4 Occurrence, Fate, Transport, and Exposure Pathways
This chapter reviews the fate, transport, and occurrence of tire particles containing 6PPD and 6PPD‑q in the environment. More research is needed on the fate, transport, occurrence, and persistence of 6PPD‑q once released from tires and other rubber products to inform toxic reduction actions.
In the context of what is known regarding 6PPD and 6PPD‑q occurrence, this section also discusses different pathways by which human or ecological receptors may be exposed to 6PPD and 6PPD‑q.
Recent studies have investigated the occurrence of 6PPD and 6PPD-q in various environmental matrices across the globe. This section summarizes the current state of knowledge but is not intended to represent a comprehensive review of occurrence data. Additionally, reliability evaluation and comparison of analytical methods have not been performed for studies discussed in this document. The following index presents tables, organized by medium type, where summaries of peer-reviewed studies can be found.
| Table | Media Type | Available Publications | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-1 | Surface Water | 12 |  |
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| 4-2 | Stormwater | 9 | |
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| 4-3 | Wastewater, water treatment plants, and tap water | 8 | |
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| 4-4 | Groundwater | 1 | |
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| 4-5 | Roadside soil | 1 | |
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| 4-6 | Sediment | 3 | |
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| 4-7 | Outdoor air | 8 | |
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| 4-8 | Road dust and roadside snow | 7 | |
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| 4-9 | Indoor and nonroad settled dust | 6 | |
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| 4-10 | Aquatic organisms and food | 9 | |
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Note: PDF versions of each table are provided for the reader to view information in a visual format that is consistent across browsers and platforms. Executable files are provided to allow readers to sort information by the column of their choice, but may appear different visually depending on the software used to view this file. Instructions on how to sort information in a document formatted for a word processor format are widely available by internet search.
The studies listed in these tables are from peer-reviewed literature and do not capture the preliminary monitoring efforts by state, tribal, and local agencies. Some of these studies also measured a range of other PPDs and transformation products, which are not addressed in this document. Recent efforts have been made to summarize occurrence data in peer-reviewed literature ( Mayer et al. 2024[ZTAVFS9G] Mayer, Paul, Kelly Moran, Ezra Miller, Susanne Brander, Stacey Harper, Manuel Garcia-Jaramillo, Victor Carrasco-Navarro, et al. 2024. “Where the Rubber Meets the Road: Emerging Environmental Impacts of Tire Wear Particles and Their Chemical Cocktails.” Science of The Total Environment 927 (June):171153. https://doi.org/10.1016/j.scitotenv.2024.171153.Benis et al. 2023[XXB6GPKJ] Benis, Khaled Zoroufchi, Ali Behnami, Shahab Minaei, Markus Brinkmann, Kerry N. McPhedran, and Jafar Soltan. 2023. “Environmental Occurrence and Toxicity of 6PPD Quinone, an Emerging Tire Rubber–Derived Chemical: A Review.” Environmental Science & Technology Letters, September. https://doi.org/10.1021/acs.estlett.3c00521.Chen et al. 2023[39YBXWMI] Chen, Xiaoli, Tao He, Xinlu Yang, Yijing Gan, Xian Qing, Jun Wang, and Yumei Huang. 2023. “Analysis, Environmental Occurrence, Fate and Potential Toxicity of Tire Wear Compounds 6PPD and 6PPD-Quinone.” Journal of Hazardous Materials 452 (June):131245. https://doi.org/10.1016/j.jhazmat.2023.131245.Hua and Wang 2023[3FVXFWPE] Hua, Xin, and Dayong Wang. 2023. “Tire-Rubber Related Pollutant 6-PPD Quinone: A Review of Its Transformation, Environmental Distribution, Bioavailability, and Toxicity.” Journal of Hazardous Materials 459:132265. https://doi.org/10.1016/j.jhazmat.2023.132265. ). Given the regional-specific conditions that impact the fate and transport of 6PPD-q (and TRWP and 6PPD), considerations should be made when interpreting these data. Further, the sampling and analytical methods used should be reviewed (for example, collecting and storing material, use of a commercially available standard) given the continued advancements made in quantifying this compound in environmental matrices (see also Section 5: Measuring, Mapping, and Modeling). The transport pathways and exposure risk to aquatic, terrestrial, and human systems is poorly understood (see Section 8.2).
Occurrence, Fate, and Transport
- Friction between the road and a tire during driving, braking, and turning leads to the generation and emission of TRWP.
- TRWP are carried beyond the road surface via stormwater or other pathways, such as air dispersion and deposition, and can enter aquatic or terrestrial systems.
- Fate and transport mechanisms that control occurrence vary among landscapes, geography, climate, and environmental conditions.
- The occurrence and persistence of 6PPD, 6PPD-q, TWP, and TRWP in the environment is poorly understood.
- More research is needed to standardize methods and fill in data gaps because initial studies have only scratched the surface.
- In addition, reused and recycled whole tires are a potential source of 6PPD and 6PPD-q, so more research is needed to understand the half-life of 6PPD in tires across varying environments (marine, freshwater, and terrestrial, etc.).
Understanding the mechanisms of toxic exposure, including the duration and mode of action, is needed to characterize the environmental risk (see also Section 2: Effects Characterization and Toxicity).
6PPD and 6PPD-q can be released to the environment from whole tires, TRWP, and reused/recycled/repurposed consumer products. For example, recycled tires can be used to modify asphalt, and 6PPD and 6PPD-q can be released from or sorbed to that material on the roadways ( Lokesh et al. 2023[LBXMIT5W] Lokesh, Srinidhi, Siththarththan Arunthavabalan, Elie Hajj, Edgard Hitti, and Yu Yang. 2023. “Investigation of 6PPD-Quinone in Rubberized Asphalt Concrete Mixtures.” ACS Environmental Au, July. https://doi.org/10.1021/acsenvironau.3c00023. ). Although whole-tire reuse and disposal has not been the main focus of the available occurrence data, it is important to note that more research is needed to investigate whole tires as a continued source of 6PPD and 6PPD-q. Whole tires are reused in many ways, such as marine reef structures, boat bumpers on docks, and landscaping materials. Regulation and guidance to provide proper storage, transport, and disposal of tires varies across states and should be evaluated to address 6PPD and 6PPD-q exposure concerns. The following subsections, which discuss environmental media, do not discuss whole-tire pollution, tire piles, or tires submerged or on land; these tires represent a source of 6PPD and 6PPD-q that is not well investigated or understood. See Section 1.2 for a brief discussion of tire life cycle and both Section 1.2 and Section 8.2 for discussion and summary of information needs and data gaps regarding sources of 6PPD and 6PPD-q.
TRWP are heterogeneous particles generated at the road surface by the friction of the tire on the road surface during driving. TRWP can be dispersed into the air or onto the roadway and includes wear particles from both the tire tread (i.e., TWP) and the road surface (i.e., road component) ( Baensch-Baltruschat et al. 2020[SG7DEPVC] Baensch-Baltruschat, Beate, Birgit Kocher, Friederike Stock, and Georg Reifferscheid. 2020. “Tyre and Road Wear Particles (TRWP)—A Review of Generation, Properties, Emissions, Human Health Risk, Ecotoxicity, and Fate in the Environment.” Science of the Total Environment 733 (September):137823. https://doi.org/10.1016/j.scitotenv.2020.137823. ). The road-wear component of TRWP have an impact on the characteristics and transport of the particles in the environment ( Kreider et al. 2010[QCJY4JI9] Kreider, Marisa L., Julie M. Panko, Britt L. McAtee, Leonard I. Sweet, and Brent L. Finley. 2010. “Physical and Chemical Characterization of Tire-Related Particles: Comparison of Particles Generated Using Different Methodologies.” Science of the Total Environment 408 (3): 652–59. https://doi.org/10.1016/j.scitotenv.2009.10.016. ). Elsewhere in this document TWP will be referred to as the tire fraction of the overall TRWP. See the Tire and Road-Wear Particle Background and Related Terms inset for more details.
An estimated 4.7 kg/year (equivalent to 10.3 pounds/year) per capita of TRWP is released to the environment in the United States ( Kole et al. 2017[NZZMY6WC] Kole, Pieter Jan, Ansje J. Löhr, Frank G. A. J. Van Belleghem, and Ad M. J. Ragas. 2017. “Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment.” International Journal of Environmental Research and Public Health 14 (10): 1265. https://doi.org/10.3390/ijerph14101265. ). The mass loading of 6PPD and 6PPD-q in the environment is expected to vary spatially given the differences in 6PPD-q and TRWP release rates, tire manufacturing, tire age, and vehicle attributes and operation (for example, weight, speed, and braking) ( Kreider et al. 2010[QCJY4JI9] Kreider, Marisa L., Julie M. Panko, Britt L. McAtee, Leonard I. Sweet, and Brent L. Finley. 2010. “Physical and Chemical Characterization of Tire-Related Particles: Comparison of Particles Generated Using Different Methodologies.” Science of the Total Environment 408 (3): 652–59. https://doi.org/10.1016/j.scitotenv.2009.10.016.Baensch-Baltruschat et al. 2020[SG7DEPVC] Baensch-Baltruschat, Beate, Birgit Kocher, Friederike Stock, and Georg Reifferscheid. 2020. “Tyre and Road Wear Particles (TRWP)—A Review of Generation, Properties, Emissions, Human Health Risk, Ecotoxicity, and Fate in the Environment.” Science of the Total Environment 733 (September):137823. https://doi.org/10.1016/j.scitotenv.2020.137823.Wagner et al. 2018[4UCJI65Q] Wagner, Stephan, Thorsten Hüffer, Philipp Klöckner, Maren Wehrhahn, Thilo Hofmann, and Thorsten Reemtsma. 2018. “Tire Wear Particles in the Aquatic Environment — A Review on Generation, Analysis, Occurrence, Fate and Effects.” Water Research 139 (August):83–100. https://doi.org/10.1016/j.watres.2018.03.051. ). Further, once released to roads and parking lots, the fate and transport of 6PPD-q and TRWP depends on many factors including tire particle characteristics (for example, size, shape, and density), road characteristics, regional weather, and environmental characteristics ( Unice et al. 2019[TLVMH289] Unice, K.M., M.P. Weeber, M.M. Abramson, R.C.D. Reid, J.A.G. van Gils, A.A. Markus, A.D. Vethaak, and J.M. Panko. 2019. “Characterizing Export of Land-Based Microplastics to the Estuary — Part I: Application of Integrated Geospatial Microplastic Transport Models to Assess Tire and Road Wear Particles in the Seine Watershed.” Science of the Total Environment 646:1639–49. https://doi.org/10.1016/j.scitotenv.2018.07.368.Wagner et al. 2018[4UCJI65Q] Wagner, Stephan, Thorsten Hüffer, Philipp Klöckner, Maren Wehrhahn, Thilo Hofmann, and Thorsten Reemtsma. 2018. “Tire Wear Particles in the Aquatic Environment — A Review on Generation, Analysis, Occurrence, Fate and Effects.” Water Research 139 (August):83–100. https://doi.org/10.1016/j.watres.2018.03.051. ). Road characteristics include traffic amount and type, vehicle type (for example, car, truck, electric vehicle), road surface, road size, road type (for example, local roads vs. highways), grade (steepness), and roadside slope and type ( Wagner et al. 2018[4UCJI65Q] Wagner, Stephan, Thorsten Hüffer, Philipp Klöckner, Maren Wehrhahn, Thilo Hofmann, and Thorsten Reemtsma. 2018. “Tire Wear Particles in the Aquatic Environment — A Review on Generation, Analysis, Occurrence, Fate and Effects.” Water Research 139 (August):83–100. https://doi.org/10.1016/j.watres.2018.03.051. ). Watershed characteristics and stormwater conveyance are expected to influence the transport of TRWP and 6PPD-q, including attributes such as roadside slope and conveyance (curb and gutter, grass ditch, paved ditch, presence/absence of stormwater drain and pipe, presence/absence of stormwater filtration or catchment), land use, seasonal traffic trends, seasonal weather patterns, regional stormwater and wastewater management practices, soil types, watershed size, and flood risk. Section 5.3.5.3 describes how these characteristics can be used in the USEPA Visualizing Ecosystem Land Management Assessments (VELMA) tool to identify potential hotspots of 6PPD-q contamination as a means of prioritizing locations to investigate and potentially mitigate impacts to the environment. Section 5: Measuring, Mapping, and Modeling provides additional discussion on how these factors can be measured, mapped, and used in models of TRWP, 6PPD, and 6PPD-q fate and transport.
Although TRWP containing 6PPD are mostly transported by surface water and stormwater, TRWP are also released and transported by atmospheric processes. In a road dust and sediment study ( Klöckner et al. 2020[9B7NCVNZ] Klöckner, Philipp, Bettina Seiwert, Paul Eisentraut, Ulrike Braun, Thorsten Reemtsma, and Stephan Wagner. 2020. “Characterization of Tire and Road Wear Particles from Road Runoff Indicates Highly Dynamic Particle Properties.” Water Research 185 (October):116262. https://doi.org/10.1016/j.watres.2020.116262. ), more coarse particles were found closer to the roadway, while smaller particles were more readily transported away from the road. As mentioned previously, TRWP are generally found more frequently and in higher quantities near roadways and in urban areas, particularly those with high-volume traffic patterns ( Unice, Kreider, and Panko 2013[EU6MQU9K] Unice, K.M., Marisa L. Kreider, and Julie M. Panko. 2013. “Comparison of Tire and Road Wear Particle Concentrations in Sediment for Watersheds in France, Japan, and the United States by Quantitative Pyrolysis GC/MS Analysis.” Environmental Science & Technology 47 (15): 8138–47. https://doi.org/10.1021/es400871j. ). Roadside vegetation and solid structures have been shown to mitigate the dispersion of airborne particles near roadways ( Baldauf 2016[I5U5RA8E] Baldauf, Richard W. 2016. “Recommendations for Constructing Roadside Vegetation Barriers to Improve Near-Road Air Quality.” 321772. https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NRMRL&dirEntryId=321772&simpleSearch=1&searchAll=Recommendations+for+constructing+roadside+vegetation+barriers+to+improve+near+road+air+quality.Greenwald, Sarnat, and Fuller 2024[ECNMWXRZ] Greenwald, Roby, Jeremy A. Sarnat, and Christina H. Fuller. 2024. “The Impact of Vegetative and Solid Roadway Barriers on Particulate Matter Concentration in Urban Settings.” PLOS ONE 19 (1): e0296885. https://doi.org/10.1371/journal.pone.0296885. ) (see also Section 6.4: Air Particulate Migration). By volume, most TRWP are less than 100 µm ( Kreider et al. 2010[QCJY4JI9] Kreider, Marisa L., Julie M. Panko, Britt L. McAtee, Leonard I. Sweet, and Brent L. Finley. 2010. “Physical and Chemical Characterization of Tire-Related Particles: Comparison of Particles Generated Using Different Methodologies.” Science of the Total Environment 408 (3): 652–59. https://doi.org/10.1016/j.scitotenv.2009.10.016. ). TRWP have been observed in ambient air monitoring stations in the PM2.5 fraction ( Unice et al. 2019[TLVMH289] Unice, K.M., M.P. Weeber, M.M. Abramson, R.C.D. Reid, J.A.G. van Gils, A.A. Markus, A.D. Vethaak, and J.M. Panko. 2019. “Characterizing Export of Land-Based Microplastics to the Estuary — Part I: Application of Integrated Geospatial Microplastic Transport Models to Assess Tire and Road Wear Particles in the Seine Watershed.” Science of the Total Environment 646:1639–49. https://doi.org/10.1016/j.scitotenv.2018.07.368.Baensch-Baltruschat et al. 2020[SG7DEPVC] Baensch-Baltruschat, Beate, Birgit Kocher, Friederike Stock, and Georg Reifferscheid. 2020. “Tyre and Road Wear Particles (TRWP)—A Review of Generation, Properties, Emissions, Human Health Risk, Ecotoxicity, and Fate in the Environment.” Science of the Total Environment 733 (September):137823. https://doi.org/10.1016/j.scitotenv.2020.137823.Panko et al. 2019[H57M387X] Panko, Julie M., Kristen M. Hitchcock, Gary W. Fuller, and David Green. 2019. “Evaluation of Tire Wear Contribution to PM2.5 in Urban Environments.” Atmosphere 10 (2): 99. https://doi.org/10.3390/atmos10020099.Panko et al. 2013[UQNTNRM4] Panko, Julie M., Jennifer Chu, Marisa L. Kreider, and Ken M. Unice. 2013. “Measurement of Airborne Concentrations of Tire and Road Wear Particles in Urban and Rural Areas of France, Japan, and the United States.” Atmospheric Environment 72 (June):192–99. https://doi.org/10.1016/j.atmosenv.2013.01.040. ). Because TRWP (or TWP) are categorized as microplastics, they are also discussed in Section 2.2.2.1 of the ITRC Microplastics Guidance Document ( ITRC 2023[LLWEHVCX] ITRC. 2023. “Microplastics.” Washington, D.C.: Interstate Technology & Regulatory Council, MP Team. https://mp-1.itrcweb.org. ).
Specific surface area is a key indicator of potential for release and/or transformation of chemicals contained in environmental particles like 6PPD in TRWP ( Moran et al. 2023[9FSZ84KX] Moran, Kelly, Alicia Gilbreath, Miguel Mendez, Diana Lin, and Rebecca Sutton. 2023. “Tire Wear: Emissions Estimates and Market Insights to Inform Monitoring Design.” SFEI Technical Report SFEI Contribution #1049. Richmond, CA: San Francisco Estuary Institute. ). For example, copper has been shown to leach from vehicle brake-pad wear particles at a high rate compared to some high-copper reference materials, likely because the wear-particle surface area is more than 150 times greater than the powdered reference materials ( Hur, Yim, and Schlautman 2003[XN47T74F] Hur, Jin, Soobin Yim, and Mark A. Schlautman. 2003. “Copper Leaching from Brake Wear Debris in Standard Extraction Solutions. Electronic Supplementary Information (ESI) Available: Thermodynamic Equilibrium Speciation Results from the Geochemical Model MINTEQ. See Http://Www.Rsc.Org/Suppdata/Em/B3/B303820c/.” Journal of Environmental Monitoring 5 (5): 837. https://doi.org/10.1039/b303820c. ). The greater the surface area, the greater the potential for formation of transformation products, including 6PPD-q, and for chemical release from the particle into the environment. Scanning electron micrographs (such as Figure 1-6) and focused ion beam images of TRWP reveal rough, irregular surfaces, which suggest that TRWP may have high surface areas ( Kreider et al. 2010[QCJY4JI9] Kreider, Marisa L., Julie M. Panko, Britt L. McAtee, Leonard I. Sweet, and Brent L. Finley. 2010. “Physical and Chemical Characterization of Tire-Related Particles: Comparison of Particles Generated Using Different Methodologies.” Science of the Total Environment 408 (3): 652–59. https://doi.org/10.1016/j.scitotenv.2009.10.016.Milani et al. 2004[6N5I8G73] Milani, M., F.P. Pucillo, M. Ballerini, M. Camatini, M. Gualtieri, and S. Martino. 2004. “First Evidence of Tyre Debris Characterization at the Nanoscale by Focused Ion Beam.” Materials Characterization 52 (4–5): 283–88. https://doi.org/10.1016/j.matchar.2004.06.001. ). Surface area typically has an inverse correlation with particle size (that is, smaller particles typically have greater total surface area per unit mass). The absence of TRWP surface-area data means that the portion of the particle size distribution (and associated transport pathway) with the greatest potential to release tire-related chemicals, including 6PPD and 6PPD-q, into the environment is unknown.
Modeling in the Seattle area found that the correlation between the presence of 6PPD-q in stormwater and vehicle miles traveled in the area’s subwatersheds was slightly stronger than the correlation with subwatershed impervious area ( Feist et al. 2017[4PSDP2BG] Feist, Blake E., Eric R. Buhle, David H. Baldwin, Julann A. Spromberg, Steven E. Damm, Jay W. Davis, and Nathaniel L. Scholz. 2017. “Roads to Ruin: Conservation Threats to a Sentinel Species across an Urban Gradient.” Ecological Applications 27 (8): 2382–96. https://doi.org/https://doi.org/10.1002/eap.1615. ). If this correlation is observed in other locations, it would suggest that large tire particles, which deposit near roads, might release more chemicals into surface runoff than tire particles that are PM10 or smaller in diameter, which deposit throughout watersheds.
4.1 Water
4.1.1 Surface Water and Stormwater
Surface runoff (Table 4-1) and stormwater (Table 4-2) are presumed to be major transport pathways of TRWP, 6PPD, and 6PPD-q, which can then result in URMS ( McIntyre et al. 2021[MVL2LKBM] McIntyre, Jenifer K., Jasmine Prat, James Cameron, Jillian Wetzel, Emma Mudrock, Katherine T. Peter, Zhenyu Tian, et al. 2021. “Treading Water: Tire Wear Particle Leachate Recreates an Urban Runoff Mortality Syndrome in Coho but Not Chum Salmon.” Environmental Science & Technology 55 (17): 11767–74. https://doi.org/10.1021/acs.est.1c03569.McIntyre et al. 2023[F7NAIVJ4] McIntyre, Jenifer, Julann Spromberg, James Cameron, John P. Incardona, Jay W. Davis, and Nathaniel L. Scholz. 2023. “Bioretention Filtration Prevents Acute Mortality and Reduces Chronic Toxicity for Early Life Stage Coho Salmon (Oncorhynchus kisutch) Episodically Exposed to Urban Stormwater Runoff.” Science of the Total Environment 902 (December):165759. https://doi.org/10.1016/j.scitotenv.2023.165759.McIntyre et al. 2018[G7QW7PSD] McIntyre, Jenifer, Jessica Lundin, James Cameron, Michelle Chow, Jay Davis, John Incardona, and Nathaniel Scholz. 2018. “Interspecies Variation in the Susceptibility of Adult Pacific Salmon to Toxic Urban Stormwater Runoff.” Environmental Pollution 238:196–203. https://doi.org/https://doi.org/10.1016/j.envpol.2018.03.012.Hiki et al. 2021[WZF69GXC] Hiki, Kyoshiro, Kenta Asahina, Kota Kato, Takahiro Yamagishi, Ryo Omagari, Yuichi Iwasaki, Haruna Watanabe, and Hiroshi Yamamoto. 2021. “Acute Toxicity of a Tire Rubber–Derived Chemical, 6PPD Quinone, to Freshwater Fish and Crustacean Species.” Environmental Science & Technology Letters 8 (9): 779–84. https://doi.org/10.1021/acs.estlett.1c00453.Johannessen, Helm, and Metcalfe 2021[U9BWIDJ5] Johannessen, Cassandra, Paul Helm, and Chris D. Metcalfe. 2021. “Detection of Selected Tire Wear Compounds in Urban Receiving Waters.” Environmental Pollution 287 (October):117659. https://doi.org/10.1016/j.envpol.2021.117659.Tian et al. 2021[X8BRFG3P] Tian, Zhenyu, Haoqi Zhao, Katherine T. Peter, Melissa Gonzalez, Jill Wetzel, Christopher Wu, Ximin Hu, et al. 2021. “A Ubiquitous Tire Rubber–Derived Chemical Induces Acute Mortality in Coho Salmon.” Science 371 (6525): 185–89. https://doi.org/10.1126/science.abd6951.Seiwert et al. 2020[XRNTFZ69] Seiwert, Bettina, Philipp Klöckner, Stephan Wagner, and Thorsten Reemtsma. 2020. “Source-Related Smart Suspect Screening in the Aqueous Environment: Search for Tire-Derived Persistent and Mobile Trace Organic Contaminants in Surface Waters.” Analytical and Bioanalytical Chemistry 412 (20): 4909–19. https://doi.org/10.1007/s00216-020-02653-1.French et al. 2022[3GCU2L57] French, B. F., D. H. Baldwin, J. Cameron, J. Prat, K. King, J. W. Davis, J. K. McIntyre, and N. L. Scholz. 2022. “Urban Roadway Runoff Is Lethal to Juvenile Coho, Steelhead, and Chinook Salmonids, But Not Congeneric Sockeye.” Environmental Science & Technology Letters 9 (9): 733–38. https://doi.org/10.1021/acs.estlett.2c00467.Chow et al. 2019[7RMZ3UNQ] Chow, Michelle I., Jessica I. Lundin, Chelsea J. Mitchell, Jay W. Davis, Graham Young, Nathaniel L. Scholz, and Jenifer K. McIntyre. 2019. “An Urban Stormwater Runoff Mortality Syndrome in Juvenile Coho Salmon.” Aquatic Toxicology 214 (September):105231. https://doi.org/10.1016/j.aquatox.2019.105231.Du et al. 2017[56E8Y27X] Du, Bowen, Jonathan M. Lofton, Katherine T. Peter, Alexander D. Gipe, C. Andrew James, Jenifer K. McIntyre, Nathaniel L. Scholz, Joel E. Baker, and Edward P. Kolodziej. 2017. “Development of Suspect and Non-Target Screening Methods for Detection of Organic Contaminants in Highway Runoff and Fish Tissue with High-Resolution Time-of-Flight Mass Spectrometry.” Environmental Science: Processes & Impacts 19 (9): 1185–96. https://doi.org/10.1039/C7EM00243B.Scholz et al. 2011[S8ASEIXU] Scholz, Nathaniel L., Mark S. Myers, Sarah G. McCarthy, Jana S. Labenia, Jenifer K. McIntyre, Gina M. Ylitalo, Linda D. Rhodes, et al. 2011. “Recurrent Die-Offs of Adult Coho Salmon Returning to Spawn in Puget Sound Lowland Urban Streams.” PLOS ONE 6 (12): e28013. https://doi.org/10.1371/journal.pone.0028013.Peter et al. 2020[5CPFCBQT] Peter, Katherine T., Fan Hou, Zhenyu Tian, Christopher Wu, Matt Goehring, Fengmao Liu, and Edward P. Kolodziej. 2020. “More than a First Flush: Urban Creek Storm Hydrographs Demonstrate Broad Contaminant Pollutographs.” Environmental Science & Technology 54 (10): 6152–65. https://doi.org/10.1021/acs.est.0c00872.Spromberg et al. 2016[GI97QYN4] Spromberg, Julann A., David H. Baldwin, Steven E. Damm, Jenifer K. McIntyre, Michael Huff, Catherine A. Sloan, Bernadita F. Anulacion, Jay W. Davis, and Nathaniel L. Scholz. 2016. “Coho Salmon Spawner Mortality in Western US Urban Watersheds: Bioinfiltration Prevents Lethal Storm Water Impacts.” Journal of Applied Ecology 53 (2): 398–407. https://doi.org/https://doi.org/10.1111/1365-2664.12534. ). Rain and snow melt (Table 4-8) pick up and transport dissolved and particulate contaminants from impervious surfaces and deliver them to natural waterbodies (Table 4-1), stormwater treatment structures (Table 4-2), or WWTP (Table 4-3) ( Seiwert et al. 2022[QDRRVWMW] Seiwert, Bettina, Maolida Nihemaiti, Mareva Troussier, Steffen Weyrauch, and Thorsten Reemtsma. 2022. “Abiotic Oxidative Transformation of 6-PPD and 6-PPD Quinone from Tires and Occurrence of Their Products in Snow from Urban Roads and in Municipal Wastewater.” Water Research 212:118122. https://doi.org/10.1016/j.watres.2022.118122.Challis et al. 2021[T8TEWPCL] Challis, J. K., H. Popick, S. Prajapati, P. Harder, J. P. Giesy, K. McPhedran, and M. Brinkmann. 2021. “Occurrences of Tire Rubber–Derived Contaminants in Cold-Climate Urban Runoff.” Environmental Science & Technology Letters 8 (11): 961–67. https://doi.org/10.1021/acs.estlett.1c00682. ). Throughout the United States, MS4 are geographically more common, where stormwater is discharged to natural water bodies or stormwater treatment structures. In combined sewer systems, TRWP and associated chemicals are transported to WWTP under normal conditions but may be discharged to natural waterbodies under excessive runoff conditions as combined sewer overflow. In regard to the persistence of 6PPD-q, Foscari et al. ( Foscari et al. 2024[JBUK7E3P] Foscari, Aurelio, Bettina Seiwert, Daniel Zahn, Matthias Schmidt, and Thorsten Reemtsma. 2024. “Leaching of Tire Particles and Simultaneous Biodegradation of Leachables.” Water Research 253 (April):121322. https://doi.org/10.1016/j.watres.2024.121322. ) demonstrated in lab experiments the biodegradation of 6PPD-q and a corresponding concentration decrease when TRWP were not present, while 6PPD-q concentrations are relatively stable or may even increase when particles are present. The development of methods to accurately measure 6PPD and 6PPD-q in surface water (see Section 5.1.5: Sampling 6PPD-q in Water) will help address the fate, transport, and occurrence data gaps to inform management actions (see Section 8.2).
Occurrence in Water
- Surface runoff and stormwater are major mechanisms for transporting TRWP, 6PPD and 6PPD‑q to receiving surface water.
- More studies are needed to understand how environmental, landscape, and stormwater characteristics effect the fate and transport of TRWP, 6PPD, and 6PPD‑q.
- More studies are needed to understand what stormwater, wastewater and drinking water treatment technologies are most effective at preventing the transport of TRWP, 6PPD, and 6PPD‑q.
- More studies are needed to understand if 6PPD or 6PPD‑q are transported by surface water and groundwater.
6PPD and 6PPD-q can be released directly from tires and readily bind to particulates ( Hu et al. 2023[BFCN5BLS] Hu, Ximin, Haoqi (Nina) Zhao, Zhenyu Tian, Katherine T. Peter, Michael C. Dodd, and Edward P. Kolodziej. 2023. “Chemical Characteristics, Leaching, and Stability of the Ubiquitous Tire Rubber–Derived Toxicant 6PPD-Quinone.” Environmental Science: Processes & Impacts 25 (5): 901–11. https://doi.org/10.1039/D3EM00047H. ); these particulates are transported by stormwater ( Seiwert et al. 2022[QDRRVWMW] Seiwert, Bettina, Maolida Nihemaiti, Mareva Troussier, Steffen Weyrauch, and Thorsten Reemtsma. 2022. “Abiotic Oxidative Transformation of 6-PPD and 6-PPD Quinone from Tires and Occurrence of Their Products in Snow from Urban Roads and in Municipal Wastewater.” Water Research 212:118122. https://doi.org/10.1016/j.watres.2022.118122. ). 6PPD and 6PPD-q can also be released from TRWP that are generated during driving, deposited along roadways, and transported by stormwater to waterbodies or treatment facilities. Detected environmental concentrations of 6PPD and 6PPD-q have been highest in urban runoff ( Zhu et al. 2024[WEPL88BC] Zhu, Jianqiang, Ruyue Guo, Shengtao Jiang, Pengfei Wu, and Hangbiao Jin. 2024. “Occurrence of p-Phenylenediamine Antioxidants (PPDs) and PPDs-Derived Quinones in Indoor Dust.” Science of the Total Environment 912:169325. https://doi.org/10.1016/j.scitotenv.2023.169325.Tian et al. 2021[X8BRFG3P] Tian, Zhenyu, Haoqi Zhao, Katherine T. Peter, Melissa Gonzalez, Jill Wetzel, Christopher Wu, Ximin Hu, et al. 2021. “A Ubiquitous Tire Rubber–Derived Chemical Induces Acute Mortality in Coho Salmon.” Science 371 (6525): 185–89. https://doi.org/10.1126/science.abd6951.Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376.Zhang et al. 2023[JWRBWTKN] Zhang, Hai-Yan, Zheng Huang, Yue-Hong Liu, Li-Xin Hu, Liang-Ying He, You-Sheng Liu, Jian-Liang Zhao, and Guang-Guo Ying. 2023. “Occurrence and Risks of 23 Tire Additives and Their Transformation Products in an Urban Water System.” Environment International 171 (January):107715. https://doi.org/10.1016/j.envint.2022.107715. ). Studies that measure and compare dissolved and suspended fractions of 6PPD and 6PPD-q in water are needed to understand the fate and transport of these contaminants’ pathways from impervious surfaces to natural waterbodies.
Potential aquatic ecological receptors, which include freshwater and marine organisms (vertebrates, invertebrates, and plants), may be exposed to 6PPD-q and 6PPD through direct uptake of water through respiratory surfaces, ingestion, and absorption. The route of exposure may vary among species or life stage. People who engage in subsistence or recreational activities such as swimming, fishing, or boating may be exposed to 6PPD and 6PPD-q through incidental ingestion of and dermal contact with water contaminated by surface runoff. Human exposure to 6PPD and 6PPD-q during water-based activities is an emerging area of concern that also needs further study.
Waterbodies located near roadways and used as sources for drinking water may be vulnerable to environmental contamination with 6PPD and 6PPD-q. Surface water in streams and lakes that are drinking water sources may be affected by surface runoff and stormwater or, depending on the setting, effluent from WWTP. Two published studies report analyses of drinking water source samples connected to surface water, one in China and the other in the greater Toronto area in Canada (Table 4-3). As of May 2024, the ITRC Tire Anti-degradants team did not identify reports or studies for 6PPD and 6PPD-q detection in finished drinking water in the United States.
In China, H.-Y. Zhang et al. ( Zhang et al. 2023[JWRBWTKN] Zhang, Hai-Yan, Zheng Huang, Yue-Hong Liu, Li-Xin Hu, Liang-Ying He, You-Sheng Liu, Jian-Liang Zhao, and Guang-Guo Ying. 2023. “Occurrence and Risks of 23 Tire Additives and Their Transformation Products in an Urban Water System.” Environment International 171 (January):107715. https://doi.org/10.1016/j.envint.2022.107715. ) detected 6PPD in 30%–48% of filtered river source water samples and 6PPD-q in 100% of samples. Concentrations were in the low ng/L range, and 6PPD-q concentrations were higher than the parent chemical 6PPD. Within the drinking water treatment plant (DWTP), neither chemical was detected in samples drawn at each of six treatment stages. A Canadian study sampled for 6PPD-q in four WWTP and two DWTPs which use Lake Ontario as the source water ( Johannessen and Metcalfe 2022[6AEMVTD8] Johannessen, Cassandra, and Chris D. Metcalfe. 2022. “The Occurrence of Tire Wear Compounds and Their Transformation Products in Municipal Wastewater and Drinking Water Treatment Plants.” Environmental Monitoring and Assessment 194 (10): 731. https://doi.org/10.1007/s10661-022-10450-9. ). 6PPD-q was detected in WWTP influent and effluent but not in drinking water (pre- or post-treatment).
4.1.2 Groundwater
There is little information regarding the transport of 6PPD and 6PPD-q from surface water to groundwater (Table 4-4). Groundwater contamination by 6PPD and 6PPD-q, along with other PPD chemicals, was reported in a shallow aquifer in China ( Zhang et al. 2023[D6T6D4JP] Zhang, Ruiling, Shizhen Zhao, Xin Liu, Lele Tian, Yangzhi Mo, Xin Yi, Shiyang Liu, Jiaqi Liu, Jun Li, and Gan Zhang. 2023. “Aquatic Environmental Fates and Risks of Benzotriazoles, Benzothiazoles, and p-Phenylenediamines in a Catchment Providing Water to a Megacity of China.” Environmental Research 216 (January):114721. https://doi.org/10.1016/j.envres.2022.114721. ). Samples from civil wells and household water directly connected to groundwater were analyzed as proxies for groundwater samples. The authors describe the hydrogeology of the aquifer as unconfined and highly permeable to the nearby river water. An important data gap in the United States is whether groundwater that serves as drinking water sources could be vulnerable to a similar contamination pathway. The transport potential will ultimately depend on the organic content and soil type present. More research is needed to evaluate the assumptions that 6PPD and 6PPD-q stay bound to particulates and do not readily move through soil ( Cunningham and Schalk 2011[N2W34PYI] Cunningham, W.L., and C.W. Schalk. 2011. “Groundwater Technical Procedures of the U.S. Geological Survey: U.S. Geological Survey Techniques and Methods 1–A1.” https://pubs.usgs.gov/tm/1a1/. ).
4.1.3 Tap Water
One study that analyzed drinking water at the point of exposure (tap water) was identified during preparation of the current document (Table 4-3). In that study, 6PPD was detected in 25% of drinking water samples collected from 20 buildings in Singapore, while 6PPD-q was not detected ( Marques dos Santos and Snyder 2023[GEI8HFLB] Marques dos Santos, Mauricius, and Shane Allen Snyder. 2023. “Occurrence of Polymer Additives 1,3-Diphenylguanidine (DPG), N-(1,3-Dimethylbutyl)-N′-Phenyl-1,4-Benzenediamine (6PPD), and Chlorinated Byproducts in Drinking Water: Contribution from Plumbing Polymer Materials.” Environmental Science & Technology Letters, September. https://doi.org/10.1021/acs.estlett.3c00446. ). The source of 6PPD in the drinking water samples was not identified but may be due to leaching of the chemical from plumbing components (for example, rubber gaskets, O-rings). The original sources of the drinking water sampled in the study were not identified as surface water, groundwater, or otherwise. More research is needed to understand potential exposures from drinking water broadly, including water at the point of use.
4.1.4 Wastewater and Biosolids
In some cities in the United States, stormwater is diverted to WWTP through combined sewer systems. Studies investigating 6PPD-q removal in WWTP have had mixed results (Table 4-3). Several studies showed a strong reduction or removal of 6PPD-q to nondetect levels in water ( Seiwert et al. 2022[QDRRVWMW] Seiwert, Bettina, Maolida Nihemaiti, Mareva Troussier, Steffen Weyrauch, and Thorsten Reemtsma. 2022. “Abiotic Oxidative Transformation of 6-PPD and 6-PPD Quinone from Tires and Occurrence of Their Products in Snow from Urban Roads and in Municipal Wastewater.” Water Research 212:118122. https://doi.org/10.1016/j.watres.2022.118122.Maurer et al. 2023[TJQR62IC] Maurer, Loïc, Eric Carmona, Oliver Machate, Tobias Schulze, Martin Krauss, and Werner Brack. 2023. “Contamination Pattern and Risk Assessment of Polar Compounds in Snow Melt: An Integrative Proxy of Road Runoffs.” Environmental Science & Technology 57 (10): 4143–52. https://doi.org/10.1021/acs.est.2c05784.Zhang et al. 2023[JWRBWTKN] Zhang, Hai-Yan, Zheng Huang, Yue-Hong Liu, Li-Xin Hu, Liang-Ying He, You-Sheng Liu, Jian-Liang Zhao, and Guang-Guo Ying. 2023. “Occurrence and Risks of 23 Tire Additives and Their Transformation Products in an Urban Water System.” Environment International 171 (January):107715. https://doi.org/10.1016/j.envint.2022.107715. ), and another study showed an increased mass of 6PPD-q in the effluent from the WWTP ( Johannessen and Metcalfe 2022[6AEMVTD8] Johannessen, Cassandra, and Chris D. Metcalfe. 2022. “The Occurrence of Tire Wear Compounds and Their Transformation Products in Municipal Wastewater and Drinking Water Treatment Plants.” Environmental Monitoring and Assessment 194 (10): 731. https://doi.org/10.1007/s10661-022-10450-9. ). More research is needed to follow up on this.
Biosolids are a necessary byproduct of our WWTP. WWTP may receive TRWP, 6PPD, and 6PPD-q from upstream sources, and as such they can end up in biosolids. Both 6PPD and 6PPD-q were detected in 100% of biosolid samples from WWTP in Hong Kong ( Cao et al. 2023[D5FPK9YB] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Han Qiao, Huankai Li, Gefei Huang, Zhu Yang, and Zongwei Cai. 2023. “Occurrence and Fate of Substituted P-Phenylenediamine-Derived Quinones in Hong Kong Wastewater Treatment Plants.” Environmental Science & Technology, October. https://doi.org/10.1021/acs.est.3c03758. ), though more research is needed to characterize these occurrences in other localities. 6PPD-q has been detected in biosolids from a WWTP in Irvine, California (35.3±2.9 and 18.8±1.5 µg/kg, n=2) ( Dennis, Braun, and Gan 2024[RNAS6357] Dennis, Nicole M., Audrey J. Braun, and Jay Gan. 2024. “A High-Throughput Analytical Method for Complex Contaminant Mixtures in Biosolids.” Environmental Pollution 345:123517. https://doi.org/10.1016/j.envpol.2024.123517. ).6
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-1 | Surface Water | |
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| 4-2 | Stormwater | |
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| 4-3 | Wastewater, water treatment plants, and tap water | |
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| 4-4 | Groundwater | |
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Note: PDF versions of each table are provided for the reader to view information in a visual format that is consistent across browsers and platforms. Executable files are provided to allow readers to sort information by the column of their choice, but may appear different visually depending on the software used to view this file. Instructions on how to sort information in a document formatted for a word processor format are widely available by internet search.
4.2 Soil
Modeled and measured physicochemical characteristics of 6PPD and 6PPD-q suggest that these compounds readily bind to soil and organics ( Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376.OSPAR Commission 2006[SVMKJM7X] OSPAR Commission. 2006. “Hazardous Substances Series 4-(Dimethylbutylamino)Diphenylamine (6PPD) 2005 (2006 Update).” Publication Number: 271/2006. https://www.ospar.org/documents?v=7029. ); however, occurrence data are limited and more studies are needed (Table 4-5). Sampling protocols for measuring and understanding the occurrence and persistence of tire contaminants in soils along roadways are needed to address data gaps. Biodegradation of 6PPD and 6PPD-q in soil has been observed ( Xu et al. 2023[4P2E4JLJ] Xu, Qiao, Gang Li, Li Fang, Qian Sun, Ruixia Han, Zhe Zhu, and Yong-Guan Zhu. 2023. “Enhanced Formation of 6PPD-Q during the Aging of Tire Wear Particles in Anaerobic Flooded Soils: The Role of Iron Reduction and Environmentally Persistent Free Radicals.” Environmental Science & Technology, March. https://doi.org/10.1021/acs.est.2c08672. ). Additional research is needed to understand other degradation pathways and overall stability in soil.
Exposure in terrestrial organisms (vertebrates, invertebrates, and plants) is poorly characterized, but it is possible that terrestrial receptors may be exposed via ingestion, inhalation, or absorption. The primary pathways through which humans may be exposed to 6PPD and 6PPD-q in soil are ingestion and dermal contact. Incidental ingestion can occur when people come into direct contact with contaminated soil and engage in hand-to-mouth behaviors.
An exposure assessment conducted by Cao et al. ( Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376. ), using concentrations of total PPD compounds and their quinone transformation products in roadside soil, surface runoff, and air particles in Hong Kong, estimated that ingestion of roadside soil could be the primary contributor of human exposure to PPDs and PPDquinones, followed by dermal contact, and then inhalation of ambient air particulate. The relative importance of the exposure pathway reflected the lower concentrations of PPD chemicals and their transformation products in ambient air particulate as compared to concentrations in roadside soil and roadway runoff samples ( Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376. ). It is unknown whether these exposure pathway trends are applicable to people in the United States. Additional research is needed to understand the potential human health impacts from soil exposure pathways.
Beneficial use of biosolids, or land application, returns nutrients to the soil in place of commercial fertilizers. In California, 6PPD-q was detected in biosolids from a WWTP (see Section 4.1.4: Wastewater and Biosolids). More research is needed to understand the fate, transport, and exposure risk of 6PPD-q in biosolids used for agriculture and landscaping.
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-5 | Roadside soil | |
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Note: The PDF version of this table is provided for the reader to view information in a visual format that is consistent across browsers and platforms. The executable file is provided to allow readers to sort information by the column of their choice, but may appear different visually depending on the software used to view this file. Instructions on how to sort information in a document formatted for a word processor format are widely available by internet search.
4.3 Sediment
Occurrence in Sediment
- Tire, road, and soil particles are transported by stormwater and surface water. The allocation between what stays suspended in water and what is deposited in the sediments is unknown.
- Standardized methods for measuring small TRWP in water and sediments are challenging. 6PPD q may provide a proxy for tire-derived microplastics that could represent a continued source of 6PPD and 6PPD q.
- More studies are needed to understand the fate and transport of TRWP in sediments including deposition, composition, biodegradation, and transformation processes.
In hydrology practices, sediment often refers to the benthic media at the bottoms of streams, rivers, estuaries, oceans, and lakes. In stormwater practices, the dirt and debris transported from impervious surfaces to stormwater catchments is referred to as sediment as well. In the San Francisco Bay Area alone, an estimated 0.3 to 2.4 million kg of TRWP wash off roads and parking lots into stormwater systems and enter San Francisco Bay via small tributaries annually ( Moran et al. 2023[9FSZ84KX] Moran, Kelly, Alicia Gilbreath, Miguel Mendez, Diana Lin, and Rebecca Sutton. 2023. “Tire Wear: Emissions Estimates and Market Insights to Inform Monitoring Design.” SFEI Technical Report SFEI Contribution #1049. Richmond, CA: San Francisco Estuary Institute. ). These estimates do not include contributions from San Francisco’s combined sewer system or from California’s Central Valley. Other urban areas likely have similar wash-off rates. As with soil, 6PPD and 6PPD-q will readily bind to sediment instead of the water phase; however, occurrence data are limited, and more studies are needed (Table 4-6). Studies conducted in the Jiaojiang River and the Pearl River Delta and Estuary in China found both 6PPD and 6PPD-q as the most dominant PPD and PPDquinones quantified in a large-scale survey of urban rivers, estuaries, coasts, and deep-sea sediments ( Zhu et al. 2024[3FETIQAB] Zhu, Jianqiang, Ruyue Guo, Fangfang Ren, Shengtao Jiang, and Hangbiao Jin. 2024. “Occurrence and Partitioning of p-Phenylenediamine Antioxidants and Their Quinone Derivatives in Water and Sediment.” Science of the Total Environment 914 (March):170046. https://doi.org/10.1016/j.scitotenv.2024.170046.Zeng et al. 2023[TK5YR8WJ] Zeng, Lixi, Yi Li, Yuxin Sun, Liang-Ying Liu, Mingjie Shen, and Bibai Du. 2023. “Widespread Occurrence and Transport of p-Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions.” Environmental Science & Technology, January, acs.est.2c07652. https://doi.org/10.1021/acs.est.2c07652. ). The concentration of 6PPD and 6PPD-q decreased with distance from the urban areas. The detection of 6PPD may suggest TRWP are a source and/or that the half-life of 6PPD varies between air and water, environmental conditions, and chemical phases (dissolved and suspended fractionation) ( Zhu et al. 2024[3FETIQAB] Zhu, Jianqiang, Ruyue Guo, Fangfang Ren, Shengtao Jiang, and Hangbiao Jin. 2024. “Occurrence and Partitioning of p-Phenylenediamine Antioxidants and Their Quinone Derivatives in Water and Sediment.” Science of the Total Environment 914 (March):170046. https://doi.org/10.1016/j.scitotenv.2024.170046.Zeng et al. 2023[TK5YR8WJ] Zeng, Lixi, Yi Li, Yuxin Sun, Liang-Ying Liu, Mingjie Shen, and Bibai Du. 2023. “Widespread Occurrence and Transport of p-Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions.” Environmental Science & Technology, January, acs.est.2c07652. https://doi.org/10.1021/acs.est.2c07652. ). More studies are needed to continue understanding the variability in partitioning and other physicochemical characteristics. Klöckner, Seiwert, Wagner, et al. ( Klöckner et al. 2021[Y49MVKMM] Klöckner, Philipp, Bettina Seiwert, Stephan Wagner, and Thorsten Reemtsma. 2021. “Organic Markers of Tire and Road Wear Particles in Sediments and Soils: Transformation Products of Major Antiozonants as Promising Candidates.” Environmental Science & Technology 55 (17): 11723–32. https://doi.org/10.1021/acs.est.1c02723. ) suggested using organic markers (specifically, the 6PPD transformation products N-formyl-6-PPD, hydroxylated N-1,3-dimethylbutyl-N-phenyl quinone diimine, and 6PPD-q) to measure TRWP. For 6PPD, a greater fraction of release was found in sediment compared to water following aging and biodegradation processes; however, more studies are needed to estimate leaching rates ( Unice et al. 2015[NS59BGK2] Unice, K.M., Jennifer Bare, Marisa Kreider, and Julie Panko. 2015. “Experimental Methodology for Assessing the Environmental Fate of Organic Chemicals in Polymer Matrices Using Column Leaching Studies and OECD 308 Water/Sediment Systems: Application to Tire and Road Wear Particles.” Science of the Total Environment 533 (July):476–87. https://doi.org/10.1016/j.scitotenv.2015.06.053.Xu et al. 2023[4P2E4JLJ] Xu, Qiao, Gang Li, Li Fang, Qian Sun, Ruixia Han, Zhe Zhu, and Yong-Guan Zhu. 2023. “Enhanced Formation of 6PPD-Q during the Aging of Tire Wear Particles in Anaerobic Flooded Soils: The Role of Iron Reduction and Environmentally Persistent Free Radicals.” Environmental Science & Technology, March. https://doi.org/10.1021/acs.est.2c08672. ). Anaerobic sediment conditions have been shown to produce more 6PPD-q ( Xu et al. 2023[4P2E4JLJ] Xu, Qiao, Gang Li, Li Fang, Qian Sun, Ruixia Han, Zhe Zhu, and Yong-Guan Zhu. 2023. “Enhanced Formation of 6PPD-Q during the Aging of Tire Wear Particles in Anaerobic Flooded Soils: The Role of Iron Reduction and Environmentally Persistent Free Radicals.” Environmental Science & Technology, March. https://doi.org/10.1021/acs.est.2c08672. ).
As with soil, the primary pathways through which humans may be exposed to 6PPD and 6PPD-q in sediment are also ingestion and dermal contact. Sediment disturbance due to human activities such as wading and swimming can resuspend sediment particles in the water column, making them available for dermal contact or incidental ingestion. Incidental ingestion can also occur when people come into direct contact with contaminated sediment and engage in hand-to-mouth behaviors. It is unknown whether skin absorption of the chemicals from these particles is possible. Additional research is needed to understand the potential human health impacts from sediment exposure pathways. Likewise, more research is needed to understand the ecological risks associated with TRWP, 6PPD, and 6PPD-q in sediments.
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-6 | Sediment | |
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Note: The PDF version of this table is provided for the reader to view information in a visual format that is consistent across browsers and platforms. The executable file is provided to allow readers to sort information by the column of their choice, but may appear different visually depending on the software used to view this file. Instructions on how to sort information in a document formatted for a word processor format are widely available by internet search.
4.4 Air
Over time, as tires wear down, particles containing 6PPD and 6PPD-q are released into the environment (outdoor air), and present different potential exposure pathways. Additionally, these particles could infiltrate indoor environments, settling as dust on various surfaces. 6PPD and 6PPD-q have been detected in particulate matter in outdoor air; dusts collected from roads, tunnels, and paved parking; and settled indoor dusts. To date, much, but not all, of the data on 6PPD and 6PPD-q in air and dust were collected in China. This section reviews airborne TRWP and associated particle size, dust, 6PPD, and 6PPD-q.
TRWP, 6PPD, and 6PPD-q Occurrence in Air
- 6PPD and 6PPD-q have been observed in outdoor ambient air and fine particulate matter.
- 6PPD and 6PPD-q have been observed in dust along roads and highways, parking lots and garages, rubber playgrounds, recycling facilities, and homes.
- Tire dust has been observed in snow along roadways.
- More research is needed to understand the airborne exposure pathways for tire dust and related chemicals to humans and terrestrial and aquatic ecosystems.
In laboratory simulations of tire wear, TWPs are generated in multiple size fractions, including below 10 µm in diameter, with a large fraction of the total number of particles emitted below 0.1 µm, known as ultrafine particulate matter or PM0.1 ( Dahl et al. 2006[PTHCYGGU] Dahl, Andreas, Arash Gharibi, Erik Swietlicki, Anders Gudmundsson, Mats Bohgard, Anders Ljungman, Göran Blomqvist, and Mats Gustafsson. 2006. “Traffic-Generated Emissions of Ultrafine Particles from Pavement–Tire Interface.” Atmospheric Environment 40 (7): 1314–23. https://doi.org/10.1016/j.atmosenv.2005.10.029.Park, Kim, and Lee 2018[HIY3Z76P] Park, Inyong, Hongsuk Kim, and Seokhwan Lee. 2018. “Characteristics of Tire Wear Particles Generated in a Laboratory Simulation of Tire/Road Contact Conditions.” Journal of Aerosol Science 124 (October):30–40. https://doi.org/10.1016/j.jaerosci.2018.07.005. ). PM0.1 can readily be inhaled and can pass directly into the body. Particles, including TRWP, less than 10 µm in diameter are generally recognized to be respirable; the smaller they are, the deeper they can penetrate the lungs. The term “dust” can comprise different particle types and size fractions. We use the term here in the context of human health to indicate particles that are greater than 10 µm in diameter and therefore not respirable into the deep lung.
4.4.1 Outdoor Air
In outdoor air, 6PPD has been observed in respirable fine particulate matter (PM2.5), while 6PPD-q has been found in both outdoor ambient air and in the PM2.5 fraction (Table 4-7). In PM2.5, 6PPD has been observed at concentrations ranging from about 0.02–9,340 pg/m3 ( Zhang et al. 2022[G77DTKD6] Zhang, Yanhao, Caihong Xu, Wenfen Zhang, Zenghua Qi, Yuanyuan Song, Lin Zhu, Chuan Dong, Jianmin Chen, and Zongwei Cai. 2022. “p‑Phenylenediamine Antioxidants in PM2.5: The Underestimated Urban Air Pollutants.” Environmental Science & Technology 56 (11): 6914–21. https://doi.org/https://doi.org/10.1021/acs.est.1c04500.Wang et al. 2022[TMV9VLRS] Wang, Wei, Guodong Cao, Jing Zhang, Pengfei Wu, Yanyan Chen, Zhifeng Chen, Zenghua Qi, Ruijin Li, Chuan Dong, and Zongwei Cai. 2022. “Beyond Substituted p-Phenylenediamine Antioxidants: Prevalence of Their Quinone Derivatives in PM2.5.” Environmental Science & Technology, July, acs.est.2c02463. https://doi.org/10.1021/acs.est.2c02463.Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376. ). 6PPD has also been observed at concentrations ranging from less than 0.02 to 0.41 pg/m3 and detected in 70% of ambient air samples (particle size not specified) from Chicago; 6PPD-q was not measured in this study ( Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114. ). 6PPD was not detected in ambient air during a three-month study where passive air samplers were deployed across 18 major cities that compose the Global Atmospheric Passive Sampling (GAPS) Network ( Johannessen et al. 2022[RYBDCBV4] Johannessen, Cassandra, Amandeep Saini, Xianming Zhang, and Tom Harner. 2022. “Air Monitoring of Tire-Derived Chemicals in Global Megacities Using Passive Samplers.” Environmental Pollution 314 (December):120206. https://doi.org/10.1016/j.envpol.2022.120206. ). 6PPD-q has been observed at concentrations ranging from about 0.1–7,250 pg/m3 in PM2.5 ( Zhang et al. 2022[G77DTKD6] Zhang, Yanhao, Caihong Xu, Wenfen Zhang, Zenghua Qi, Yuanyuan Song, Lin Zhu, Chuan Dong, Jianmin Chen, and Zongwei Cai. 2022. “p‑Phenylenediamine Antioxidants in PM2.5: The Underestimated Urban Air Pollutants.” Environmental Science & Technology 56 (11): 6914–21. https://doi.org/https://doi.org/10.1021/acs.est.1c04500.Wang et al. 2022[TMV9VLRS] Wang, Wei, Guodong Cao, Jing Zhang, Pengfei Wu, Yanyan Chen, Zhifeng Chen, Zenghua Qi, Ruijin Li, Chuan Dong, and Zongwei Cai. 2022. “Beyond Substituted p-Phenylenediamine Antioxidants: Prevalence of Their Quinone Derivatives in PM2.5.” Environmental Science & Technology, July, acs.est.2c02463. https://doi.org/10.1021/acs.est.2c02463.Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376. ) and 0.17–1.75 pg/m3 in ambient air ( Johannessen et al. 2022[RYBDCBV4] Johannessen, Cassandra, Amandeep Saini, Xianming Zhang, and Tom Harner. 2022. “Air Monitoring of Tire-Derived Chemicals in Global Megacities Using Passive Samplers.” Environmental Pollution 314 (December):120206. https://doi.org/10.1016/j.envpol.2022.120206. ). High detection frequencies of 6PPD and 6PPD-q in urban PM2.5 indicate a widespread prevalence, but some observed variability suggests that seasonal, geographic, and economic conditions may impact 6PPD and 6PPD-q occurrence in urban air ( Zhang et al. 2022[G77DTKD6] Zhang, Yanhao, Caihong Xu, Wenfen Zhang, Zenghua Qi, Yuanyuan Song, Lin Zhu, Chuan Dong, Jianmin Chen, and Zongwei Cai. 2022. “p‑Phenylenediamine Antioxidants in PM2.5: The Underestimated Urban Air Pollutants.” Environmental Science & Technology 56 (11): 6914–21. https://doi.org/https://doi.org/10.1021/acs.est.1c04500.Wang et al. 2022[TMV9VLRS] Wang, Wei, Guodong Cao, Jing Zhang, Pengfei Wu, Yanyan Chen, Zhifeng Chen, Zenghua Qi, Ruijin Li, Chuan Dong, and Zongwei Cai. 2022. “Beyond Substituted p-Phenylenediamine Antioxidants: Prevalence of Their Quinone Derivatives in PM2.5.” Environmental Science & Technology, July, acs.est.2c02463. https://doi.org/10.1021/acs.est.2c02463.Cao et al. 2022[VBAMJHA7] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Xingchen Zhao, Zhu Yang, Di Hu, and Zongwei Cai. 2022. “New Evidence of Rubber-Derived Quinones in Water, Air, and Soil.” Environmental Science & Technology 56 (7): 4142–50. https://doi.org/10.1021/acs.est.1c07376. ). Y. Zhang et al. ( Zhang et al. 2022[G77DTKD6] Zhang, Yanhao, Caihong Xu, Wenfen Zhang, Zenghua Qi, Yuanyuan Song, Lin Zhu, Chuan Dong, Jianmin Chen, and Zongwei Cai. 2022. “p‑Phenylenediamine Antioxidants in PM2.5: The Underestimated Urban Air Pollutants.” Environmental Science & Technology 56 (11): 6914–21. https://doi.org/https://doi.org/10.1021/acs.est.1c04500. ) used standard exposure assumptions to compute average daily inhalation intakes for residents of each city from the measured concentrations in PM2.5. Estimated daily inhalation intakes ranged from 0.3–7.2 pg/day for 6PPD and 2.2–18 pg/day for 6PPD-q.
There is some indication that compounds associated with tire particles are more highly associated with coarse particulate matter, though this association was not measured specifically for 6PPD or 6PPD-q ( Zhang et al. 2022[GHLGNCHV] Zhang, Ying-Jie, Ting-Ting Xu, Dong-Min Ye, Ze-Zhao Lin, Fei Wang, and Ying Guo. 2022. “Widespread N‑(1,3-Dimethylbutyl)‑N′‑phenyl‑p‑phenylenediamine Quinone in Size-Fractioned Atmospheric Particles and Dust of Different Indoor Environments.” Environmental Science & Technology Letters 9 (5): 420–25. https://doi.org/https://doi.org/10.1021/acs.estlett.2c00193.Wang et al. 2023[MVLEXF2I] Wang, Xiaoliang, Steven Gronstal, Brenda Lopez, Heejung Jung, L.-W. Antony Chen, Guoyuan Wu, Steven Sai Hang Ho, et al. 2023. “Evidence of Non-Tailpipe Emission Contributions to PM2.5 and PM10 near Southern California Highways.” Environmental Pollution 317:120691. https://doi.org/10.1016/j.envpol.2022.120691. ). More investigation is needed to determine potential human exposure to 6PPD and 6PPD-q due to coarse particulate matter.
TRWP are regularly found in PM2.5 air monitoring stations along roadways ( USEPA 2023[7MLIS2WE] USEPA. 2023. “Air Data: Air Quality Data Collected at Outdoor Monitors Across the US.” Collections and Lists. November 9, 2023. https://www.epa.gov/outdoor-air-quality-data. ). Studies have demonstrated the transport of TRWP by resuspension and deposition along roadways ( Mayer et al. 2024[ZTAVFS9G] Mayer, Paul, Kelly Moran, Ezra Miller, Susanne Brander, Stacey Harper, Manuel Garcia-Jaramillo, Victor Carrasco-Navarro, et al. 2024. “Where the Rubber Meets the Road: Emerging Environmental Impacts of Tire Wear Particles and Their Chemical Cocktails.” Science of The Total Environment 927 (June):171153. https://doi.org/10.1016/j.scitotenv.2024.171153. ). Air monitoring is needed along transportation corridors and in near-road environments to estimate the mass loading, chemical composition, and transport of TRWP nonexhaust emissions and volatile chemicals from tires.
Measurements of 6PPD and 6PPD-q in respirable particulate matter samples of indoor air were not identified by the ITRC Tire Anti-degradants team during the preparation of this document. The relevance of exposure through indoor air remains a data gap.
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-7 | Outdoor air | |
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4.4.2 Road Dust and Roadside Snow
6PPD and 6PPD-q have been observed along roads (Table 4-5 and Table 4-8), including urban roads, a parking garage, and in roadside snow ( Deng et al. 2022[Y9G3IQTU] Deng, Chengliang, Jialing Huang, Yunqing Qi, Da Chen, and Wei Huang. 2022. “Distribution Patterns of Rubber Tire–Related Chemicals with Particle Size in Road and Indoor Parking Lot Dust.” Science of the Total Environment 844:157144. https://doi.org/10.1016/j.scitotenv.2022.157144.Huang et al. 2021[EZEWIV8E] Huang, Wei, Yumeng Shi, Jialing Huang, Chengliang Deng, Shuqin Tang, Xiaotu Liu, and Da Chen. 2021. “Occurrence of Substituted p-Phenylenediamine Antioxidants in Dusts.” Environmental Science & Technology Letters 8 (5): 381–85. https://doi.org/10.1021/acs.estlett.1c00148.Hiki and Yamamoto 2022[VQ3M4AFW] Hiki, Kyoshiro, and Hiroshi Yamamoto. 2022. “Concentration and Leachability of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine (6PPD) and Its Quinone Transformation Product (6PPD-Q) in Road Dust Collected in Tokyo, Japan.” Environmental Pollution 302 (June):119082. https://doi.org/10.1016/j.envpol.2022.119082.Jin et al. 2023[P9WXQJUR] Jin, Ruihe, Yan Wu, Qun He, Pei Sun, Qiqing Chen, Chunjie Xia, Ye Huang, Jing Yang, and Min Liu. 2023. “Ubiquity of Amino Accelerators and Antioxidants in Road Dust from Multiple Land Types: Targeted and Nontargeted Analysis.” Environmental Science & Technology 57 (28): 10361–72. https://doi.org/10.1021/acs.est.3c01448.Maurer et al. 2023[TJQR62IC] Maurer, Loïc, Eric Carmona, Oliver Machate, Tobias Schulze, Martin Krauss, and Werner Brack. 2023. “Contamination Pattern and Risk Assessment of Polar Compounds in Snow Melt: An Integrative Proxy of Road Runoffs.” Environmental Science & Technology 57 (10): 4143–52. https://doi.org/10.1021/acs.est.2c05784.Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114.Klöckner et al. 2021[DDXBMPC5] Klöckner, Philipp, Bettina Seiwert, Steffen Weyrauch, Beate I. Escher, Thorsten Reemtsma, and Stephan Wagner. 2021. “Comprehensive Characterization of Tire and Road Wear Particles in Highway Tunnel Road Dust by Use of Size and Density Fractionation.” Chemosphere. https://doi.org/https://doi.org/10.1016/j.chemosphere.2021.130530. ). In road dust, 6PPD concentrations have been found ranging from 11.4–5,359 ng/g, and 6PPD-q concentrations have been found ranging from 4.02–2,369 ng/g ( Deng et al. 2022[Y9G3IQTU] Deng, Chengliang, Jialing Huang, Yunqing Qi, Da Chen, and Wei Huang. 2022. “Distribution Patterns of Rubber Tire–Related Chemicals with Particle Size in Road and Indoor Parking Lot Dust.” Science of the Total Environment 844:157144. https://doi.org/10.1016/j.scitotenv.2022.157144.Hiki and Yamamoto 2022[VQ3M4AFW] Hiki, Kyoshiro, and Hiroshi Yamamoto. 2022. “Concentration and Leachability of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine (6PPD) and Its Quinone Transformation Product (6PPD-Q) in Road Dust Collected in Tokyo, Japan.” Environmental Pollution 302 (June):119082. https://doi.org/10.1016/j.envpol.2022.119082. ). Higher concentrations of 6PPD and 6PPD-q were found on finer particles than coarser particles in road dust collected within a highway tunnel ( Klöckner et al. 2021[DDXBMPC5] Klöckner, Philipp, Bettina Seiwert, Steffen Weyrauch, Beate I. Escher, Thorsten Reemtsma, and Stephan Wagner. 2021. “Comprehensive Characterization of Tire and Road Wear Particles in Highway Tunnel Road Dust by Use of Size and Density Fractionation.” Chemosphere. https://doi.org/https://doi.org/10.1016/j.chemosphere.2021.130530. ). 6PPD and 6PPD-q were detected in street-sweeping debris in Germany, prior to decantation of water; more research is needed to understand the management of street-sweeping waste and the potential resuspension of fine TRWP during the treatment process (Klöckner, Seiwert, Wagner, et al. 2021). See Section 6.3.1.6: Street Sweeping and Other Road Maintenance Activities for discussion of street sweeping as a mitigation measure for TRWP.
A pilot study was conducted to test whether funnels placed inside near-roadway passive samplers (Sigma-2) could successfully capture TRWP in the approximately 1 μm to 80 μm size range ( Olubusoye et al. 2023[R8HSVPUG] Olubusoye, Boluwatife S., James V. Cizdziel, Matthew Bee, Matthew T. Moore, Marco Pineda, Viviane Yargeau, and Erin R. Bennett. 2023. “Toxic Tire Wear Compounds (6PPD-Q and 4-ADPA) Detected in Airborne Particulate Matter Along a Highway in Mississippi, USA.” Bulletin of Environmental Contamination and Toxicology 111 (6): 68. https://doi.org/10.1007/s00128-023-03820-7. ). The authors reported that TRWP concentrations increased with proximity to the road with deposition rates (TRWPs/cm2/day) of 23, 47, and 63 at 30 m, 15 m, and 5 m from the highway, respectively. 6PPD-q was detected in each of the near-road sampling locations, suggesting the possibility of inhalation exposure.
In another study, road dust samples were collected from Guiyu Town, which is an area with extensive e-waste recycling activity, and Haojiang, a neighboring municipality without e-waste recycling activity ( Zhang et al. 2024[ZQPREK6H] Zhang, Zhuxia, Xijin Xu, Ziyi Qian, Qi Zhong, Qihua Wang, Machteld N. Hylkema, Harold Snieder, and Xia Huo. 2024. “Association between 6PPD-Quinone Exposure and BMI, Influenza, and Diarrhea in Children.” Environmental Research 247:118201. https://doi.org/10.1016/j.envres.2024.118201. ). Median levels in road dust were higher in Haojiang than in Guiyu Town, the e-waste recycling area. The higher levels of 6PPD-q in road dust from Haojiang were presumably due to higher traffic flow in that municipality compared to Guiyu, according to the authors.
Jin et al. ( Jin et al. 2023[P9WXQJUR] Jin, Ruihe, Yan Wu, Qun He, Pei Sun, Qiqing Chen, Chunjie Xia, Ye Huang, Jing Yang, and Min Liu. 2023. “Ubiquity of Amino Accelerators and Antioxidants in Road Dust from Multiple Land Types: Targeted and Nontargeted Analysis.” Environmental Science & Technology 57 (28): 10361–72. https://doi.org/10.1021/acs.est.3c01448. ) used their measured concentrations of 6PPD-q and 6PPD in dust samples from urban/suburban, agricultural, and forest areas to derive estimates of daily intake from ingestion and dermal contact for adults and children for each of these area types. Using standard exposure factors, Jin et al. estimated that ingestion may be the primary exposure route for roadside dust for all scenarios and age groups. The highest estimated daily intake by both adults and children was predicted to occur in urban/suburban regions ( Jin et al. 2023[P9WXQJUR] Jin, Ruihe, Yan Wu, Qun He, Pei Sun, Qiqing Chen, Chunjie Xia, Ye Huang, Jing Yang, and Min Liu. 2023. “Ubiquity of Amino Accelerators and Antioxidants in Road Dust from Multiple Land Types: Targeted and Nontargeted Analysis.” Environmental Science & Technology 57 (28): 10361–72. https://doi.org/10.1021/acs.est.3c01448. ).
6PPD and 6PPD-q have also been found in roadside snow (Table 4-8). 6PPD concentrations have been detected up to 783.79 ng/L, and 6PPD-q concentrations have been detected from about 110–428 ng/L ( Seiwert et al. 2022[QDRRVWMW] Seiwert, Bettina, Maolida Nihemaiti, Mareva Troussier, Steffen Weyrauch, and Thorsten Reemtsma. 2022. “Abiotic Oxidative Transformation of 6-PPD and 6-PPD Quinone from Tires and Occurrence of Their Products in Snow from Urban Roads and in Municipal Wastewater.” Water Research 212:118122. https://doi.org/10.1016/j.watres.2022.118122.Maurer et al. 2023[TJQR62IC] Maurer, Loïc, Eric Carmona, Oliver Machate, Tobias Schulze, Martin Krauss, and Werner Brack. 2023. “Contamination Pattern and Risk Assessment of Polar Compounds in Snow Melt: An Integrative Proxy of Road Runoffs.” Environmental Science & Technology 57 (10): 4143–52. https://doi.org/10.1021/acs.est.2c05784. ). Studies to date have not evaluated the relative contribution in snow between potential atmospheric sources and road dust sources. Snow melt has been shown to transport the road dust deposition into surface water ( Challis et al. 2021[T8TEWPCL] Challis, J. K., H. Popick, S. Prajapati, P. Harder, J. P. Giesy, K. McPhedran, and M. Brinkmann. 2021. “Occurrences of Tire Rubber–Derived Contaminants in Cold-Climate Urban Runoff.” Environmental Science & Technology Letters 8 (11): 961–67. https://doi.org/10.1021/acs.estlett.1c00682. ).
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-8 | Road dust and roadside snow | |
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4.4.3 Indoor Settled Dust
We use the term “dust” in the context of human health to indicate particles that are greater than 10 µm in diameter and therefore not respirable into the deep lung. Exposure to chemicals in dust can occur through direct emissions or when wind or turbulence resuspends dust particles in air. Dust particles greater than 10 µm deposit in the upper airways and can cause effects at the site of deposition or can be transported by mucociliary action and swallowed, resulting in ingestion exposure. Dusts can also be ingested when they get onto hands or food, or in the case of young children, objects that are mouthed. Dermal contact with chemicals in dust may also occur.
6PPD and 6PPD-q have been observed in dust in indoor environments, including e-waste facilities, homes, dormitories, buses, and shopping malls (Table 4-9). 6PPD and 6PPD-q in indoor dust have been detected up to 1,020 ng/g and 2,850 ng/g, respectively ( Zhu et al. 2024[WEPL88BC] Zhu, Jianqiang, Ruyue Guo, Shengtao Jiang, Pengfei Wu, and Hangbiao Jin. 2024. “Occurrence of p-Phenylenediamine Antioxidants (PPDs) and PPDs-Derived Quinones in Indoor Dust.” Science of the Total Environment 912:169325. https://doi.org/10.1016/j.scitotenv.2023.169325.Zhang et al. 2022[GHLGNCHV] Zhang, Ying-Jie, Ting-Ting Xu, Dong-Min Ye, Ze-Zhao Lin, Fei Wang, and Ying Guo. 2022. “Widespread N‑(1,3-Dimethylbutyl)‑N′‑phenyl‑p‑phenylenediamine Quinone in Size-Fractioned Atmospheric Particles and Dust of Different Indoor Environments.” Environmental Science & Technology Letters 9 (5): 420–25. https://doi.org/https://doi.org/10.1021/acs.estlett.2c00193.Huang et al. 2021[EZEWIV8E] Huang, Wei, Yumeng Shi, Jialing Huang, Chengliang Deng, Shuqin Tang, Xiaotu Liu, and Da Chen. 2021. “Occurrence of Substituted p-Phenylenediamine Antioxidants in Dusts.” Environmental Science & Technology Letters 8 (5): 381–85. https://doi.org/10.1021/acs.estlett.1c00148.Liu et al. 2019[TL9MEY5E] Liu, Runzeng, Yiling Li, Yongfeng Lin, Ting Ruan, and Guibin Jiang. 2019. “Emerging Aromatic Secondary Amine Contaminants and Related Derivatives in Various Dust Matrices in China.” Ecotoxicology and Environmental Safety 170 (April):657–63. https://doi.org/10.1016/j.ecoenv.2018.12.036.Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114.Liang et al. 2022[SZPLJY9T] Liang, Bowen, Jiehua Li, Bibai Du, Zibin Pan, Liang-Ying Liu, and Lixi Zeng. 2022. “E-Waste Recycling Emits Large Quantities of Emerging Aromatic Amines and Organophosphites: A Poorly Recognized Source for Another Two Classes of Synthetic Antioxidants.” Environmental Science & Technology Letters, June, acs.estlett.2c00366. https://doi.org/10.1021/acs.estlett.2c00366. ). The referenced indoor dust studies varied in the sample collection and processing methods they used and so are not directly comparable to each other, but the results taken together serve to illustrate the potential for human exposure via indoor dust.
In four solid waste recycling facilities in China, 6PPD-q was found in airborne particulate matter and settled dusts ( Zhang et al. 2022[G77DTKD6] Zhang, Yanhao, Caihong Xu, Wenfen Zhang, Zenghua Qi, Yuanyuan Song, Lin Zhu, Chuan Dong, Jianmin Chen, and Zongwei Cai. 2022. “p‑Phenylenediamine Antioxidants in PM2.5: The Underestimated Urban Air Pollutants.” Environmental Science & Technology 56 (11): 6914–21. https://doi.org/https://doi.org/10.1021/acs.est.1c04500. ). 6PPD was reported in dust samples from electronic waste recycling facilities, one in China and one in Canada ( Du et al. 2022[DWFYR89F] Du, Bibai, Bowen Liang, Yi Li, Mingjie Shen, Liang-Ying Liu, and Lixi Zeng. 2022. “First Report on the Occurrence of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine (6PPD) and 6PPD-Quinone as Pervasive Pollutants in Human Urine from South China.” Environmental Science & Technology Letters, November. https://doi.org/10.1021/acs.estlett.2c00821.Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114. ). Neither of the electronic waste studies analyzed for 6PPD-q. Overall, there is a lack of data relevant to determining occupational exposure levels for U.S. workers with potential exposure to 6PPD-q. Biomonitoring and evaluation of health impacts to worker population groups is a data gap ( DTSC 2022[2M3Z8Z4F] DTSC. 2022. “Product-Chemical Profile for Motor Vehicle Tires Containing N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine (6PPD) from the California Department of Toxic Substances Control (DTSC).” https://dtsc.ca.gov/wp-content/uploads/sites/31/2022/05/6PPD-in-Tires-Priority-Product-Profile_FINAL-VERSION_accessible.pdf. ).
6PPD was detected in settled dust collected from the floor of U.S. and Canadian residences in Indiana and Ontario, respectively. Median 6PPD levels in dust were slightly higher in residences located in Indiana compared to Ontario ( Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114. ). 6PPD-q was not analyzed in this study, but other studies on indoor dust, as described below, detected 6PPD-q where 6PPD was present.
Composite dust samples collected from 97 residences in a large city in eastern China contained both 6PPD and 6PPD-q, with a 100% detection frequency ( Zhu et al. 2024[WEPL88BC] Zhu, Jianqiang, Ruyue Guo, Shengtao Jiang, Pengfei Wu, and Hangbiao Jin. 2024. “Occurrence of p-Phenylenediamine Antioxidants (PPDs) and PPDs-Derived Quinones in Indoor Dust.” Science of the Total Environment 912:169325. https://doi.org/10.1016/j.scitotenv.2023.169325. ). Average levels in the dust were 17 ng of 6PPD and 14 ng of 6PPD-q per gram of dust. In another study, indoor dust samples were collected from Guiyu Town, which is an area with extensive e-waste recycling activity, and Haojiang, a neighboring municipality without e-waste recycling activity ( Zhang et al. 2024[ZQPREK6H] Zhang, Zhuxia, Xijin Xu, Ziyi Qian, Qi Zhong, Qihua Wang, Machteld N. Hylkema, Harold Snieder, and Xia Huo. 2024. “Association between 6PPD-Quinone Exposure and BMI, Influenza, and Diarrhea in Children.” Environmental Research 247:118201. https://doi.org/10.1016/j.envres.2024.118201. ). Median levels of 6PPD-q in house dust and dust in kindergartens in the e-waste town were 3.2 ng and 7.5 ng per gram of dust, respectively. Median levels in Haojiang were lower, with 1.4 ng and 1.3 ng of 6PPD-q per gram of dust sampled from houses and kindergartens, respectively. The authors conducted an exposure assessment and determined that children living in Guiyu Town have a higher daily intake from ingestion and inhalation of 6PPD-q from house or kindergarten dust than children living in Haojiang. Daily intakes of 6PPD-q were higher from kindergarten dust compared to house dust for both towns.
6PPD and 6PPD-q were detected in settled dust samples (25 µm–250 µm particle size fraction) taken from the interior of 11 vehicles and 18 residences in Guangzhou, China ( Huang et al. 2021[EZEWIV8E] Huang, Wei, Yumeng Shi, Jialing Huang, Chengliang Deng, Shuqin Tang, Xiaotu Liu, and Da Chen. 2021. “Occurrence of Substituted p-Phenylenediamine Antioxidants in Dusts.” Environmental Science & Technology Letters 8 (5): 381–85. https://doi.org/10.1021/acs.estlett.1c00148. ). Median levels of 6PPD in dust samples taken from vehicle interiors were two orders of magnitude higher than median levels of 6PPD in dust samples taken from residences. For 6PPD-q, concentrations were higher in interior vehicle dusts than in dusts sampled outdoors in parking lots and roadways. 6PPD-q was not detected in residences above the limit of quantitation in this study.
In another study relevant to vehicle interiors, 6PPD-q concentrations were higher in dust sampled from surfaces in buses compared to indoor dust sampled from shopping malls, residential bedrooms, and air conditioner filters inside college dormitories and houses ( Zhang et al. 2022[GHLGNCHV] Zhang, Ying-Jie, Ting-Ting Xu, Dong-Min Ye, Ze-Zhao Lin, Fei Wang, and Ying Guo. 2022. “Widespread N‑(1,3-Dimethylbutyl)‑N′‑phenyl‑p‑phenylenediamine Quinone in Size-Fractioned Atmospheric Particles and Dust of Different Indoor Environments.” Environmental Science & Technology Letters 9 (5): 420–25. https://doi.org/https://doi.org/10.1021/acs.estlett.2c00193. ).
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-9 | Indoor and nonroad settled dust | |
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4.5 Potential Food Sources and Human Consumption of 6PPD and 6PPD-q
The development of consistent and reliable standardized methods for measuring 6PPD and 6PPD-q in biological matrices will allow us to investigate biological uptake and, potentially, exposure history. These methods are still in development. The following section summarizes what is known about the potential for exposure to 6PPD-q and 6PPD from consumption of fish and other potential food sources. Table 4-10 presents the available data relevant to 6PPD and 6PPD-q in aquatic organisms and food. Section 3.5 describes biological uptake and accumulation of 6PPD and 6PPD-q. In general, more research is needed to understand biological uptake and bioaccumulation processes of 6PPD and 6PPD-q (see Section 8.1 for additional detail on information gaps and research needs on this topic).
| Table | Media Type | Link to PDF | Link to Executable File (Word Processor Format) |
| 4-10 | Aquatic organisms and food | |
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4.5.1 Aquatic Food Sources
6PPD-q and 6PPD have been detected in the organs of several different species of fish; however, a substantial knowledge gap exists regarding the presence of 6PPD-q and 6PPD within edible fish tissues and other aquatic biota consumed by humans, as well as whether these levels are sufficiently elevated to pose a risk to human health. Additionally, more research is needed to understand human exposure potential to 6PPD-q and 6PPD from consumption of the whole body of the fish. Results described below indicate measurements done on uncooked fish tissue. It is unknown how heat applied during cooking processes may impact 6PPD-q and 6PPD concentrations in fish tissue and other aquatic food sources.
Ji, Li, et al. ([Exploration of emerging environmental pollutants]) analyzed for 6PPD-q and 6PPD in a small sample of fish from a market in China. Freshwater species tested included snakehead fish, mandarin fish, tilapia, crucian, yellow-head catfish, and blunt-snout bream; marine species were Spanish mackerel, weever, silver pomfret, and large yellow croaker. 6PPD was detected in snakehead and weever. 6PPD-q was detected in the Spanish mackerel, but at a level below the limit of quantitation. Ji, Li, et al. ([Exploration of emerging environmental pollutants]) did not indicate which tissue types of the fish were analyzed, nor the source water they were fished from.
Frozen capelin caught in Canada and purchased at a supermarket contained 6PPD-q DNA adducts in liver, gill, and roe ( Wu et al. 2023[PYQQU7AG] Wu, Jiabin, Guodong Cao, Feng Zhang, and Zongwei Cai. 2023. “A New Toxicity Mechanism of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine Quinone: Formation of DNA Adducts in Mammalian Cells and Aqueous Organisms.” Science of the Total Environment 866:161373. https://doi.org/10.1016/j.scitotenv.2022.161373. ). The authors did not state where in Canada the capelin were caught.
6PPD-q was detected in the brain and gill of white spotted char (Salvelinus leucomaenis pluvius), southern Asian Dolly Varden (Salvelinus curilus), and masu (Oncorhynchus masou masou) exposed under laboratory conditions ( Hiki and Yamamoto 2022[VQE4EZWI] Hiki, Kyoshiro, and Hiroshi Yamamoto. 2022. “The Tire-Derived Chemical 6PPD-Quinone Is Lethally Toxic to the White-Spotted Char Salvelinus leucomaenis pluvius but Not to Two Other Salmonid Species.” Environmental Science & Technology Letters 9 (12): 1050–55. https://doi.org/10.1021/acs.estlett.2c00683. ), but the relevance of this finding to human exposure through consumption of these organs is unclear. The monohydroxylated metabolite of 6PPD-q was also detected in the same tissues ( Hiki and Yamamoto 2022[VQE4EZWI] Hiki, Kyoshiro, and Hiroshi Yamamoto. 2022. “The Tire-Derived Chemical 6PPD-Quinone Is Lethally Toxic to the White-Spotted Char Salvelinus leucomaenis pluvius but Not to Two Other Salmonid Species.” Environmental Science & Technology Letters 9 (12): 1050–55. https://doi.org/10.1021/acs.estlett.2c00683. ). People who consume the whole body of the fish, such as in soup stock, may be exposed to higher concentrations of 6PPD-q due to its concentration in the brain of the fish. It is unknown whether 6PPD-q and/or 6PPD are present in the tissues of fish that are more likely to be consumed, such as the skin and fillet.
In addition to the lack of data concerning 6PPD-q and 6PPD levels in edible fish tissue, a notable data gap concerns other aquatic biota consumed by people, such as shellfish. In addition, limited attention has been directed toward understanding how cooking processes might alter levels in the edible portions of these aquatic organisms. Determining concentrations of 6PPD-q and 6PPD in aquatic food sources and assessing the impact of various cooking techniques is important for evaluating any potential human exposure risks and establishing informed consumption guidelines.
4.5.2 Other Potential Food Sources
Sherman, Hämmerle, et al. ( Sherman et al. 2024[QBL568VF] Sherman, Anya, Luzian Elijah Hämmerle, Evyatar Ben Mordechay, Benny Chefetz, Thorsten Hüffer, and Thilo Hofmann. 2024. “Uptake of Tire-Derived Compounds in Leafy Vegetables and Implications for Human Dietary Exposure.” Frontiers in Environmental Science 12 (May). https://doi.org/10.3389/fenvs.2024.1384506. ) detected 6PPD in 7 of 28 leafy vegetables sampled from grocery stores in Switzerland and Israel. The average concentration of the seven positive vegetables was 0.26 ng/g; the highest measurement was 0.4 ng/g. 6PPD-q was not detected in the vegetables. The uptake of 6PPD into hydroponically grown lettuce has been examined ( Castan et al. 2023[3RBDETGD] Castan, Stephanie, Anya Sherman, Ruoting Peng, Michael T. Zumstein, Wolfgang Wanek, Thorsten Hüffer, and Thilo Hofmann. 2023. “Uptake, Metabolism, and Accumulation of Tire Wear Particle–Derived Compounds in Lettuce.” Environmental Science & Technology 57 (1): 168–78. https://doi.org/10.1021/acs.est.2c05660. ). The results provide some general context for whether food crops can uptake 6PPD and translocate it through the plant. Interpretation is limited to the laboratory conditions, however, and are not necessarily relevant for field-grown crops. Exposing corn salad (Valerianella locusta) roots to 6PPD and 6PPD-q via tire particulates in hydroponic nutrient solution resulted in the detection of small amounts of 6PPD and 6PPD-q in the leaves of this edible plant. These experiments show that uptake and translocation of 6PPD-q into consumable plants is possible under some conditions. The exposure relevance of this study is limited because hydroponics is not a predominant cultivation method employed in agriculture, so it is uncertain whether the uptake behavior of 6PPD shown in this study would occur similarly in crops grown in agricultural soil (see also Section 4.1.4: Wastewater and Biosolids). Further, dosing levels used in the study were higher than environmentally relevant concentrations. Although a conjugated 6PPD-glucoside form and three stable biotransformation products of 6PPD-q were detected, the human health relevance of these products is unknown ( Castan et al. 2023[3RBDETGD] Castan, Stephanie, Anya Sherman, Ruoting Peng, Michael T. Zumstein, Wolfgang Wanek, Thorsten Hüffer, and Thilo Hofmann. 2023. “Uptake, Metabolism, and Accumulation of Tire Wear Particle–Derived Compounds in Lettuce.” Environmental Science & Technology 57 (1): 168–78. https://doi.org/10.1021/acs.est.2c05660. ).
Only one other study examining the presence of 6PPD and 6PPD-q in food other than fish was located by the ITRC Tire Anti-degradants team during preparation of this document ( Ji et al. 2022[LDBNLUJS] Ji, Jiawen, Changsheng Li, Bingjie Zhang, Wenjuan Wu, Jianli Wang, Jianhui Zhu, Desheng Liu, et al. 2022. “Exploration of Emerging Environmental Pollutants 6PPD and 6PPDQ in Honey and Fish Samples.” Food Chemistry 396:133640. https://doi.org/10.1016/j.foodchem.2022.133640. ). Ten samples of honey purchased at a supermarket in Beijing, China, were analyzed for 6PPD and 6PPD-q; neither 6PPD nor 6PPD-q was detected in any of the samples ( Ji et al. 2022[LDBNLUJS] Ji, Jiawen, Changsheng Li, Bingjie Zhang, Wenjuan Wu, Jianli Wang, Jianhui Zhu, Desheng Liu, et al. 2022. “Exploration of Emerging Environmental Pollutants 6PPD and 6PPDQ in Honey and Fish Samples.” Food Chemistry 396:133640. https://doi.org/10.1016/j.foodchem.2022.133640. ).
It is unknown whether the ingestion of non-aquatic food sources will expose the consumer to 6PPD or 6PPD-q. More studies are needed to fill this knowledge gap.
4.6 Consumer Products
In addition to exposure to tire-derived 6PPD and 6PPD-q via environmental media pathways (air, soil/dust, food, and water), there is also potential for exposure from manufactured consumer products other than tires that can also contain 6PPD and 6PPD-q. Consumer products are manufactured from new materials that contain 6PPD as an additive, as well as from recycled tires. Potential routes of human exposure from contact with consumer products may include dermal contact, inhalation of particles, and incidental ingestion of particles, depending upon the product and usage patterns. Exposure to 6PPD and 6PPD-q from consumer products has not been characterized. Further, testing of consumer products for 6PPD and 6PPD-q may not be comparable among the studies cited herein due to the lack of standardized methods and intra- and inter-laboratory validation procedures. The extent of human exposure and importance of the sources and products discussed below is not known.
4.6.1 Recycled Tire Rubber Products
An estimated five million tons of scrap tires were generated in the United States in 2021 ( USTMA 2022[WYGSX5UU] USTMA, U.S. Tire Manufacturers Association. 2022. “2021 US Scrap Tire Management Summary.” October 25. https://www.ustires.org/sites/default/files/21%20US%20Scrap%20Tire%20Management%20Report%20101722.pdf. ). Approximately 32% (equal to 1.6 million tons in 2021) of scrap tires are made into ground rubber in the United States. Ground rubber is then repurposed for a range of applications and manufactured products, including asphalt rubber paving, sports surfaces, loose mulch, and automotive and consumer products ( USTMA 2022[WYGSX5UU] USTMA, U.S. Tire Manufacturers Association. 2022. “2021 US Scrap Tire Management Summary.” October 25. https://www.ustires.org/sites/default/files/21%20US%20Scrap%20Tire%20Management%20Report%20101722.pdf. ).
The U.S. Tire Manufacturers Association (USTMA) estimates that 11% of the ground rubber from scrap tires is used to manufacture molded and extruded consumer and industrial products. Some of these products may be present in indoor environments, including flooring materials such as rubber mats and tiles and accessibility ramps ( CalRecycle 2023[9MZ55M8A] CalRecycle. 2023. “California Tire Derived Products Catalog.” 2023. https://www.e-productcatalog.com/TDPCatalog/. ). A small sample (n=2) of doormats made from recycled rubber were reported to contain 6PPD at an average of 630 µg/g and 6PPD-q at an average of 18 µg/g ( Zhao et al. 2023[NMVDB224] Zhao, Haoqi Nina, Ximin Hu, Melissa Gonzalez, Craig A. Rideout, Grant C. Hobby, Matthew F. Fisher, Carter J. McCormick, et al. 2023. “Screening P-Phenylenediamine Antioxidants, Their Transformation Products, and Industrial Chemical Additives in Crumb Rubber and Elastomeric Consumer Products.” Environmental Science & Technology, February. https://doi.org/10.1021/acs.est.2c07014. ).
Some products can also be stamped or cut directly from scrap tire tread. No analytic data on samples of stamped products was located.
As of 2021, 7% of recycled tire rubber is used for sports surfaces, for example as crumb rubber infill material for artificial turf fields and as playground surfaces. The potential for exposure to chemicals bound in recycled tire rubber requires additional investigation.
The U.S. National Toxicology Program detected 6PPD in recycled tire-crumb rubber samples from manufacturing facilities ( National Toxicology Program, Public Health Service U.S. Department of Health and Human Services 2019[SAUIHJFK] National Toxicology Program, Public Health Service U.S. Department of Health and Human Services. 2019. “NTP Research Report on the Chemical and Physical Characterization of Recycled Tire Crumb Rubber.” Research Report 11. https://ntp.niehs.nih.gov/go/rr11abs. ). The National Toxicology Program did not test for 6PPD-q (6PPD-q had not yet been discovered at the time the study was conducted). In a series of publications from a European study of crumb rubber, 86 samples from playing fields across Europe (Figure 4-1) contained 6PPD at an average concentration of 571 mg/kg for all samples. Levels were higher, averaging 1,000 mg/kg (0.1% by weight), for granules that had not been coated with polyurethane ( Schneider et al. 2020[QN7GCPX2] Schneider, Klaus, Manfred De Hoogd, Maria Pelle Madsen, Pascal Haxaire, Anne Bierwisch, and Eva Kaiser. 2020. “ERASSTRI — European Risk Assessment Study on Synthetic Turf Rubber Infill — Part 1: Analysis of Infill Samples.” Science of The Total Environment 718:137174. https://doi.org/10.1016/j.scitotenv.2020.137174. ). Again, concentrations of the quinone were not measured. More recently, both 6PPD and 6PPD-q were found in samples of crumb rubber from recreational facilities in Europe ( Armada et al. 2023[AHX5WWPV] Armada, Daniel, Antia Martinez-Fernandez, Maria Celeiro, Thierry Dagnac, and Maria Llompart. 2023. “Assessment of the Bioaccessibility of PAHs and Other Hazardous Compounds Present in Recycled Tire Rubber Employed in Synthetic Football Fields.” Science of the Total Environment 857:159485. https://doi.org/10.1016/j.scitotenv.2022.159485. ). Further, both compounds could be extracted from crumb-rubber samples with synthetic digestive fluids.
Figure 4-1. Crumb rubber, from recycled tires, is added as a cushioning infill in artificial turf, creating the potential for athletes to be exposed.
Image attribution: Football in the City Stadium, by Steve Daniels, https://commons.wikimedia.org/wiki/File:Football_in_the_City_Stadium_-_geograph.org.uk_-_5459076.jpg
Both 6PPD and 6PPD-q were detected in crumb rubber from a small sample (n=9) of synthetic turf athletic fields in Washington state ( Cao et al. 2023[D5FPK9YB] Cao, Guodong, Wei Wang, Jing Zhang, Pengfei Wu, Han Qiao, Huankai Li, Gefei Huang, Zhu Yang, and Zongwei Cai. 2023. “Occurrence and Fate of Substituted P-Phenylenediamine-Derived Quinones in Hong Kong Wastewater Treatment Plants.” Environmental Science & Technology, October. https://doi.org/10.1021/acs.est.3c03758. ). Concentrations were highly variable. Median concentrations for 6PPD and 6PPD-q were 1.2 mg/kg and 9.8 mg/kg, respectively. The difference in concentration levels noted in Washington compared to the European samples is not yet understood.
In another study, researchers collected samples of crumb rubber from 40 school artificial turf fields in China. 6PPD and 6PPD-q were detected in fields using classical black rubber but not in fields using ethylene propylene diene monomer or thermoplastic elastomer. The results varied widely among fields using classical black rubber, with 6PPD concentrations ranging from 0.18 to 12.87 µg/g. 6PPD-q was detected in 24 out of 28 fields using classical black rubber, with concentrations ranging from less than the limit of quantitation to 28.05 µg/g ( Zhao et al. 2024[LZFSSS5F] Zhao, Feng, Jingzhi Yao, Xinyu Liu, Man Deng, Xiaojia Chen, Changzhi Shi, Lei Yao, Xiaofei Wang, and Mingliang Fang. 2024. “Occurrence and Oxidation Kinetics of Antioxidant p-Phenylenediamines and Their Quinones in Recycled Rubber Particles from Artificial Turf.” Environmental Science & Technology Letters 11 (4): 335–41. https://doi.org/10.1021/acs.estlett.3c00948. ).
The study of recreational facilities in Europe did not detect 6PPD above the limit of quantitation in air monitoring of particulates conducted at field locations with crumb rubber playing surfaces ( Schneider et al. 2020[7AVSU3G7] Schneider, Klaus, Manfred De Hoogd, Pascal Haxaire, Arne Philipps, Anne Bierwisch, and Eva Kaiser. 2020. “ERASSTRI — European Risk Assessment Study on Synthetic Turf Rubber Infill — Part 2: Migration and Monitoring Studies.” Science of The Total Environment 718:137173. https://doi.org/10.1016/j.scitotenv.2020.137173. ).
4.6.2 Manufactured Products from New Materials Containing 6PPD
The vast majority of 6PPD in the United States is used in the manufacturing of tires; however, other consumer products made from elastomeric materials may also include 6PPD as an anti-degradant additive. Manufactured products that contain 6PPD and are used indoors could contribute to human exposure to 6PPD-q. People spend most of their time indoors, and young children are particularly vulnerable to exposure from household dusts due to behavioral factors such as crawling and playing on the floor ( USEPA 2011[PL79ZTRH] USEPA. 2011. “Exposure Factors Handbook 2011 Edition (Final Report).” U.S. Environmental Protection Agency, Washington, DC. ). We were unable to evaluate data on the extent of 6PPD additives in products. The lack of data and of studies that determine the rate of production of 6PPD-q from 6PPD in indoor environments represent information needed to adequately characterize human exposure levels.
A small sample of rubber consumer products including laboratory stoppers (n=3), sneaker soles (n=3), and a rubber garden hose (n=1) were reported to contain 6PPD ( Zhao et al. 2023[NMVDB224] Zhao, Haoqi Nina, Ximin Hu, Melissa Gonzalez, Craig A. Rideout, Grant C. Hobby, Matthew F. Fisher, Carter J. McCormick, et al. 2023. “Screening P-Phenylenediamine Antioxidants, Their Transformation Products, and Industrial Chemical Additives in Crumb Rubber and Elastomeric Consumer Products.” Environmental Science & Technology, February. https://doi.org/10.1021/acs.est.2c07014. ). The authors also tested for 6PPD-q. The laboratory stoppers and sneaker soles contained detectable levels of 6PPD-q ( Zhao et al. 2023[NMVDB224] Zhao, Haoqi Nina, Ximin Hu, Melissa Gonzalez, Craig A. Rideout, Grant C. Hobby, Matthew F. Fisher, Carter J. McCormick, et al. 2023. “Screening P-Phenylenediamine Antioxidants, Their Transformation Products, and Industrial Chemical Additives in Crumb Rubber and Elastomeric Consumer Products.” Environmental Science & Technology, February. https://doi.org/10.1021/acs.est.2c07014. ).
Marques dos Santos and Snyder ( Marques dos Santos and Snyder 2023[GEI8HFLB] Marques dos Santos, Mauricius, and Shane Allen Snyder. 2023. “Occurrence of Polymer Additives 1,3-Diphenylguanidine (DPG), N-(1,3-Dimethylbutyl)-N′-Phenyl-1,4-Benzenediamine (6PPD), and Chlorinated Byproducts in Drinking Water: Contribution from Plumbing Polymer Materials.” Environmental Science & Technology Letters, September. https://doi.org/10.1021/acs.estlett.3c00446. ) analyzed samples of plumbing fittings to determine whether these products can be a source of 6PPD in drinking water. Seven different plumbing devices (o-rings and polymer seals) were tested. An oscillating kit for a faucet filter made of acrylonitrile butadiene rubber leached the highest amount of 6PPD (1 ng/mg of material). Silicone-based seals did not leach 6PPD. 6PPD-q was not detected in these samples.
6PPD and 6PPD-q were found in the dust of e-waste recycling facilities in China ( Liang et al. 2022[SZPLJY9T] Liang, Bowen, Jiehua Li, Bibai Du, Zibin Pan, Liang-Ying Liu, and Lixi Zeng. 2022. “E-Waste Recycling Emits Large Quantities of Emerging Aromatic Amines and Organophosphites: A Poorly Recognized Source for Another Two Classes of Synthetic Antioxidants.” Environmental Science & Technology Letters, June, acs.estlett.2c00366. https://doi.org/10.1021/acs.estlett.2c00366. ) and Ontario ( Wu, Venier, and Hites 2020[F7XN9GAC] Wu, Yan, Marta Venier, and Ronald A. Hites. 2020. “Broad Exposure of the North American Environment to Phenolic and Amino Antioxidants and to Ultraviolet Filters.” Environmental Science & Technology 54 (15): 9345–55. https://doi.org/10.1021/acs.est.0c04114. ). Liang et al. suggest that the sources may be materials in consumer electronics ( Liang et al. 2022[SZPLJY9T] Liang, Bowen, Jiehua Li, Bibai Du, Zibin Pan, Liang-Ying Liu, and Lixi Zeng. 2022. “E-Waste Recycling Emits Large Quantities of Emerging Aromatic Amines and Organophosphites: A Poorly Recognized Source for Another Two Classes of Synthetic Antioxidants.” Environmental Science & Technology Letters, June, acs.estlett.2c00366. https://doi.org/10.1021/acs.estlett.2c00366. ). Whether consumer electronics expose people in the United States to 6PPD or 6PPD-q is unknown as of March 2024.
