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Review

6PPD and 6PPD-Quinone in the Urban Environment: Assessing Exposure Pathways and Human Health Risks

by
Stanley Chukwuemeka Ihenetu
1,2,3,
Qiao Xu
1,2,
Li Fang
4,*,
Muhamed Azeem
1,
Gang Li
1,2,3 and
Christian Ebere Enyoh
5,*
1
State Key Laboratory of Regional and Urban Ecology, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Zhejiang Key Laboratory of Urban Environmental Processes and Pollution Control, CAS Haixi Industrial Technology Innovation Center in Beilun, Ningbo 315830, China
4
Zhoushan Municipal Center for Disease Control and Prevention, Zhoushan Municipal Health Supervision Institute, Zhoushan 316021, China
5
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
*
Authors to whom correspondence should be addressed.
Urban Sci. 2025, 9(6), 228; https://doi.org/10.3390/urbansci9060228
Submission received: 22 April 2025 / Revised: 4 June 2025 / Accepted: 11 June 2025 / Published: 16 June 2025
(This article belongs to the Special Issue Innovations in Land Resource and Environmental Governance for Future Cities)
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Abstract

In recent years, tires have become a prominent concern for researchers and environmentalists in regard to their potential threat of tire-derived pollutants (TDPs) to human health. Among these pollutants, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and its oxidized form, 6PPD-quinone (6PPD-Q), have been of primary interest due their ubiquity in urban environments, and their potential negative effects on human health. This review provides a summary of human health implications of TDPs, including 6PPD and 6PPD-Q. For the methodology, datasets were collected from the literature sources, including sources, formations and ecological effects of these pollutants, and pathways of human exposure and public health significance. Urban soils are key for services including carbon storage, water filtration, and nutrient cycling, underpinning urban ecosystem resilience. Soil degradation through compaction, sealing, and pollution, particularly by pollutants from tire wear, destroys these functions, however. These pollutants disturb the soil microbial communities, leading to a loss of diversity, an increase in pathogenic species, and changes in metabolism, which in turn can impact human health by increasing disease transmission and diseases of the respiratory systems. Incorporating green-infrastructure practices can enhance the ecosystem service potentials of urban soils and contribute to sustainable, climate-resilient urban city development. These findings underscore the pressing need for a coordinated international campaign to study chronic health effects and science informed policy frameworks to address this ubiquitous environmental health concern—an issue that crosses urban water quality, environmental justice, and global management of tire pollution.

1. Introduction

The fast-paced development of urbanization has intensified challenges of managing microplastic (MP) pollution in urban areas, especially with the increasing contribution of tire wear particles (TWPs) due to a larger number of vehicles in urban areas [1,2]. MPs are now ubiquitous in urban environments, contaminating soils and waterways via various routes, including the mismanagement of wastes, stormwater runoff, atmospheric deposition, and traffic on roads [3]. Given the heightened global awareness regarding the challenges faced by the environment and the rising scale of environmental degradation, particularly in urban areas, integrating the environment into planning and development processes has become imperative [1,4]. The rise in greenhouse gases, a key factor in global warming, has been linked not only to increased mortality from cardiovascular and respiratory diseases but also to adverse pregnancy outcomes, including low birth weight, preterm birth, and congenital disabilities [5]. These health risks show the critical need to address the impact of environmental pollutants, such as tire wear particles (TWPs), which contribute to both air pollution and associated health hazards [6]. The widespread use of rubber tires in transportation and industry has undoubtedly transformed modern life, enhanced mobility, and facilitated economic development [7]. However, this convenience brings the risk of unintended environmental impacts and adverse effects on human health due to tire pollutants and their transformation products [8,9]. 6PPD is a vital component in tire manufacturing, serving as an antioxidant that protects tires from oxidative degradation, thereby extending their lifespan [10]. While this function is essential for tire durability, recent research has unveiled a conversion aspect of 6PPD: its capacity to form toxic transformation products upon reaction with ozone in the environment [9]. For a long time, the environmental consequence of tire wear has been unnoticed, until new studies showed that MPs, especially originating from tire wear, had disseminated through different ecosystems, and even to urban and aquatic areas [11,12]. MPs from tires are now understood to be a significant source of environmental pollution, with 6PPD, an antioxidant used in tires to prevent degradation, being one of the key pollutants [12]. 6PPD is released during tire wear and undergoes transformation into 6PPD-Q when exposed to ozone, which is particularly concerning due to its persistence and toxicity in the environment [13].
The release of tires is primarily driven by frictional wear, a process that has been occurring since the widespread adoption of rubber tires in the early 20th century [14]. As vehicles move, the tires make contact with the road surface, and the resulting friction and mechanical stress gradually wear the tires down, producing tiny tire particles [15]. This phenomenon became more significant as automobile use increased globally, leading to a rise in tire wear and the subsequent release of MPs into the environment. The design and composition of tire treads are crucial factors in MP release [12]. Treads are engineered for specific conditions, such as summer, winter, or off-road driving, with patterns designed to enhance traction and durability [16]. However, these same patterns, including grooves and sipes, can affect the rate at which tires are released. Rough or abrasive surfaces, such as those found on poorly maintained roads or construction zones, can accelerate tire wear, leading to higher tire generation [17]. Driving behaviors and environmental conditions influence tire release. Aggressive driving, characterized by rapid acceleration, hard braking, and high speeds, intensifies tire wear, increasing the emission [18]. Environmental factors like weather and temperature also play a role; for instance, colder temperatures can make tire rubber more brittle, leading to increased wear [2]. The vehicle’s weight is another critical factor, as heavier vehicles or those carrying substantial loads experience greater tire wear [7]. Proper tire maintenance, including regular checks for inflation and alignment, can mitigate uneven wear and extend tire life, thereby reducing MP release. During tire manufacturing and usage, 6PPD may migrate to the surface and be released into the environment as the tire wears down. 6PPD is then carried by rainwater or stormwater runoff, leading to contamination of water bodies [14]. Moreover, 6PPD-Q, a derivative formed from 6PPD through reactions with ozone, has been detected in atmospheric particles, highlighting the broader environmental and health risks associated with tire-derived MPs [19].
The release of 6PPD from TWPs leads to the generation of 6PPD-Q, a toxic compound formed through complex degradation and transformation processes [8]. These transformation products, including 6PPD-Q and other derivatives, have been detected in TWPs and runoff from roadways, raising questions about their potential impact on human health [20]. These compounds are subsequently dispersed across diverse ecosystems globally via surface runoff and atmospheric transport, exerting adverse effects on various organisms [21]. In 2020, a group of scientists based in Washington State made a significant breakthrough by finding and characterizing 6PPD-Q as the chemical causative behind the urban runoff mortality syndrome that had been affecting coho salmon (Oncorhynchus kisutch). It arises because 6PPD in tire rubber is converted to 6PPD-Q as it reacts with the ozone; the oxidized quinone is strongly toxic to salmon, inducing fatal respiratory stress within hours of contact [13]. The widespread presence of 6PPD and 6PPD-Q can be primarily attributed to the production of rubber particles, a consequence of high-frequency transportation and traffic activity [22].
As emerging contaminants, 6PPD and 6PPD-Q have been detected in a range of environmental samples, including water [23], the atmosphere [9], dust [24], and soil [23]. Additionally, Ji et al. [25] identified for the first time the presence of 6PPD and 6PPD-Q in fish samples, raising concerns about potential harm to human health through the consumption of contaminated fish. Furthermore, Du et al. [5] reported the presence of 6PPD and 6PPD-Q in human urine samples, showing the need for urgent attention to the potential human health risks associated with exposure to these compounds. Currently, several studies have documented the toxic effects of 6PPD and 6PPD-Q on various organisms, including fish [25,26,27,28,29], mussels [30], and crustacean species [31]. Inhalation of particulate matter 2.5 represents a common method of human exposure to various pollutants, in comparison to other contaminants bound to PM2.5, such as Organophosphate Esters [32] and Polybrominated Diphenyl Ethers [33]. Recent studies have confirmed the presence of 6PPD-Q in atmospheric particles, particularly in coarse fractions (9–10 μm), with concentrations ranging from 7.78 to 23.2 pg m−3 in samples collected from waste recycling plants and indoor environments in South China [34]. Model simulations further revealed high deposition efficiency of particle-bound 6PPD-Q in the respiratory tract, especially within the head airways (89–91%) [23,34]. This suggests that, despite their relatively lower levels, their cumulative effects from frequent inhalation may still raise concerns about potential impacts on human health.
This review explores the occurrence and effects of tire-derived pollutants (6PPD and 6PPD-Q) in the urban environment, where dense vehicular traffic drives rapid tire wear and accumulation of pollutants. While previous research has explored various aspects of tire-derived pollutants, this review consolidates and synthesizes this knowledge into a cohesive nature, placing focus on 6PPD, shedding light on its chemical characteristics, release mechanisms, environmental behavior, and potential health risks. In the context of rapid urban city development, the accumulation and transformation of these pollutants present emerging challenges for environmental health. For the review preparation, five sets of literature data were collected: (1) sources and environmental fate of tire 6PPD in urban systems, (2) transformation of 6PPD to 6PPD-Q, (3) human health effects of tire-derived products, (4) exposure pathways prevalent in urban populations, (5) significance of investigating TWP’s effect on human health impacts. Considering the global use of tires and the growing world-wide urbanization trends, knowledge of the environment and health effects of 6PPD and 6PPD-Q is an important consideration for international regulatory actions and mitigation efforts.

2. Methodology

We performed an extensive literature search following the PRISMA framework [35] across the Web of Science, Scopus, and Google Scholar databases using relevant keywords including 6PPD, 6PPD-Q within urban environments, human health, exposure pathways, toxicology, respiratory effects, cardiovascular effects, and dermal exposure. All titles and abstracts were reviewed for studies pertaining to the health effects of 6PPD and 6PPD-Q, with exclusion criteria applied to remove duplicates, irrelevant studies, and papers with inadequate study design or small dataset. To ensure that the selected literature produced high quality results, articles with poorly described data collection methods, flawed data analysis, or not published in peer-reviewed journals were excluded. Full-text articles that fulfilled the inclusion criteria were obtained after screening for in-depth scientific analysis. The search yielded a compendium of investigation articles, literature reviews, and various other publications analyzing the toxicological profiles of 6PPD and 6PPD-Q and the health risks associated with exposure. The identified articles were screened for relevance to the topic and 65 studies were identified for full synthesis. This information was extracted and included in the manuscript as part of the textual narrative, figures, and tables (similar approach to [36]). This enables a robust evaluation of human health impacts from exposure to tire-derived pollutants, particularly 6PPD and its transformation product 6PPD-Q, in densely populated urban settings.

3. Occurrence of 6PPD and 6PPD-Q

3.1. Sources of 6PPD in Urban Environments

6PPD is a chemical used in tire rubber to protect it from the harmful effects of environmental factors like ozone and sunlight, which can deteriorate the rubber and cause it to crack. 6PPD protects against radicals and ROS in a similar way, so 6PPD plays this role by quenching free radicals and reactive oxygen species that otherwise can start chain reactions to degrade the rubber [37]. Consequently, it maintains a fundamental function in prolonging tire service-life and maintaining their structural stability [38,39]. In the rubber industry, 6PPD is generally used in tire production with the mass concentrations of 0.4–2% [40]. After becoming incorporated in rubber materials, 6PPD has been shown to slowly migrate to the tire surface, where it becomes available for gas-phase processes. These reactions underlie the antioxidant activity of 6PPD, protecting the rubber from oxidative damage [40]. Industrial activities such as manufacturing and construction release 6PPD and 6PPD-Q into the environment through machinery wear and particulate emissions. These particles can be absorbed directly by humans from dermal absorption, inhalation, or ingestion. The contamination is widespread, including wastewater treatment facilities and farmlands, where tire particles are collected, indicating how ubiquitous these pollutants are and how they may affect human health [41]. Figure 1 demonstrates these exposure routes and emphasizes the urgency of addressing the environmental and health impact of tire-derived 6PPD. Through the processes of surface runoff, seepage, percolation, and deposition of dust, point sources (example, a stormwater drain outlet or a tire manufacturing facility) become a source of environmental contamination with potential exposure to wildlife and human population. In the aquatic environment, 6PPD is oxidized and photodegraded to 6PPD-Q [42]. In terrestrial environments, 6PPD reacts with microorganisms, oxygen, and soil minerals to form toxic derivatives, too.

3.2. Characteristics and Metabolism of 6PPD

6PPD exhibits specific chemical properties; it is a crystalline solid with a molecular weight of approximately 268.4 g/mol (Table 1), showing limited solubility in water but readily dissolving in organic solvents [27]. Once discharged into the environment and absorbed into the human body, 6PPD undergoes several transformation processes. These processes, influenced by both biological and environmental factors, play a detailed role in its conversion into other compounds. This metabolic process may involve enzymatic reactions within the body or non-enzymatic reactions in the environment, ultimately leading to the formation of various pollutants, including 6PPD-Q, as well as other harmful compounds like polycyclic aromatic hydrocarbons and volatile organic compounds, which are known to contribute to environmental degradation and pose risks to human health [14]. Cao et al. [23] used EPI Suite software (version 4.11, US EPA) to determine the physicochemical characteristics of natural forms of para-phenylenediamine (PPD) that exist in the environment or in their original chemical form before any transformation or reaction occurs, and their quinone variants. Cao et al. [23] formulated equations to estimate daily exposure to 6PPD and related quinones through three primary pathways: inhalation (DI_inh), dermal contact with soil (DI_der), and oral ingestion via soil dust (DI_ing).

3.3. Formation of 6PPD-Q from 6PPD

Although 6PPD plays a key role in supply chain production of tire rubber and performance as an antioxidant, it is not chemically locked in the rubber matrix. Conversely, it is blended into the rubber as an independent ingredient [13]. Gradually, as tires wear down from driving and environmental exposure, the 6PPD in the rubber can leach out [47]. It should be noted, however, that there are a variety of factors that affect how much 6PPD leaches from tires. While driving, the friction between the tire and the road surface causes abrasion and wear of the tire tread. The mechanical wear can result in the release of small particles with 6PPD into the environment. High temperatures or exposure to UV radiation accelerate rubber decay and may help release 6PPD. Rainwater may run off 6PPD particles from tire surfaces and carry them to the ambient environment, including water bodies and stormwater drainage systems [8]. The environmental transformation of 6PPD-Q is a complex process influenced by multiple factors such as light, pH, and the presence of reactive species. 6PPD can undergo oxidative reactions in the presence of atmospheric oxygen or other oxidizing agents. This process creates something called 6PPD-Q. 6PPD-Q and other decomposition products may be distributed throughout the environment, including soil, water, and sediments [14,26,45].
Figure 2 shows the formation of 6PPD-Q from 6PPD. Of interest is the more recent study by Fohet et al. [48] which discovered 6PPD-quinone imine (6PPD-QI) existing during both the photo-aging and thermal-aging that occurred, suggesting that temperature differences might play a considerable part in the conversion of 6PPD. 6PPD-QI, by virtue of its relatively high instability, reacts with ozone to add an oxygen atom and generate Hydroxy-6PPD-quinone imine (6PPDQI-OH). Next, 6PPDQI-OH is oxidized again, this time with an additional incorporation of an oxygen atom, to yield 6PPDQ as described by Seiwert et al. (2022) [8]. 6PPD and 6PPD-QI would tend to decrease at the same time during photo-aging, and the 6PPD-Q concentrations would increase [27,48].

4. Human Exposure Pathways and Associated Health Risks

4.1. Potential Human Health Risk

6PPD and its derivative 6PPD-Q exhibit considerable risks to human health, especially so-called inhalational or dermal exposure [39]. These pollutants can enter the human body through inhalation of contaminated air, then deposited in the respiratory tract. 6PPD-Q exposure in the lungs has been associated with the generation of oxidative stress, a mechanism known to induce inflammation and potentially contribute to lung tissue damage [34]; this is particularly concerning for respiratory diseases like asthma and chronic obstructive pulmonary disease (COPD). It is particularly important that 6PPD-Q aggregates on larger air particle sizes, which are preferentially inhaled and may cause oxidative stress and other health problems in humans [9].
There are multiple pathways through which individuals encounter 6PPD, including dietary intake [49], inhalation of fine particles [50], and possible consumption of contaminated water [39,51]. Human exposure to TDPs was assessed by Cao et al. [23]. The findings suggested that the primary source of human exposure to PPDs and PPD-Qs was the ingestion of roadside soil dust, followed by dermal absorption, which accounted for approximately 15% of the oral ingestion intake rate. Additionally, inhalation was identified as a lesser contributor to human intake, likely due to the relatively low levels of these contaminants detected in air particles in Hong Kong. For adults, daily intake of 6PPD-Qs (1.08 ng/(kg·day)) surpasses parent compounds (0.71 ng/(kg·day) under the same exposure. Among children, ingestion of roadside soil dust results in higher daily doses, with 6PPD-Q (7.30 ng/(kg·day)) slightly exceeding parent compounds (4.85 ng/(kg·day). These findings underline health risks from 6PPD-Q exposure, urging further research and mitigation strategies for TDP-related hazards [23].
According to Wang et al. [9], daily intake of 6PPD varies from 0.16 to 1.25 ng/(kgbwday) between different subgroups and exposures, similar to that of the parent PPDs, showing a range from 0.19 to 1.41 ng/(kgbwday), representing a quantitatively significant exposure previously neglected. Similarly to other PM2.4 and 5, 6PPD might have comparable effects [52]. Intake estimates based on oral consumption showed that the daily oral dose of 6PPD-Q was much higher than the daily intake dose of its parent compounds for adults and children in Hong Kong, China. The cellular dithiothreitol (DTT) assay found that 6PPD, especially 6PPD-Q, increased oxidative potential of PM2.↑5 [9]. Recent studies have also shown the bioaccumulation of 6PPD and 6PPD-Q in the liver and the disturbance of lipid metabolism and inflammatory reactions in mice [44]. Analysis of urine samples showed that both compounds were detected in a high proportion of the samples analyzed, with the highest concentration of 6PPDQ in pregnant women; the concentrations of both 6PPD and 6PPD-Q (median 0.068 and 2.91 ng/mL) were substantially elevated in pregnant women compared with adults (0.018 and 0.40 ng/mL) and children (0.015 and 0.076 ng/mL). In addition, the daily urinary excretion rate of 6PPD-Q in pregnant women was also significantly higher, estimated to be 273 (ng/kgbw)/day [5]. Currently, there is limited research regarding the biological effects of exposure to 6PPD-Q, highlighting the need for further investigation to elucidate its toxicities and potential human health effects.

Tire Wear Particles: A Cross-Domain Analysis of Ecological Impact

In soil health, these particles lead to soil contamination and potentially toxic effects on plant life. These substances may interfere with the natural processes of root uptake, potentially altering the mere balance of nutrient acquisition by plants [37]. There is also signs of TWPs affecting allelopathy, the biochemical interaction between plants, which could have broader implications for plant competition and survival. Stephanie et al. [43] investigated the uptake, metabolism, and accumulation of TWP in derived compounds in lettuce and found that the concentrations in leaves, roots, and nutrient solutions were quantified by triple quadrupole mass spectrometry, and metabolites in the leaves were identified by Orbitrap high-resolution mass spectrometry. Their research illustrates the uptake of TWP-derived compounds by lettuce, with peak leaf concentrations ranging from approximately 0.75 (6PPD) to 20 μg/g hexamethoxymethylmelamine (HMMM). While these compounds underwent metabolism within the plant, the study also revealed the presence of various transformation products of the original five TWP-derived compounds including Benzothiazole, 6PPD, 6PPD-Q, and HMMM, many of which exhibited greater stability in lettuce leaves compared to the original compounds.
According to Castan et al. [53], TWPs amended into farmland soils tend to release their compounds predominately to the upper soil layers as opposed to migrating to deeper soil horizons. Once in the root zone, these compounds are readily available for absorption by the plant roots, as previously demonstrated with a range of substances including pharmaceuticals and plastic-derived phthalates [54,55]. The chemical nature of the compounds affects the uptake process across the plant root system, with hydrophobic neutral compounds being absorbed while negatively charged compounds are mostly repulsed by the negatively charged membranes of root cells [56,57]. These compounds are then transported through root tissues via the symplastic pathway (cytoplasm through the plasmodesmata) or apoplastic pathway (the intercellular space, i.e., cell wall) [53]. The possible uptake, retention, and subsequent transfer of TWP-derived compounds from the ambient water into plant leaves by lettuce suggests a potential risk for consumers, particularly since lettuce is most often eaten raw [43].
Zhang et al. [50] reported the ubiquitous distributions of 6PPD-Q and its parent compound in fine particulate matter (PM2.5). TWPs have their roles in contributing to atmospheric particulate matter, which has a complex relationship with the earth’s energy balance and climate system [9,58]. Figure 3 shows the lifecycle and environmental impact of TWPs, which begins with tire manufacturing, where raw materials are processed into tires, which are then used on urban roads, resulting in TWPs from vehicle wear. These particles can be dispersed into the air and transported via surface runoff, especially during rainfall. Airports also contribute to TWP generation through aircraft tire wear. Some TWPs enter wastewater systems, reaching wastewater treatment plants (WWTPs), but not all are removed, leading to environmental contamination. TWPs can deposit in farmlands, potentially harming soil health and crop productivity. Aquatic ecosystems are affected as TWPs accumulate in water bodies, impacting aquatic organisms.

4.2. Routes of Exposure to Tire 6PPD-Derived Products

Pathways of human exposure to tire-derived products (6PPD/6PPD-Q) include various routes, falling into three distinct categories. Inhalation represents a significant route of exposure, primarily through airborne particles generated from tire wear, which may include 6PPD-Q. Zhang et al. [50] found 6PPDQ widely present in atmospheric particles and dust. Their study estimated how efficiently this contaminant is deposited in the human respiratory system upon inhalation, particularly in workers’ upper airways. Du et al. [5] researched the prevalence of 6PPD and 6PPD-Q as prevalent pollutants in human urine collected from South China. Their analysis of urine samples from individuals unveiled that both 6PPD and 6PPD-Q were present, with detection frequencies spanning from 60% to 100%. Notably, concentrations of urinary 6PPD-Q surpassed those of 6PPD significantly and exhibited a robust positive correlation (p < 0.01), indicating simultaneous exposure to both compounds in humans. In in vitro metabolic experiments employing human liver microsomes, a rapid depletion of 6PPD was observed, potentially elucidating the lower observed concentrations of 6PPD in human urine. Thus, when TWPs arrive at agricultural fields, they may continue releasing tire additives and become bioavailable to plants and enter the human food chain. In general, for other TWP-derived compounds, with constant transformation products, there may be potential human exposure to these transformed compounds, whose toxicological endpoints are unknown. Further investigation is required to evaluate the toxicity of TWP-initiated compounds and their transformation derivatives, as well as to determine the availability of such compounds in agricultural products [43]. Small fractions of these particles can aerosolize and people, especially those who live next to busy roads or perform tire-wear tasks, can inhale them [58]. Lastly, indirect oral exposure can be through the intake of food, water, or soil containing 6PPD-Q. Runoff from roads, where tire wear particles accumulate, has the potential to contaminate water bodies.

4.3. Occupational Exposure Consequences in Tire Manufacturing

Workers in tire manufacturing and related industries have definite potential for occupational exposure to 6PPD and 6PPD-Q. First, during the tire manufacturing process, workers who handle the raw materials, such as rubber compounds containing 6PPD, may be exposed to dermal exposure and inhalation of dust/aerosols formed during mixing and processing. According to the simulation results [50], nearly all particle-bound 6PPD-Q (89–91%: 10.8–39.1 pg h−1) was deposited in the head airways of the workers. Decreased deposition was found in the tracheobronchial (3.2–3.8%, 0.45–1.64 pg h−1) and pulmonary alveolar regions (6.0–6.9%, 0.80–2.85 pg h−1) of the respiratory tract. Later, in an even more recent study by Wang et al. [9], 6PPD-Q was identified as a novel contributor to the oxidative potential of fine particulate matter. Second, maintenance personnel in tire manufacturing facilities could be exposed to 6PPD-Q contamination when they service machinery and equipment that encounter 6PPD-containing rubber materials. TWPs, once assimilated by organisms, are likely retained within tissues, which may increase the probability of exposure and transfer throughout the food chain [10]. Lastly, workers performing roles in the waste management industry to dispose of or recycle waste tires and discarded rubber material may also be exposed to 6PPD and its derivatives when handling or processing tire waste. Human health is affected by TWP emissions: TWP emissions can undergo physical, chemical, and biological changes, and can be in the form of direct contact with materials or breathing in airborne particles (Figure 4). In addition, these pollutants can bioaccumulate along the food chain [10,59].

4.4. Significance of Investigating Human Health Impacts

The human health implications of tire rubber-derived contaminants like 6PPD and its oxidized form 6PPD-Q is an important area for investigation given their potential health hazards. Additionally, this compound has been detected in urban stormwater, suggesting ubiquitous environmental spread [6]. Heavy rains carry these particles into urban stormwater systems, contaminating surface waters. Humans can subsequently be exposed to this compound by consuming potable water containing 6PPD-Q, ingesting bioaccumulated aquatic organisms with 6PPD-Q, or through dermal contact during recreational swimming in contaminated water. 6PPD-Q has been found in diverse environmental matrices, including water, air, dust, and soil, demonstrating its ubiquitous nature and risk for human exposure through ingestion, inhalation, and dermal contact. Its lipophilic property and bioaccumulation potential pose significant concern for persistence in the environment and risk to human health [60,61]. The study published by Fang et al. [51] reported that 6PPD and its derivative 6PPD-Q induced hepatotoxicity by an oral route of exposure in mice. This could be problematic because these compounds are widespread in the environment, and 6PPD being a staining and environmentally unfriendly substance has also been reported, which is alarming as it poses long-term environmental and health threats [50].
Figure 5 schematically summarizes the key features of 6PPD and 6PPD-Q as public health threats with respect to their chemical identity, environmental presence, exposure modes, and health implications. The role and impact types of these data are grouped and summarized as follows: significance, health research, and potential risk. The figure illustrates an ongoing necessity for broad toxicity measurements and performed/calculated remedial measures (environmental monitoring, public awareness) that may be imposed to tackle oncoming threats of these tire-generated pollutants. The schema is consistent with the observations of our review, especially with respect to the extensive widespread of urban exposures (e.g., soil and water) and the potential for 6PPD-Q to be a carcinogen, reinforcing the need for regulatory action and research efforts to protect human health. This synthesis in imagery supports our narrative, further illuminating the interconnected routes through which these substances threaten city-dwellers.

5. Role of Urban Soils and Tire Wear-Derived Pollutants in Ecosystem Services and Urban Environmental Resilience

Urban soils play a vital role in sustaining ecosystem services such as carbon sequestration, water filtration, and nutrient cycling, all of which are integral in maintaining the resilience of the urban environment. Urban soils are crucial for biodiversity, food security, and climate resilience; however, urban soil degradation is further aggravated by processes such as sealing, compaction, and pollution from anthropogenic sources, with tire wear-derived pollutants being among the top seven contaminants of urban soils [4,14]. These conditions challenge soil function and the fundamental services urban ecosystems rely on. However, urban soil management strategies are necessary to recover and improve the ecosystem supply of urban soils. Methods like organic amendments, phytoremediation, and conservation tillage have been shown to improve the soil structure, increase the number of microorganisms, and preserve nutrients. These soil fertility practices also strengthen the resilience of urban ecological systems [62].
Integrating green infrastructure such as green roofs, permeable pavements, and vegetated urban areas within urban planning can further maximize the benefits provided by urban soils. By improving stormwater infiltration, combating urban heat island effects, and providing habitats for other species, these interventions increase the adaptive capacity of urban settings against climate-related pressures [4]. In addition, the idea of urban soils as critical components of a city’s brown infrastructure highlights their basic role for supporting vital ecological health. This approach would integrate soil health into urban development policies, enabling cities to maximize the delivery of ecosystem services and support constructive urban forms capable of proceeding with sustainable and resilient development. As shown in Figure 6, the net effects of TWP pollution on the urban soil microbiome indicate that tire wear introduces pollutants into the urban environment, and urbanization leads to transformation of land use, dominating pollution that is responsible for altering the soil landscape. These changes may change the soil environment, disturb the microbial communities involved, reduce microbial diversity, increase the occurrence of potentially pathogenic species, and alter microbial metabolism and gene expression. In urban environments, where close contact with infected soils and airborne contaminants is common, such dysbiosis could promote increased disease transmission risk, reduced respiratory tract function, and other adverse health outcomes via the loss of microbial bioregulatory capacity and the accumulation of toxic microbial metabolites [60,63,64].

6. Conclusions

Among the widely studied TDPs, 6PPD and its transformation product 6PPD-Q have gained attention as environmental contaminants with implications for ecosystem integrity and human health. In urban environments that experience TWPs being continuously deposited through a combination of dense traffic and infrastructure, the transformation of 6PPD to 6PPD-Q, primarily driven via oxidative environmental processes, becomes a critical question. These contaminants spread through air, water, and soil, leading to ubiquitous exposure to the urban environment. The derivative of 6PPD, known as 6PPD-Q, has been found in many different systems, from aquatic species to human tissue, and suggests bioaccumulation and systemic toxicity. The hepatotoxicity profile of 6PPD-Q has been exposed in animal studies, necessitating the need for complete toxicological evaluations. Considering the ubiquitous use of tires and the concentrated presence of TWPs in urban areas, targeted research efforts are needed to better understand the fate, exposure routes, and health impacts associated with 6PPD and 6PPD-Q, especially in urban-dwelling populations in which most exposure is likely to occur.
It is crucial to tackle TDPs, most notably 6PPD and its derivative, 6PPD-Q, which represent one of the many emerging contaminants in the environment, particularly in urban areas where exposure is compounded. The first step is to conduct extensive research to address these knowledge gaps on the toxicity, environmental fate, and human health risks of such chemicals. Such research is essential to inform evidence-based public health policy and strong environmental regulation to reduce risks from TDP exposure in urban environments. The tire manufacturers also need to embrace sustainability, as do urban TWP management practices. Reasonable strategies would comprise advanced waste treatment and the use of safer tire additives that further limit the discharge of 6PPD and its derivatives into the cityscape. Thus, this study highlights the critical demand to elucidate the human health relevance of TDP exposure in cities and instigate targeted management actions to safeguard human health and urban environment from emerging super pollutants.

Author Contributions

S.C.I.: whole paper idea, figures, tables, categorization, methodology, and draft writing; Q.X.: editing, table construction, modifications; L.F.: editing, table construction, modifications; G.L.: supervising and conceptualization, writing—original draft and revision; M.A.: editing, table construction, modifications; C.E.E.: editing, table construction, modifications. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of Fujian Province, China (2023J05078) and the National Natural Science Foundation of China (42207010), and the Science and Technology Project of Zhejiang Province Disease Control and Prevention (No. 2025JK0127).

Institutional Review Board Statement

All authors have read, understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.

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Urbansci 09 00228 g001
Figure 1. Sources, chemical exposure, and toxic impacts of 6PPD and 6PPD-Q in urban environments.
Figure 1. Sources, chemical exposure, and toxic impacts of 6PPD and 6PPD-Q in urban environments.
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Figure 2. Transformation mechanism of 6PPD-Q from 6PPD. (A) Photodegradation of 6PPD generates intermediates like 6PPD-Q and 6PPD-OOH via oxygen reaction and hydroxyl radicals. (B) Microbial degradation in water converts 6PPD to intermediates and ultimately to 6PPD-Q, highlighting biodegradation’s significance. (C) Soil transformation involves direct photodegradation and microbial action, with sunlight converting 6PPD to intermediates, aided by soil microorganisms. (D) Air-driven reactions in the atmosphere produce hydroxyl radicals and 6PPD-OH, leading to 6PPD-Q formation through UV radiation and gas interactions.
Figure 2. Transformation mechanism of 6PPD-Q from 6PPD. (A) Photodegradation of 6PPD generates intermediates like 6PPD-Q and 6PPD-OOH via oxygen reaction and hydroxyl radicals. (B) Microbial degradation in water converts 6PPD to intermediates and ultimately to 6PPD-Q, highlighting biodegradation’s significance. (C) Soil transformation involves direct photodegradation and microbial action, with sunlight converting 6PPD to intermediates, aided by soil microorganisms. (D) Air-driven reactions in the atmosphere produce hydroxyl radicals and 6PPD-OH, leading to 6PPD-Q formation through UV radiation and gas interactions.
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Figure 3. TWP: a cross-domain analysis of ecological impact.
Figure 3. TWP: a cross-domain analysis of ecological impact.
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Figure 4. Pathways and health impacts of tire wear emissions.
Figure 4. Pathways and health impacts of tire wear emissions.
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Figure 5. Summary of tire chemical toxicity: urban health impacts.
Figure 5. Summary of tire chemical toxicity: urban health impacts.
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Figure 6. Impact of tire wear and urbanization on urban soil microbial communities.
Figure 6. Impact of tire wear and urbanization on urban soil microbial communities.
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Table 1. Summary of 6PPD and 6PPDQ properties.
Table 1. Summary of 6PPD and 6PPDQ properties.
Property/ImpactDescriptionSource
Chemical StructureUrbansci 09 00228 i001
6PPD-Q (298.39)		Urbansci 09 00228 i002
6PPD(268.40)		[27,43]
Chemical Formula6PPD: C18H24N2; 6PPD-Q: C18H20N2O2[27]
Impact on MiceOral exposure to 6PPD and 6PPD-Q causes hepatotoxicity in mice, affecting hepatic metabolism and immune response[44]
Effect on Rubber PropertiesAdding 0.5 wt % 6PPD to rubber improves thermal stability, lowers glass transition temperature, increases ionic conductivity[27]
Toxicity in Aquatic Life6PPD-Q is highly toxic to coho salmon and other fish species, causing acute mortality[27]
Environmental Occurrence6PPD and 6PPD-Q are commonly found in urban watersheds [6,20]
Photodegradation Pathway6PPD undergoes photodegradation to form 6PPD-Q when exposed to sunlight in water[45]
Bioaccumulation Potential6PPD-Q has been shown to bioaccumulate in aquatic organisms, leading to long-term exposure risks[20]
Persistence in Soil6PPD and 6PPD-Q persist in roadside soils, with slow degradation due to low microbial activity[46]
Oxidative Stress Induction6PPD-Q exposure induces oxidative stress and mitochondrial damage in aquatic organisms[39]
Impact on Algae6PPD and 6PPD-Q reduce algal photosynthetic efficiency and impair growth[40]
Effects on AmphibiansExposure to 6PPD-Q disrupts reproductive hormones and impairs development in amphibians[31]
Transformation in Air6PPD can form secondary pollutants like quinones when exposed to atmospheric oxidants[34]
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Ihenetu, S.C.; Xu, Q.; Fang, L.; Azeem, M.; Li, G.; Enyoh, C.E. 6PPD and 6PPD-Quinone in the Urban Environment: Assessing Exposure Pathways and Human Health Risks. Urban Sci. 2025, 9, 228. https://doi.org/10.3390/urbansci9060228

AMA Style

Ihenetu SC, Xu Q, Fang L, Azeem M, Li G, Enyoh CE. 6PPD and 6PPD-Quinone in the Urban Environment: Assessing Exposure Pathways and Human Health Risks. Urban Science. 2025; 9(6):228. https://doi.org/10.3390/urbansci9060228

Chicago/Turabian Style

Ihenetu, Stanley Chukwuemeka, Qiao Xu, Li Fang, Muhamed Azeem, Gang Li, and Christian Ebere Enyoh. 2025. "6PPD and 6PPD-Quinone in the Urban Environment: Assessing Exposure Pathways and Human Health Risks" Urban Science 9, no. 6: 228. https://doi.org/10.3390/urbansci9060228

APA Style

Ihenetu, S. C., Xu, Q., Fang, L., Azeem, M., Li, G., & Enyoh, C. E. (2025). 6PPD and 6PPD-Quinone in the Urban Environment: Assessing Exposure Pathways and Human Health Risks. Urban Science, 9(6), 228. https://doi.org/10.3390/urbansci9060228

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