Comparative toxicity of tire wear particle leachates: Zinc as a key toxicant affecting development and motility in zebrafish larvae

Tire wear particles (TWPs) are a major contributor to environmental microplastic pollution, posing risks to aquatic ecosystems. This study evaluated the toxicity of leachates derived from TWPs generated from tires with different treadwear grades (TWGs), a standardized index of tire abrasion resistance. TWP 250, 350, and 700 refer to particles generated from tires with TWGs of 250, 350, and 700, respectively. TWPs were dispersed in water (500 mg/L) for 72 h, and the resulting leachates were tested on zebrafish (Danio rerio) larvae. Among inorganic elements, zinc showed marked variation, with concentrations of 0.15, 0.20, and 2.66 ppm in TWP 250, 350, and 700, respectively. Organic analysis detected trace levels of N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine (6PPD; <2 ppb), no 6PPD-quinone. 1,3-Diphenyl guanidine (DPG) was present at high levels in TWP 250 and 350 (<1654 ppb), but < 27 ppb in TWP 700, while benzothiazole (BT) was consistently detected at ∼30 ppb across samples. Among the three leachates, only TWP 700 induced behavioral impairment, with larvae exhibiting reduced locomotion and delayed adaptation to darkness. Transcriptomic analysis further revealed disruptions in developmental pathways related to the eye, skin, and nervous system. Among the inorganic and organic chemicals measured in the leachates, zinc showed a clear elevation in TWP 700 and appears to play a primary role in its toxicity. Nevertheless, contributions from other organic chemicals cannot be excluded and may also underlie the observed effects. These findings highlight the need for future studies using diverse tire types and experimental assessments of mixture toxicity from individual tire-related chemicals.