Researcher: California tire-wear emissions are large and should be included in microplastics monitoring

Dr. Kelly Moran, a researcher at the San Francisco Estuary Institute, told the California Water Quality Monitoring Council's Microplastics Monitoring Subcommittee that tire wear is a major and undercounted source of microplastic material in California.

Moran summarized two independent estimation approaches (vehicle-travel-based wear rates and tire-sales/tread-loss calculations) that converged on statewide annual emissions of more than 100 million kilograms of tire-wear material, roughly 2.6'—3 kg per person per year. She said an estimated 2% to 6% of that material reaches surface waters each year via urban stormwater runoff, and that sampling methods historically missed much of the material because many particles are smaller than 125 microns and therefore omitted by coarser filters.

Moran emphasized the chemical hazards associated with tires as well as the particles. She highlighted 6PPD and 6PPD quinone, chemicals associated with rubber that are formed when a tire additive reacts with ozone; the quinone has been linked in laboratory and field studies to acute mortality in salmon species. "Tire particles do release toxic chemicals like 6PPD quinone and that matters in the environment," Moran said, noting that tire-related chemicals have been detected in nearshore and mid-bay water samples in San Francisco Bay, with higher concentrations in the wet season.

The presentation described methodological drivers of underestimation: many monitoring programs historically used a 125-micron filter that captures little tire material, whereas smaller filters (for example, 20 microns) capture far more black tire-derived material in stormwater samples. Moran also noted a very large number of particles at sub-10-micron sizes that are more likely to move quickly in stormwater and to be inhaled if airborne—areas where current monitoring and toxicology data remain incomplete.

Subcommittee members asked about where the unaccounted-for material is sequestered. Moran and other participants pointed to catch-basin sediment, gutters, soils adjacent to roads, and municipal catch-basin cleanings as major sinks; municipal street sweeping and vacuum cleaning capture some material but are not fully effective across the full particle size range. Moran recommended that microplastic monitoring programs include tire particles and that mass-based analyses be pursued to better quantify transported material.

Next steps discussed by the subcommittee included forming a tire-wear working group to help coordinate methods, data sharing, and pilot sampling in stormwater and catch-basin matrices. Moran said additional chemical-monitoring work led by colleagues (including Dr. Ezra Miller) is underway to refine understanding of human- and ecological-exposure pathways.

The subcommittee did not take formal votes on policy in this meeting; Moran's presentation and the discussion focused on research priorities and monitoring design. The group recorded interest in creating a dedicated tire-wear subgroup to move methods and pilot sampling forward.