Tire Wear Particles' Environmental Distribution and Potential Threats to Human Health

Tire wear particles (TWPs), stand out as the largest contributor to primary microplastics in the environment. Recent estimates put global annual releases at around 5.9 million tons. Unlike commodity microplastics (PE, PP, PS) from degrading products, tire wear particles possess a distinct rubber matrix reinforced with 30–40% carbon black and silica, combined with unique tire-specific additives like 6PPD that do not exist in conventional microplastics. These particles account for roughly 60–70% of all primary microplastic emissions reaching natural systems. These particles form mainly from the friction between tires and roads, and they also emerge through weathering over time. Once released, TWPs spread widely, they linger in the air, settle on land, and flow into water bodies. Along the way, they carry unique tire-specific additives, notably 6PPD and its oxidation product 6PPD-quinone. Critically, 6PPD-quinone shows acute toxicity to aquatic organisms at concentrations below 1 μg/L (median lethal concentration in coho salmon), a toxicity threshold substantially lower than typical microplastic-derived compounds. This compound can mobilize metals and polycyclic aromatic hydrocarbons, which adds to the concern. People encounter TWPs through inhalation, ingestion, and sometimes skin contact. These exposure routes raise worries about respiratory issues, immune system effects, and even neurological problems. However, we still lack solid data on the tiniest fractions, those under 1 µm, as well as on long-term buildup in the body and the combined toxicity of these particles with other substances. Unlike typical microplastics from sources like broken-down packaging or textiles, TWPs come from complex rubber composites. Their distinct physical and chemical properties, plus their unique mix of additives, call for specialized testing methods and toxicity evaluation approaches. Current detection technologies often fall short in sensitivity, specificity, and standardization. As a result, it remains challenging to accurately trace exposure back to tire wear sources. This review pulls together the latest evidence on how these rubber-derived particles behave in the environment. It aims to address major gaps in assessing health risks specific to TWPs. In particular, it highlights what we don't yet understand about the toxicity of submicron particles and chronic human exposure. Finally, it recommends steps forward: stronger integrated monitoring in the environment and in human bodies, plus the development of tires designed to wear less. These measures could help curb the persistence of TWPs and reduce associated health risks.