and that leachates were even more harmful than the particles themselves, with zinc identified as the primary toxicity-causing substance.
Ecotoxicological effects of micronized car tire wear particles and their heavy metals on the earthworm (Eisenia fetida) in soil
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Researchers exposed earthworms to microplastics from ground-up car tires at various concentrations and sizes over 14 and 28 days. They found that the worms preferentially consumed smaller particles, which led to increased heavy metal accumulation and significant signs of oxidative stress at higher concentrations. The study highlights that tire-derived microplastics in soil can harm terrestrial organisms through both physical ingestion and the release of associated toxic metals.
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Tiny bits of worn tire rubber that wash into rivers and lakes don't just pollute water on their own, this study found that when combined with nickel (a metal common in urban runoff), they team up to harm aquatic plants more than either does alone, stunting growth and disrupting nutrient balance. This matters because it's a reminder that real-world pollution rarely comes from a single source, and the mix of everyday contaminants like tire dust and metals may pose bigger risks to waterways, and the food chains and drinking water they support, than testing pollutants one at a time would suggest.
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