Published research

6-PPDQ, an oxidation product of the antioxidant 6-PPD added to tire rubber, reaches the environment with tire abrasion on roads. Firm data on the environmental persistence of 6-PPDQ in different compartments is lacking. The microbial degradation of 6-PPDQ was studied in simulation tests under standardized conditions in the laboratory. Biodegradation half-life of 6-PPDQ (DT50) increased from surface water (12–17 d) over water-sediment systems including nonextractable residue (NER I) under aerobic conditions (46–55 d), soils including NER I under aerobic conditions (27–141 d, median 61 d), water–sediment systems under anaerobic conditions (140–202 d) to soil under anaerobic conditions (214–887 d, >10 000 d in one model). On this basis, 6-PPDQ would be considered nonpersistent according to commonly accepted definitions under aerobic conditions in water, in water–sediment systems, and in soil (median value and three of the four tested soils). 34% of the applied radioactivity (AR) of [¹⁴C]-6-PPDQ was mineralized under aerobic conditions in 60 d in surface water, 27% in the water–sediment system in 100 d and 34% in soil in 120 d. Under anaerobic conditions, mineralization was 4%–5% AR, only, both in water–sediment and in soil. Hydroxylated and carboxylated 6-PPDQ were identified as transformation products, of which 11% AR remained after 60 d. In sediment and soil no single transformation product exceeded 1% AR at the end of the experiment; instead, NERs were formed. Under aerobic conditions, 55% (water–sediment) and 49% (soil) of the AR of degraded 6-PPDQ occurred as NER; it increased to 67% (water–sediment) and 84% (soil) under anaerobic conditions. These first quantitative data on the extent and the rate of biodegradation and mineralization of 6-PPDQ help to understand the occurrence of 6-PPDQ in the environment and support environmental exposure modeling and risk assessment.

© 2026 The Authors. Published by American Chemical Society. This publication is licensed under a Creative Commons Attribution 4.0 International License.

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N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylendiamine (6-PPD) and one of its oxidation products, 6-PPD quinone (6-PPDQ) are chemicals released from tire and road wear particles (TRWP). The abiotic transformation of both compounds by hydrolysis and in contact with the atmosphere with and without oxygen species and simulated sunlight was studied, following relevant portions of OECD-guidelines 111 and 316, and using radiolabelled test substances. This allowed for mass balances including transformation products, those identified by liquid chromatography-mass spectrometry and those remaining unidentified. 6-PPD was effectively degraded by hydrolysis at neutral and basic pH (DT50 5 – 12 h at 25◦C), with 4-HDPA being the major reaction intermediate. 6-PPDQ remained below 0.2% of the applied radioactivity (AR) of 6-PPD, except at very acidic conditions and low temperature (10◦C) with an intermediate yield of 7.5% AR. In contact with atmosphere 6- PPD was transformed by direct photolysis (DT50 14 h). Direct reaction with ozone was slow (DT50 3.5 d) but degradation by OH-radicals (ozone/light) was very fast (DT50 3.7 h). A diverse set of products was formed; 6- PPDQ was determined in some cases, with a maximum intermediate yield of 1.3% AR and decreasing again with ongoing oxidation of 6-PPD (to levels ≤ 0.6% AR with ongoing oxidation). 6-PPDQ is not susceptible to pure hydrolysis (DT50 > 1000 d at pH 7, mean 490 d in 13 natural waters) and very limited to direct photolysis (DT50 56 d). On the contrary, reactivity towards reactive oxygen species (air/ozone and light) is in the same range as for 6-PPD (DT50 2 – 7 h). Oxidative transformation of 6-PPDQ proceeds via opening of the quinone-ring, with 20–50% AR recorded as CO2 14 within 2 d. For 6-PPDQ oxidation in the presence of light only 50 – 80% of AR could be recovered. This study with C-labelled test substances provides solid information on the abiotic transformation of both, 6-PPD and 6-PPDQ, and supports environmental risk assessment.

© 2026 by the authors. Published by Elsevier Inc. This is an open access article distributed under the terms of the Creative Commons CC-BY license.

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The discussion of potential adverse effects of particulate tire wear emissions on ecosystems and human health highlights the need for systematic evaluation of their sources, exposure pathways, and health impacts. Global urbanization may increase exposure to tire and road wear particles (TRWP) and tire-related chemicals, especially in high-traffic areas. This literature review examines current knowledge on human exposure to TRWPs and tire-related chemicals and explores whether TRWP pose a distinctive risk compared with airborne particulate matter (PM). Analytical challenges persist in identifying airborne TRWP, as most research focuses on tire wear particles (TWP) alone, due to difficulties in defining mass ratios within the TRWP aggregate. TWP constitute <5 wt % of ambient PM2.5 and PM10; however, inconsistent analytical methodologies hinder a conclusive exposure assessment. Actual data on human exposure to TWP or TRWP are scarce. Tire-related chemicals have been found in human body fluids, but their exposure pathways are unclear. Toxicological data mainly derive from in vitro studies with few harmonized designs. Comparative research suggests that TRWP are not more toxic than other PM fractions. This review emphasizes the need for harmonized methods, global and regional exposure characterization, and identification of TRWP exposure pathways for humans to address potential health implications more accurately.

© 2026 by the authors. Published by American Chemical Society. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY 4.0) license.

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Tire and road wear particles (TRWPs) are formed at the frictional interface of the tire and road surface and consist of polymer-containing tread with pavement mineral and binder encrustations. Their detection in various environmental compartments globally sparks increasing societal and regulatory interest. Solid quantitative information as a basis for managing and mitigating TRWPs in the environment is lacking, however. This paper presents and demonstrates a model approach that produces catchment-scale terrestrial and aquatic TRWP mass balances anywhere in the world. A spatially and temporally explicit modeling method was used that builds on publicly available global datasets and process-based open-source modeling frameworks to describe hydrological processes, TRWP releases, fate and transport under a wide range of climatic conditions. High-resolution (<1 km) models were implemented and evaluated by demonstrating consistency with available field data for three watersheds on different continents. The approach provides comprehensive mass balances to underpin management of TRWPs that account for socio-economic, climate, geography and stormwater management gradients. Case study results revealed strong climate-induced differences: the fraction of vehicle-generated TRWPs exported to the estuarine environment varied between 2% (Seine watershed, France) to 18% (Yodo River watershed, Japan), corresponding to an increase in the fraction released to freshwater ecosystems from 20% to 36%, respectively. The modeling framework provides a consistent comparison between watersheds across the world. Limitations of the approach are its lack of local details and the uncertainties stemming from the still-developing scientific knowledge base.

© 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.

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Para-phenylenediamines (PPDs) are antioxidants added to tires to protect the rubber. They are released from tire and road wear particles (TRWP) but the extent of their aerobic microbial degradation and the transformation products (TPs) formed are not known. Therefore, aerobic microbial degradation of seven tire-related PPDs, parent compounds as well as known transformation products, was studied for up to 28 days. Half-lives ranged from 0.2 ± 0.1 days (N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine, 6-PPD) and 0.6 ± 0.1 days (N-isopropyl-N’-phenyl-1,4-phenylenediamine, IPPD) to 3 ± 0.1 days (N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine quinone, 6-PPDQ). A total number of 48 TPs was tentatively identified by liquid chromatography-high resolution-mass spectrometry for the seven study compounds. Of these TPs, only four did not decrease in concentration when the parent compounds were degraded completely. Biotransformation in aqueous solution forms several TPs not known for abiotic, photolytic or oxidative transformation. For the PPDs with aliphatic substituents (6-PPD, IPPD) hydrolysis to 4-HDPA was the major initial transformation. Formation of 6-PPDQ from 6-PPD was not detectable. For the fully aromatic DPPD aerobic microbial transformation, likely, proceeded via a quinone diimine intermediate, leading to products different to those of the aliphatic PPDs. From 6-PPDQ, 26 TPs were detected. A suspect screening for the TPs detected from the biodegradation experiments was performed in data of a soil degradation study over 23 months with TRWP and cryo-milled tire tread (CMTT) and in data from the influent and effluent of a municipal wastewater treatment plant during a rain event. In total, 10 TPs were found in those data with variable intensities, most of which originated from 6-PPDQ. While all seven test compounds were (primary) degraded under aerobic conditions, mineralization was not studied. A number of TPs remain as suspects to search for in the environment.

This work, “Biodegradation pathways and products of tire-related phenylenediamines and phenylenediamine quinones in solution – a laboratory study” by "Han et al." was originally published in Water Research, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here.

Tire and road wear particles (TRWP) are continuously formed by automotive traffic on roads. This study reports effects of long-term degradation over 2 years in water and in soil in the presence of microbes on TRWP and on cryo-milled tire tread (CMTT). Degradation in water had little measurable effect on physical properties of TRWP; a shift towards larger particle sizes was mainly due to the mechanical stress from stirring. The total quantified extractables (TQE) of 27 chemicals and transformation products determined from tire particles were reduced by 90 % from TRWP and CMTT in water and by 85 % in soil. Most of this decrease occurs within the first months. For both materials, however, the speed of loss of TQE in water and in soil decreased drastically over time. Its kinetics was approximated by two phases of 1st order kinetics, resulting in half-lives from 17 days for diphenylguanidine (DPG) in phase 1 to 520 days for 6-PPD-quinone (6-PPDQ) in phase 2 of TRWP biodegradation in water. For soil, half-lives tend to be clearly longer in phase 2 compared to water but remained <1000 days for chemicals such as benzothiazole sulfonic acid (BTSA), N,N′-diphenyl-p-phenylendiamine (DPPD) and hydroxybenzothiazole (OH-BT). For N-(1,3-dimethylbutyl)-N′-phenyl-1,4-phenylenediamine (6-PPD) and 6-PPDQ they exceeded 2000 days from TRWP. Only 1–15 % of TQE lost from the tire materials remained detectable at the end of the experimental period in the supernatant of the suspension or in leachates of the soil. Mostly benzothiazoles were determined from solution. The biodegradation experiments show an effective reduction of a large part of the chemical burden of TRWP of polar and moderately polar compounds. Despite that, TRWP may serve as a long-term reservoir for some of the tire related chemicals or their transformation products in the environment.

This work, “Long term biodegradation study on tire and road wear particles and chemicals thereof” by "Weyrauch et al." was originally published in Science of the Total Environment, and is licensed under the Creative Commons Attribution 4.0 International License. You may view the original publication here.