Airborne Tire Wear Particles: A Critical Reanalysis of the Literature Reveals Emission Factors Lower than Expected

Tires are a ubiquitous part of on-road transport systems serving as the critical connecting component at the interface of the motive power and road surface. While tires are essential to automobile function, the wear of tires as a source of particulate air pollution is still poorly understood. The variety of reported emissions found in the secondary literature motivated us to summarize all known mass-based tire wear emission factors for light-duty vehicles in primary research. When excluding road wear and resuspension, mean emissions of 1.1 mg/km/vehicle (median 0.2 mg/km/vehicle) were found for tire wear PM₁₀ and mean emissions of 2.7 mg/km/vehicle (median 1.1 mg/km/vehicle) when including studies with resuspended tire wear. Notably, these factors are substantially lower than broadly cited and accepted factors in the secondary literature with mean emissions of 6.5 mg/km/vehicle (median 6.1 mg/km/vehicle). As revealed by our analysis, secondary literature reports emission factors systematically higher than those of the primary sources on which they are based. This divergence is due to misunderstandings and misquotations that have been prevalent since the year 1995. Currently accepted mass-based emission factors for directly emitted airborne tire wear particles need revision, including those from the United States Environmental Protection Agency and the European Environment Agency.