As electric vehicles become more common, exhaust emissions may decline, but another form of traffic pollution is receiving increasing attention: chemicals released as tires wear down.
A new study has provided the first national-scale source-to-receptor assessment of tire-derived 6PPD pollution in China, tracing emissions from roads through soil, water, air, and sediment and projecting how those emissions could change through 2060.
6PPD, short for N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, is widely added to tires to protect rubber from degradation caused by ozone and oxidation. As tires wear against road surfaces, however, 6PPD can be released into the environment along with tire wear particles. It can also form transformation products such as 6PPD-quinone, which has raised concern because of its toxicity to aquatic organisms.
The researchers estimated emissions across 333 prefecture-level cities in China from 2000 to 2020 and connected those emissions to 58 secondary river basins using a multimedia environmental fate model.
Their results show that national 6PPD emissions increased from approximately 0.0064 million metric tons in 2000 to 0.045 million metric tons in 2020, representing about a sevenfold rise.
"Our results show that tire-derived pollution is not simply an urban road problem. It moves through multiple environmental compartments and can continue growing even as transportation becomes increasingly electrified," said Qian-Qian Zhang of South China Normal University. "Understanding where these chemicals are released and how they move through the environment is essential for developing effective control strategies."
One of the study's most notable findings was the importance of rural roads. Rural roads contributed 56.5% of total tire-derived 6PPD emissions, exceeding contributions from urban roads and expressways. The researchers attributed this pattern largely to the extensive mileage and cumulative vehicle travel associated with lower-grade road networks.
Once released, much of the chemical remained on land. In 2020, approximately 57.0% of 6PPD entered soil, while 32.2% entered sewer systems, 6.86% entered air, and 3.95% directly reached surface water.
The small direct input to rivers does not mean aquatic ecosystems face little exposure. Contaminants stored in soils and drainage systems can later be transported into rivers through stormwater runoff and sewer discharge.
The study also identified strong geographic differences. Major emission hotspots occurred in economically developed and heavily trafficked regions, including the Beijing-Tianjin-Hebei region, Yangtze River Delta, and Shandong Peninsula. River basins receiving inputs from densely populated eastern regions generally showed higher modeled 6PPD concentrations.
Looking ahead, the researchers project that emissions could reach approximately 0.15 million metric tons per year by 2060 under the SSP2 baseline scenario. Importantly, increasing adoption of new energy vehicles does not remove the problem because electric vehicles still rely on tires and generate tire wear particles.
The findings suggest that reducing transportation pollution will require attention not only to tailpipe emissions but also to non-exhaust sources such as tire wear. The researchers highlight stormwater management, runoff control, environmental monitoring, and source reduction as important components of future pollution-management strategies.
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Journal reference: Teng W; Yu X; Cai Y; et al. National-scale emissions, multimedia fate, and future projections of tire-derived 6PPD in China. AI Environ. 2026, 1(3): xx-xx. DOI: 10.66178/aie-0026-0016
https://www.the-newpress.com/aie/article/doi/10.66178/aie-0026-0016
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