Retreaded tires are an overlooked source of microplastics with distinct additive leaching and ecotoxicity

Retreaded tires constitute a substantial segment of the commercial tire market and are an important source of tire wear particles (TWPs), yet the environmental risks of this major microplastic category remain uninvestigated. Here, we show that although the total additive mass is generally lower in TWPs from retreaded tires, these particles exhibit a markedly greater additive leaching potential, particularly for p-phenylenediamines (PPDs). Notably, the highly water-soluble additive N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), present at high concentrations in some retreaded-tire TWPs, is especially leachable. Correspondingly, leachates from retreaded-tire TWPs cause greater growth inhibition in Vibrio fischeri and Chlorella vulgaris than those from new or used tires. Furthermore, our numerical model projections under the Shared Socioeconomic Pathway 2 (SSP2) scenario show that global emissions of retreaded-tire TWPs could increase several hundred-fold by 2060. The substantial and growing risks identified in our study underscore the urgent need for broader investigations into the environmental impacts of these particles.