Transformation Products of Tire Rubber Antioxidant 6PPD in Heterogeneous Gas-Phase Ozonation: Identification and Environmental Occurrence

Environmental fate, toxicity, and mitigation of 6PPD and 6PPD-Quinone: Current understanding and future directions

N'-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely used antioxidant in the rubber industry, has garnered global attention due to the high toxicity and ecological-health risks posed by its environmental oxidation product, 6PPD-quinone (6PPD-Q). With the continuous release of tire wear particles (TWPs), 6PPD-Q is ubiquitously distributed in atmospheric, aquatic, and terrestrial environments, as well as within organisms, where it bioaccumulates through food chains. Notably, 6PPD-Q has been detected in human urine, serum, and cerebrospinal fluid, and its association with abnormal α-synuclein aggregation in the brains of Parkinson's patients further underscores its neurotoxic risks. This review systematically examines the environmental occurrence and migration patterns of 6PPD and 6PPD-Q, their multisystem toxicity, highly sensitive detection technologies, and pollution control strategies, while highlighting critical gaps in current research, such as chronic exposure mechanisms, combined pollution effects, and environmental safety thresholds. By synthesizing existing knowledge, this review provides a scientific foundation for elucidating the ecological and health risks of 6PPD-Q, offering critical insights to advance environmental regulatory policies, promote green transformation in the rubber industry, and safeguard global ecological security. Future research should prioritize long-term toxicity studies, refined detection techniques, and sustainable regulatory frameworks to mitigate the ecological and health risks posed by these emerging contaminants.

Diurnal impacts of tire wear and waste burning on fine particulate matter concentrations in a metropolitan area

The ecological toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), an antioxidant used in tires, has been investigated in studies of fine particulate matter (PM2.5) related to public health. Using samples collected in 3-h intervals from January to March 2024 (as part of the Asia-AQ campaign in Northeast Asia.), this study identifies 6PPD-quinone (6PPD-Q) along with terephthalic acid from waste polyethylene terephthalate using liquid chromatography-time-of-flight tandem mass spectrometry. The vehicle-tire wear (2 %) and tailpipe emissions (6 %), as well as wood burning (19 %) and waste food burning (9 %) were identified in PM2.5. Overall, waste burning contributed 28 % to PM2.5. Other identified sources included secondary sulfate and secondary nitrate. 6PPD-Q from tire wear exhibited concentrations peaking during daytime. Titanium emissions, used as a proxy for brake wear, were found to contain redox-active metals for 6PPD-Q. Terephthalic acid, levoglucosan, and cholesterol displayed strong diurnal patterns, with significant nighttime contributions from waste burning. These findings emphasize the substantial impact of primary sources on PM2.5 concentrations. Additionally, they underscore the importance of high temporal resolution for accurate source identification and highlight the urgent need to address tire- and waste-related pollution in urban environments.

A comprehensive reconnaissance and risk assessment of rubber additives and their transformation products (RATPs) in groundwater: 1,3-Diphenylguanidine (DPG) as a pressing ecological concern

The widespread detection of rubber additives and their transformation products (RATPs) in surface water environments is well-documented, but their pollution characteristics in groundwater remain unclear. This study comprehensively revealed the occurrence and distribution of 27 RATPs in groundwater across southern China (n = 212). RATPs were detected in groundwater at total levels of 1.21-2,345 ng/L. The primary compounds detected were 1,3-Diphenylguanidine (DPG), 1,3-diphenylurea (DPU), and 2-hydroxybenzothiazole (2-OH-BTH), each with a detection frequency of 99.5 % and mean concentrations of 125, 58.4, and 51.2 ng/L, respectively. The spatial distribution of RATPs in groundwater shows significant lateral variations but lacks vertical differences. Correlation analysis indicates a strong relationship between the RATPs pollution levels and both the type of groundwater and the level of urban economic development, with karst water exhibiting particularly high pollution levels. Five RATPs exhibited medium to high ecological risks in groundwater. The daily intake of RATPs via groundwater in South China is 3.61 × 10-8-7.00 × 10-5 mg/(kg·d). According to the multicriteria evaluation approach and persistence, mobility, and toxicity (PMT) assessment, six RATPs, including DPG, have been identified as high-priority pollutants that require significant attention in groundwater management. This study highlights the contamination characteristics and ecological risks associated with RATPs in groundwater, emphasizing the need for increased focus on these widely used yet inadequately evaluated chemicals in future research.

Tire Rubber Antioxidant 6PPD and 6PPD-quinone Disrupt the Energy Supply and Lipid Metabolism of Earthworms

With the increase in traffic due to urbanization, tire wear particles (TWPs) derived compounds persistently accumulate in the soil environment. This study addresses critical knowledge gaps regarding the ecotoxicological effects of TWP-derived contaminants, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its precursor, 6PPD-quinone (6PPD-Q), on soil-dwelling organisms. The findings demonstrated that 6PPD-Q accumulated at a higher concentration (6.77 ± 0.124 ng/g) in earthworms (Eisenia fetida) compared to 6PPD (5.41 ± 0.002 ng/g), triggering more severe oxidative stress and cellular homeostatic imbalance. Specifically, 100 ng/g of 6PPD-Q in soil significantly elevated reactive oxygen species (ROS) levels by 180.77% and suppressed acetylcholinesterase (AchE) and Ca2+-ATPase activities by 17.14% and 44.70%, respectively. Notably, 6PPD-Q uniquely disrupted the nitrogen balance and disturbed energy supply by strongly inhibiting fatty acid degradation and peroxisome proliferator-activated receptor (PPAR) signaling pathways. Additionally, 6PPD-Q profoundly altered the abundance of key microbes and microbial network stability, affecting intestinal microbial functions such as bile secretion, hormone synthesis, and lipid digestion, thus exacerbating the energy metabolic imbalance in earthworms. This study deciphers the molecular toxicity mechanisms of TWP-derived contaminants in earthworms, providing crucial insights for developing risk assessment frameworks and mitigation strategies in soil ecosystems.

Enantioselectivity in human urinary excretion of N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine (6PPD) and 6PPD-quinone

Human exposure to of N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine (6PPD) has raised global concerns due to its documented toxic effects, including hepatotoxicity, metabolic disruption, and potential contributions to organ damage. 6PPD-quinone (6PPD-Q), a ubiquitous transformation product of 6PPD, has been identified as a major toxicant linked to acute mortality in aquatic species, underscoring its ecological and human health risks. While these compounds exist as enantiomers with demonstrated differences in environmental behavior and toxicity, their enantiomer-specific distribution and excretion patterns in humans remain unknown limiting accurate risk assessments. This study analyzed 6PPD and 6PPD-Q in 109 pairs of whole blood and urine from general Chinese adults at the enantiomer-specific level. Results showed that 6PPD (range < LOD-0.60 ng/mL) and 6PPD-Q ( Collapse

Key Words

• Enantioselectivity
• Human behavior
• Human exposure
• Renal clearance
• Rubber additives

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Affiliation(s)

Hanfeng Chen Department of Neurology, The First Affiliated Hospital, School of Medicine, Zhejiang University, Hangzhou, Zhejiang, 324000, PR China
Hangbiao Jin Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang, 310014, PR China
Fangfang Ren Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang, 310014, PR China
Ruyue Guo Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang, 310014, PR China
Jianqiang Zhu Department of Environmental Engineering, Taizhou University, Taizhou, Zhejiang, 318000, PR China
Kaiyuan Huang Department of Neurosurgery, The First Affiliated Hospital, School of Medicine, Zhejiang University, Zhejiang, 310003, PR China.

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The Invisible Footprint of Climbing Shoes: High Exposure to Rubber Additives in Indoor Facilities

There is growing concern about rubber-derived compounds (RDCs), predominantly originating from tire and road wear particles. Other consumer products, including sports equipment, also contain RDCs, and human exposure to these compounds is of particular interest due to demonstrated toxicity to animal species. In this study, we investigated RDCs intentionally incorporated into climbing shoes for enhanced performance. We found high concentrations of 15 RDCs in shoe sole samples (Σ15 RDCs: 25-3405 μg/g), aerosol particulate matter (Σ15 RDCs: 2.6-37 μg/g), and settled dust (Σ15 RDCs: 1.5-55 μg/g) in indoor climbing halls. The estimated daily intake via inhalation/ingestion of Σ15 RDCs for climbers and employees in some of these facilities ranged from 1.7 to 48 ng/kg/day, exceeding known intake levels of RDCs from other sources. Abrasion powder resulting from friction between climbing shoes and footholds is the likeliest source of high concentrations of RDCs observed in aerosol particulate matter and settled dust. These findings reveal a previously unknown human exposure route of RDCs.

A Rapid Testing Kit for 6PPD Screening in Rubber Products

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) has received increasing attention due to its ubiquitous environmental occurrence and extreme aquatic toxicity of its oxidation product 6PPD-quinone. Given 6PPD's application as an antioxidant in a wide array of rubber products, cost-effective detection of 6PPD is important for product and waste management. We herein developed a rapid testing kit for the detection (<10 min) of 6PPD in rubber products with low costs (<$1/sample), which was inspired by the observation of a red-colored product after 6PPD exposure to oxidants. The selectivity of this reaction was evaluated by 14 structurally diverse phenols and anilines. Interestingly, the protonated N-1,3-dimethyl butyl-N'-phenyl quinone diimine (6QDI+H+), rather than its neutral or radical counterparts, appeared to be the red-colored product with λmax = 490 nm. We further developed a "green" and user-friendly 6PPD rapid testing kit, which was further applied to a wide array of 50 rubber products. 6PPD was detected in 22 out of 50 samples, with a strong agreement (R2 = 0.896) observed between the results of the rapid testing kit and LC-MS. In summary, this study introduces a 6PPD rapid testing kit for users without requiring expertise in chemistry.

p-Phenylenediamines and their derived quinones: A review of their environmental fate, human exposure, and biological toxicity

p-Phenylenediamines (PPDs) are widely used as antioxidants in numerous rubber products to prevent or delay oxidation and corrosion. However, their derived quinones (PPD-Qs), generated through reactions with ozone, are ubiquitous in the environment and raise significant health and toxicity concerns. This review summarizes the current state of knowledge on environmental distribution and fate, human exposure, and biological toxicity of PPDs and PPD-Qs, and makes recommendations for future research directions. Although PPDs and PPD-Qs have been monitored in a variety of environmental matrices, studies on soil, sediment, and organisms remain limited. This shortcoming hinders our understanding of their distribution patterns and migration mechanisms in these specific environments. These contaminants can enter the human body through various exposure routes, but toxicological studies have not yielded sufficient results to derive risk thresholds for the assessment of human health. Most studies examining biological and toxicological effects have focused on acute exposure scenarios, which do not accurately reflect the long-term interactions that occur in natural settings. The toxic effects of PPDs and PPD-Qs on zebrafish, nematodes, and mammals include neurobehavioral changes, reproductive dysfunction, and digestive damage, which are linked to mitochondrial stress, DNA adduct formation, and disrupted lipid metabolism, respectively. However, the underlying toxicological mechanisms remain poorly understood. Future research should prioritize the investigation of the impacts of PPDs and PPD-Qs on various organizational levels within biota to provide a scientific basis for developing effective risk management measures.

Development of a quantitative analytical method for 6PPD, a harmful tire antioxidant, in biological samples for toxicity assessment

This study developed and validated a high-sensitivity analytical method for the quantification of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a hazardous antioxidant used in tires, to support the effective management of its environmental exposure. Using an HPLC-UV system with acetonitrile and deionized water as the mobile phase, the method did not require salts or acids. The calibration curve exhibited excellent linearity (R2 = 0.9999) with detection limits of 0.17 pg μL-1 (LOD) and 5.51 pg μL-1 (MDL), underscoring its ultra-trace detection capability. A pretreatment protocol was developed to quantify 6PPD in mouse liver samples. The protocol achieved a relative recovery rate of 71.89 % and high precision across all spiking concentrations (mean RSD = 2.20 ± 0.92 %). The validated methods were applied to analyze 6PPD residues in mouse liver following intratracheal instillation exposure. At 9 h post-exposure, residual 6PPD concentrations increased significantly in a dose-dependent manner (R2 = 0.9906), with a relative distribution of 0.06 ± 0.01 %. This cost-effective and reliable method provides a practical tool for quantifying 6PPD in environmental and biological matrices. The findings enhance the understanding 6PPD exposure dynamics and support the establishment of regulatory guidelines for managing its ecological and human health risks.

Acute Toxicity of 4-hydroxydiphenylamine (4-HDPA) and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPDQ), transformation products of 6PPD, to early instars of the mayfly, Neocloeon triangulifer

Our analysis of water samples collected during a rain event from two urban rivers in the Greater Toronto area, Ontario, Canada, indicated that selected transformation products (TPs) of the tire antioxidant, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), including 6PPD-quinone (6PPDQ) and 4-hydroxydiphenylamine (4-HDPA), were present at concentrations >1 µg/L. In acute (96-hr) toxicity tests with aquatic larvae of the mayfly Neocloeon triangulifer, 6PPDQ did not cause mortalities at the highest test concentration, which was just below the limit of solubility. In toxicity tests with 4-HDPA, a calculated 96-hr median lethal concentration of 339 µg/L is above environmentally relevant concentrations. However, in toxicity tests with 6PPD, there was evidence that the degradation of this tire wear compound produced unknown TPs that caused mortalities in exposed mayfly larvae. Additional work could identify other TPs of 6PPD that could be a hazard to aquatic invertebrates exposed to tire wear compounds transported into surface waters from transportation corridors.

Synthesis and Toxicity Evaluation of p-Phenylenediamine-Quinones

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), the tire rubber-derived transformation product of 6PPD, was recently discovered to cause the acute mortality of coho salmon (Oncorhynchus kisutch). Aiming to identify potential replacement antiozonants for 6PPD that do not produce toxic quinones, seven PPD-quinones with distinct side chains were synthesized to investigate their structure-related toxicities in vivo using rainbow trout (Oncorhynchus mykiss). While 6PPD-Q exerted high toxicity (96 h LC50 = 0.35 μg/L), toxicity was not observed for six other PPD-quinones despite their similar structures. The fish tissue concentrations of 6PPD-Q after sublethal exposure (0.29 μg/L) were comparable to the other PPD-quinones, which indicated that bioaccumulation levels were not the reason for the selective toxicity of 6PPD-Q. Hydroxylated PPD-quinones were detected as the predominant metabolites in fish tissue. Interestingly, a single major aromatic hydroxylation metabolite was detected for the alternate PPD-quinones, but two abundant OH-6PPD-Q isomers were detected for 6PPD-Q. MS2 spectra confirmed that hydroxylation occurred on the alkyl side chain for one isomer. The structurally selective toxicity of 6PPD-Q was also observed in a coho salmon (CSE-119) cell line, which further supports its intrinsic toxicity. This study reported the selective toxicity of 6PPD-Q and pinpointed the possibility for other PPDs to be applied as potential substitutes of 6PPD.

Environmental factors ultraviolet a and ozone exacerbate the repeated inhalation toxicity of 6PPD in mice via accelerating the aging reaction

The burden of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidized products on human health can no longer be ignored due to the detection types and concentrations in the environment continue to increase. Environmental ozone (O3) and ultraviolet A (UVA) may induce ozonation and photoaging of 6PPD to produce toxic products. However, the impact of specific environmental conditions on the aging and toxic effects of 6PPD is unclear. This study investigated the aging effects of O3 and UVA on 6PPD, and compared the repeated inhalation toxicity of differently aging 6PPDs in C57BL/6 male mice. The result showed that UVA and O3 accelerated 6PPD aging, and the aging products varied depending on the O3 and irradiation conditions. After 10 weeks of inhalation intoxication at human comparable level, mice exhibited significant neurobehavior alterations, respiratory dysfunction, and DNA damage in the blood, showing significant heterogeneity among groups. Notably, 6PPD treated with perozonation and UVA aging may be the most toxic. The study suggests inhalation health risks of transportation derived tire pollutants under the influence of ground-level ozone and ultraviolet light need more attention, and provides new insights into risk assessment and pollution control of 6PPD and other pollutants from the perspective of environmental factors.

A Nationwide Investigation of Substituted p-Phenylenediamines (PPDs) and PPD-Quinones in the Riverine Waters of China

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) has been identified as the cause of the "urban runoff mortality syndrome." Thus, the ecological risks of substituted p-phenylenediamines (PPDs) and their quinone derivatives (PPD-Qs) in water have gained global attention. However, large-scale observation of their pollution characteristics in surface water is still lacking. Herein, a nationwide investigation revealed the pervasive occurrence of PPDs and PPD-Qs in riverine waters across China, with the mean concentration of ∑5PPD-Qs being 4.9 times higher than their parent ∑5PPDs. Notably, the 6PPD-Q concentrations at eight sampling sites exceeded the median lethal concentration for coho salmon. National annual riverine fluxes were estimated at 113.1 and 276.2 tonnes/year for PPDs and PPD-Qs, respectively, with the Yangtze River contributing more than one-third of the total fluxes. The transformation of PPDs to PPD-Qs was dependent on atmospheric hydroxylation rates and the half-lives of PPDs. A combined multimedia exposure assessment revealed that water exposure accounted for 82.5% of human exposure to PPDs and PPD-Qs, surpassing the contributions from dust and air exposure. This study provides a comprehensive spatial picture of PPDs and PPD-Qs in China. The national atlas highlights their potential ecological risks and implies that targeted actions should be taken to mitigate potential exposure to PPDs and PPD-Qs.

Chemical Dissipation from Outdoor Plastics: The Significant Impact of Transformation Processes Revealed by Adjusted Mass Transfer Modeling

Chemicals in plastics raise significant concerns for potential adverse environmental and health impacts. However, dissipation kinetics and fluxes of chemicals from outdoor plastic products remain largely uncharacterized, hindering the accurate assessment of their environmental exposure. This study quantified outdoor dissipation profiles for 20 "priority" chemicals, including sunscreens (benzophenone, benzophenone-3, octyl salicylate, etc.), phthalates, benzotriazole ultraviolet stabilizers (UV-P, UV-326, UV-327, etc.), and polycyclic aromatic hydrocarbons (PAHs), from 3 types of plastic protective nets over 180 days. Results revealed that levels of sunscreens and PAHs decreased to less than 50% of their initial concentrations within 5 days. Adjusted mass transfer modeling by integrating transformation processes well reproduced the dissipation kinetics (median determination coefficients R2 = 0.82) and revealed synchronous release and transformation in the dissipation. Average dissipation half-lives of the chemicals were 19.7 days and positively correlated with the number of electron lone pairs and carbon atoms in the molecules. Unexpectedly, the transformation contributed 82.1% to the total chemical dissipation, especially for di(2-ethylhexyl) phthalate (DEHP) and UV-326. Suspect screening of formed products in PPNs suggested photolysis, ozone, and hydroxyl radical-induced oxidation as the primary transformation processes, supported by a high concentration of 2-ethylhexyl benzoate product comparable to its parent DEHP. These insights underscore the pivotal role of transformation in chemical dissipation from plastics, informing more precise emission assessment of chemicals and future efforts toward developing environmentally benign plastics.

Hydrolysis of p-Phenylenediamine Antioxidants: The Reaction Mechanism, Prediction Model, and Potential Impact on Aquatic Toxicity

While p-phenylenediamine antioxidants (PPDs) pose potential risks to aquatic ecosystems, their environmental persistence and transformation remain ambiguous due to the undefined nature of PPD C-N bond hydrolysis. Here, we investigated the hydrolysis patterns of PPDs by analyzing their hydrolysis half-lives, hydrolysis products around neutral pH (pH 6.0-7.7), and the role of atoms within the C-N bonds in PPDs. Hydrolysis preferentially targets the aromatic secondary amine N with the strongest proton affinity and the C atom of C-N with the highest nucleophilic-attack reactivity. The hydrolysis half-life (t1/2) shortens when the maximum proton affinity of N increases. These results are supported by theoretical calculations, demonstrating a hydrolysis reaction propelled by proton transfer from water to N and complemented by aromatic nucleophilic substitution of N in C-N by water hydroxyl. With the experimental results and the atom reactivity-based predictive model, the t1/2 around neutral pH for 60 PPDs (monitored in environment, commercially available, or under investigation) is determined, showing variations ranging from 2.2 h to 47 days. The model prediction of primary C-N hydrolysis is confirmed through typical PPDs. With the elucidated mechanism and developed model, this research provides new insights into PPD hydrolysis, underscoring its significance in delineating environmental impacts.

High-resolution observation per 1.5 h revealed prominent time-dependent daily contamination variations of p-phenylenediamine (PPD) antioxidants and their quinone derivatives PPDQs in PM2.5 from central China

p-Phenylenediamine (PPD) antioxidants and their quinone derivatives (PPDQs), as hot-spot novel contaminants in recent years, have been detected in air fine particulate matters (PM2.5) in multiple regions. However, current research all discussed the pollution of PPDs and PPDQs based on the collected PM2.5 samples at least in one day (23.5 h). In this work, an innovative study was conducted to investigate their precise daily pollution characteristics and health risks based on high-resolution collected PM2.5 samples every 1.5 hours in one week in Zhengzhou, a megacity continuously suffers from serious air contamination in central China. The composition patterns and sources of PPDs and PPDQs were discovered and more serious contamination of them were both found at the day-time (07:00-19:00). Almost all daily time intervals with relatively high pollution levels of PPDs and PPDQs were commuting time for majority of workers, which was possibly because of increased rubber tire abrasion induced by huge traffic volume. The highest daily adults' inhalation risks were also found in 11:30-13:00 and 16:00-17:30 for PPDs and PPDQs in PM2.5, respectively. Current study builds the relationship between pollution status of such novel pollutants and human activities, and possibly guides people in central China to take precautions to protect themselves from environmental toxicants at special daily time.

Environmental occurrence, fate, human exposure, and human health risks of p-phenylenediamines and their quinones

P-phenylenediamine antioxidants (PPDs) are widely used in the rubber industry and their release and transformation in the environment has become one of the current environmental research hotspots. PPDs are readily oxidized in the environment to form quinone transformation products (PPD-Qs), some of which (e.g. 6PPD-Q) have been shown to be highly toxic and persistent in the environment, posing a potential threat to aquatic organisms and ecosystems. The present study provides an overview of the physicochemical properties, environmental distribution, and potential human exposure and toxicological effects of PPDs and PPD-Qs. PPDs and PPD-Qs are found in water, air, dust and soil around the world, and humans are inevitably exposed to them by inhaling, ingesting and through dermal contact. There is growing evidence indicates that PPDs and PPD-Qs are present in human body fluids and tissues, where they are subject to metabolic and transformational processes in the liver and blood. Furthermore, PPDs and PPD-Qs have the potential to induce adverse health effects, including digestive, respiratory, neurotoxic and reproductive toxicity. Nevertheless, there is a paucity of evidence concerning the direct effects of PPDs and PPD-Qs on human health. Consequently, future research should concentrate on this area in order to provide quantitative support for the assessment of the risk posed by PPDs and PPD-Qs to human health.

Intensive Spatiotemporal Characterization of the Tire Wear Toxin 6PPD Quinone in Urban Waters

6PPDQ is a tire-derived contaminant toxic to coho salmon (LC50 = 41-95 ng/L) found widely distributed in urban environments. Most monitoring efforts have relied on relatively few discrete samples collected at select locations across rain events. Early work has revealed that 6PPDQ concentrations vary widely over time and space, raising questions about when and where to collect samples. Here, we employ condensed phase membrane introduction mass spectrometry, a high-throughput analysis approach to characterize spatiotemporal variability of 6PPDQ in urban streams. Analytical method detection limits of 0.3-8 ng/L and a duty cycle of 2.5 min/sample enabled high-throughput adaptive sampling. Temporal sampling revealed dynamic 6PPDQ concentrations, with significant changes occurring over minutes during peak rainfall. Spatial variability was characterized at multiple sites along three watercourses during the first significant precipitation of autumn 2023 on central Vancouver Island, B.C., Canada. Site-specific concentrations suggest attenuation of 6PPDQ after point source inputs by some combination of physical (dilution, sorption) or chemical (degradation) processes. This is the first report of an intensive sampling campaign describing the spatiotemporal distribution of 6PPDQ, highlighting the need for careful consideration of sampling strategies to evaluate the risk and impact of 6PPDQ in urban waterways.

Structurally Selective Ozonolysis of p-Phenylenediamines and Toxicity in Coho Salmon and Rainbow Trout

The tire-rubber-derived ozonation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), was recently discovered to cause acute mortality in coho salmon (Oncorhynchus kisutch). para-Phenylenediamines (PPDs) with variable side chains distinct from 6PPD have been identified as potential replacement antioxidants, but their toxicities remain unclear under environmentally relevant ozone conditions. We herein tested the multiphase gas-surface ozone reactivity of four select PPDs [6PPD, N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), N,N'-diphenyl-p-phenylenediamine (DPPD), and N-phenyl-N'-cyclohexyl-p-phenylenediamine (CPPD)] and evaluated the toxicity of their reaction mixtures in coho salmon, rainbow trout (Oncorhynchus mykiss), and fathead minnow (Pimephales promelas). 6PPD and IPPD were found to rapidly react with ozone, while no significant multiphase ozone reactivity was observed for DPPD or CPPD. The viability of coho salmon CSE-119 cells was strongly affected by the ozonolysis products of 6PPD but not by those of the other three PPDs. Acute mortality was only observed in juvenile rainbow trout that were exposed to oxidized 6PPD, suggesting a common mechanism of toxic action in the two salmonid fish species. This study reports the structurally selective ozone reactivity of PPDs and the unique toxicity of 6PPD ozonolysis mixtures, which demonstrates that other PPDs are potential alternative antioxidants.

Occurrence, sources, and human exposure assessment of amine-based rubber additives in dust from various micro-environments in South China

Despite the ubiquitous use and potential health effects of amine-based rubber additives, information regarding their occurrences in indoor environments remains scarce and is basically investigated in traffic-related environments. In this study, a total of 140 dust samples collected from eight indoor micro-environments were analyzed for twelve amine-based rubber additives. Overall, 1,3-diphenylguanidine (DPG), dicyclohexylamine (DCHA), N-(1,3-dimethylbutyl)-N'-phenyl-p-penylenediamine (6PPD), 6PPD-quinone (6PPDQ), and hexa(methoxymethyl)melamine (HMMM) were frequently detected across all micro-environments with detection frequencies of 97 %, 51 %, 71 %, 99 %, and 77 %, respectively. The highest total concentration of amine-based rubber additives was found in parking lots (median 10,300 ng/g), indicating heavier emission sources of these compounds in vehicle-related indoor environments. Despite this, amine-based rubber additives were also frequently detected in various non-vehicle-related environments, such as markets, cinemas, and hotels, probably due to the widespread use of consumer products and more frequent air exchanges with outdoor environments. Further tracking of tire rubber products and paint particles from flooring materials in parking lots revealed that paint particles might be an overlooked contributor to amine-based rubber additives in indoor environments. Finally, the highest estimated daily intakes (EDIs) of all amine-based rubber additives via dust ingestion at home were observed for toddlers (3.48 ng/kg bw/d). This research provides a comprehensive overview of human exposure to a variety of amine-based rubber additives in various indoor environments. ENVIRONMENTAL IMPLICATION: This study highlights the presence of high concentrations of amine-based additives in indoor dust from both traffic-related and non-traffic-related indoor environments. Additional efforts are needed to identify potential sources of amine-based rubber additives indoors, beyond just tire rubber. This is critical because the widespread presence of rubber products in indoor settings could pose a risk to human health.

Unveiling the environmental impact of tire wear particles and the associated contaminants: A comprehensive review of environmental and health risk

This review offers a novel perspective on the environmental fate and ecotoxicological effects of tire wear particles (TWPs), ubiquitous environmental contaminants ranging in size from micrometers to millimeters (averaging 10-100 micrometers). These particles pose a growing threat due to their complex chemical composition and potential toxicity. Human exposure primarily occurs through inhalation, ingesting contaminated food and water, and dermal contact. Our review delves into the dynamic interplay between TWP composition, transformation products (TPs), and ecological impacts, highlighting the importance of considering both individual chemical effects and potential synergistic interactions. Notably, our investigation reveals that degradation products of certain chemicals, such as diphenylguanidine (DPG) and diphenylamine (DPA), can be more toxic than the parent compounds, underscoring the need to fully understand these contaminants' environmental profile. Furthermore, we explore the potential human health implications of TWPs, emphasizing the need for further research on potential respiratory, cardiovascular, and endocrine disturbances. Addressing the challenges in characterizing TWPs, assessing their environmental fate, and understanding their potential health risks requires a multidisciplinary approach. Future research should prioritize standardized TWP characterization and leachate analysis methods, conduct field studies to enhance ecological realism, and utilize advanced analytical techniques to decipher complex mixture interactions and identify key toxicants. By addressing these challenges, we can better mitigate the environmental and health risks associated with TWPs and ensure a more sustainable future.

Evidence for the formation of 6PPD-quinone from antioxidant 6PPD by cytochrome P450

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) as a rubber antioxidant has attracted global concern, since its ozone-oxidation product 6PPD-quinone (6PPDQ) was found to be the primary toxicant responsible for urban runoff mortality syndrome in coho salmon. However, the biotransformation fate and associated toxicological mechanism of 6PPD have not received much study yet. In this work, the in vitro assays showed 6PPD can be transformed into 6PPDQ by cytochromes P450 (CYP450) in human liver microsomes (HLMs) with 0.98 % production rate, and the adducts of 6PPDQ with calf thymus DNA and the N-N coupling product between 6PPD and 6PPDQ were further identified after 6PPD incubation in HLMs. Further evidence for the 6PPDQ formation can be obtained from the in vivo assays that the 6PPDQ-DNA adducts and 6PPD-N-N-6PPDQ dimer were detected in mice by oral gavage with 6PPD, and the latter dimer species was detected as well in 6PPD exposure to zebrafish larvae. Especially, the bioaccumulation property and high reactivity of 6PPDQ result in the continuous formation of the significant DNA adducts and 6PPD-N-N-6PPDQ dimer even in case of low production rate of biotransformation of 6PPD to 6PPDQ, which may provide potentially effective biomarkers for such process. DFT computations revealed the formation mechanism of 6PPDQ is the (N)H-abstraction of 6PPD by CYP450, followed by amino radical rebound at the nearby ortho-carbon, yielding a quinol intermediate due to spin delocalization, that might readily undergo further oxidation by CYP450 into 6PPDQ.

Unveiling spatiotemporal distribution, partitioning, and transport mechanisms of tire additives and their transformation products in a highly urbanized estuarine region

Numerous tire additives are high-production volume chemicals that are used extensively worldwide. However, their presence and partitioning behavior remain largely unknown, particularly in marine environments. This study is the first to reveal the spatiotemporal distribution, multimedia partitioning, and transport processing of 22 tire additives and their transformation products (TATPs) in a highly urbanized estuary (n = 166). Nineteen, 18, and 20 TATPs were detectable in water, suspended particulate matter (SPM), and sediments, respectively, with total levels of 59.7-2021 ng/L, 164-6935 ng/g, and 4.66-58.4 ng/g, respectively. The multimedia partitioning mechanisms of TATPs are governed by their molecular weight, hydrophobicity, and biodegradation rate. Mass inventories coupled with model simulations have revealed that substantial quantities of TATPs accumulate within estuarine environments, and these compounds can be continuously transported into the ocean, particularly during the wet season. According to the multi-criteria evaluation approach, four and three TATPs were identified as high-priority pollutants during the dry and wet seasons, respectively. Unexpectedly, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone was only listed as a medium-priority pollutant. This study underscores the importance of marine surveillance and advocates for particular attention to these ubiquitous but underexplored TATPs in future studies.

Presence of N, N'-substituted p-phenylenediamine quinones in Tap Water: Implication for human exposure

Monitoring studies have demonstrated the wide presence of N, N'-substituted p-phenylenediamine-derived quinones (PPDQs) in environmental matrices. The general population may be potentially exposed to PPDQs through the consumption of tap water. While, the existence of PPDQs in tap water has not been well examined. To fill this gap, in this study we collected tap water samples from Hangzhou, China, and examined seven homologues of PPDQs in collected samples. All target PPDQs were identified in the collected tap water samples, with distinct detection frequencies (38-89%). PPDQs detected in tap water was dominated by N-(1, 3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPDQ; mean 0.56 ng/L, < LOD-4.0 ng/L). The profiles of PPDQs concentrations in tap water from the four districts of Hangzhou city were slightly different. The daily intake (DI) was found highest for 6PPDQ (mean 14-22 pg/kg bw/day, median 10-15 pg/kg bw/day) through tap water intake. The relatively higher DIs of various PPDQs were displayed for infants (mean 10-22 pg/kg bw/day, median 6.5-15 pg/kg bw/day), relative to the children (8.0-18 pg/kg bw/day, 5.4-12 pg/kg bw/day) and adults (6.7-14 pg/kg bw/day, 4.5-10 pg/kg bw/day). These data are crucial for assessing the overall human exposure to PPDQs. This study first, to our knowledge, reveals the concentrations and profiles of PPDQs in tap water.

Development and application of diffusive gradients in thin-films for in-situ monitoring of 6PPD-Quinone in urban waters

The occurrence and risk of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), derived from the oxidation of the tire antidegradant 6PPD, has raised significant concern since it was found to cause acute mortality in coho salmon when exposed to urban runoff. Given the short half-life period and low solubility of 6PPD-Q, reliable in situ measurement techniques are required to accurately understand its occurrence and behaviour in aquatic environments. Here, using the diffusive gradients in thin-films (DGT) method with HLB as a binding agent, we developed a new methodology to measure 6PPD-Q in urban waters. 6PPD-Q was rapidly and strongly adsorbed on the HLB-binding gel and was efficiently extracted using organic solvents. The HLB-DGT accumulated 6PPD-Q linearly for >7 d and its performance was not significantly affected by pH (6.5-8.5), ionic strength (0.0001-0.5 M) or dissolved organic matter (0-20 mg L-1). Field evaluation of the DGT method demonstrated its effectiveness in urban runoff, detecting 6PPD-Q levels of 15.8-39.5 ng L-1 in rivers. In snowmelt, DGT detected 6PPD-Q levels of 210 ng L-1 which is two times higher than the value obtained by grab sampling. 6PPD-Q levels were much higher in snowmelt than those in rivers. This indicates that snowfall constitutes an important transport pathway for 6PPD-Q and that DGT effectively captured the fraction continuously released from dust particles in the snow samples. 6PPD-Q posed a substantial risk to migratory fish in urban waters, and its release from tire wear particles requires further investigation. This study is the first to develop a DGT-based method for 6PPD-Q determination in urban waters, and the method can ensure an accurate measurement of the release of 6PPD-Q to the environment, particularly in rainfall or snowmelt, important pathways for its entry into the aquatic environment.

Formation of Nitrosamines from the Heterogeneous Reaction of Nitrous Acid and Organic Amines in Indoor Environments

Carcinogenic nitrosamines have been widely studied due to their risk to human health. However, the universality and evolutionary processes of their generation, particularly concerning their secondary sources, remain unclear at present. We demonstrated through laboratory flow tube experiments that corresponding nitrosamines were generated from heterogeneous reactions of nitrous acid (HONO) with five structurally diverse amines commonly found indoors, including diphenylamine (DPhA), dibenzylamine (DBzA), dioctylamine (DOtA), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), and N-phenyl-1-naphthylamine (PANA). The heterogeneous reaction rate constants of DBzA and DOtA with HONO (∼70 ppb) were 1.21 × 10-3 and 2.13 × 10-3 min-1 at 30% relative humidity (RH), resulting in a lifetime of 13.8 and 7.8 h. As compared to higher RH (∼80%), more nitrosamines were produced from the reaction of HONO with surface-sorbed DBzA, DOtA, 6PPD, and PANA at lower RH (30%), with product yields ranging from <0.1% to 0.5%. Furthermore, we observed the formation of nitroso-6PPDs and nitro-6PPDs during room air exposure of 6PPD in a genuine indoor environment, in addition to various other transformation products indicative of reactions of 6PPD with HONO, NOx, and ozone indoors. This study confirmed the universality of the heterogeneous reaction of surface-sorbed amine with HONO as a source of nitrosamines indoors.

Typical Tire Additives in River Water: Leaching, Transformation, and Environmental Risk Assessment

Tire wear particles (TWPs) released during vehicle driving can enter water bodies, leading to leaching of tire additives (TAs) in aquatic environments. However, the transformation behavior and related ecological impacts of TAs and their transformation products (TPs) remain unclear. In this study, laboratory-based simulation experiments and field investigations were conducted to explore the transformation mechanisms and ecological risks of TAs. After being placed in river water for 24 h, about 7-95% of 12 investigated TAs in TWPs were leached. Forty-eight TPs from eight TAs were tentatively identified along with different transformation pathways via suspect screening by high-resolution mass spectrometry. Semiquantitative results indicated that TPs derived from N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene-diamine (6PPD) were predominant in leachates, while aryl hydrolysis and quinone pathways were the main transformation pathways. Field investigations on urban surface water samples from 16 sites in Hong Kong revealed the occurrence of 17 TAs and 1 TP, with concentrations ranging from 13.9 to 2230 ng/L (median ± standard deviation: 226 ± 534 ng/L). Sixteen TPs from six TAs were additionally identified via suspect screening. It is estimated that 6PPD-quinone and seven TAs could pose medium to high ecological risk, while N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine, a frequently detected TP, was identified as a persistent-bioaccumulative-toxic substance.

P-phenylenediamine antioxidants and their quinone derivatives: A review of their environmental occurrence, accessibility, potential toxicity, and human exposure

Substituted p-phenylenediamines (PPDs), a class of antioxidants, have been widely used to extend the lifespan of rubber products, such as tires and pipes. During use, PPDs will generate their quinone derivatives (PPD-Qs). In recent years, PPDs and PPD-Qs have been detected in the global environment. Among them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), the oxidation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), has been identified as highly toxic to coho salmon, with the lethal concentration of 50 % (LC50) being 95 ng/L, highlighting it as an emerging pollutant of great concern. This review summarizes the physicochemical properties, global environmental distribution, bioaccessibility, potential toxicity, human exposure risk, and green measures of PPDs and PPD-Qs. These chemicals exhibit lipophilicity, bioaccumulation potential, and poor aqueous stability. They have been found in water, air, dust, soil, and sediment worldwide, indicating their significance as emerging pollutants. Notably, current studies have identified electronic waste (e-waste), such as discarded wires and cables, as a non-negligible source of PPDs and PPD-Qs, in addition to tire wear. PPDs and PPD-Qs exhibit strong bioaccumulation in aquatic organisms and mammals, with a tendency for biomagnification within the food web, posing health threats to humans. Available toxicity data indicate that PPDs and PPD-Qs have negative effects on aquatic organisms, mammals, and invertebrates. Acute exposure leads to death and acute damage, and long-term exposure can cause a series of adverse effects, including growth and development toxicity, reproductive toxicity, neurotoxicity, intestinal toxicity, and multi-organ damage. This paper discusses current research gaps and offers recommendations to understand better the occurrence, behavior, toxicity, and environmental exposure risks of PPDs and PPD-Qs.

Emerging investigator series: in-depth chemical profiling of tire and artificial turf crumb rubber: aging, transformation products, and transport pathways

Crumb rubber generated from end-of-life tires (ELTs) poses a threat to environmental and human health based on its widespread use. Of particular concern is the use of ELT crumb rubber as infill for artificial turf fields, as people are unknowingly exposed to complex mixtures of chemicals when playing on these fields. Additionally, there is concern regarding transport of rubber-related chemicals from artificial turf into the environment. However, existing knowledge does not fully elucidate the chemical profile, transformation products, and transport pathways of artificial turf crumb rubber across different ages. To address these knowledge gaps, we utilized a multi-faceted approach that consisted of targeted quantitation, chemical profiling, and suspect screening via ultra-high performance liquid chromatography-high-resolution mass spectrometry (UHPLC-HRMS). We collected and processed 3 tire and 11 artificial turf crumb rubber samples via solvent extraction, leaching, and a bioaccessibility-based extraction. Nineteen rubber-derived chemicals were quantified using parallel reaction monitoring and isotope dilution techniques. In solvent extracts, the most abundant analytes were 1,3-diphenylguanidine (0.18-1200 μg g-1), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD, 0.16-720 μg g-1), 2-mercaptobenzothiazole (0.47-140 μg g-1), and benzothiazole (0.84-150 μg g-1). Chemical profiling assessed changes in sample diversity, abundance, polarity, and molecular mass. Suspect screening identified 81 compounds with different confidence levels (16 at level 1, 53 with level 2, 7 at level 3, and 5 at level 4). The formation rate of transformation products and clustering analysis results identified time-based trends in artificial turf field samples. We found that the first two years of aging may be critical for the potential environmental impact of artificial turf fields. Our analysis provides insight into the chemical complexity of artificial turf crumb rubber samples ranging from 0-14 years in age.

Neurotoxicity from long-term exposure to 6-PPDQ: Recent advances

The recent acceleration of industrialization and urbanization has brought significant attention to N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), an emerging environmental pollutant from tire wear, due to its long-term effects on the environment and organisms. Recent studies suggest that 6-PPDQ can disrupt neurotransmitter synthesis and release, impact receptor function, and alter signaling pathways, potentially causing oxidative stress, inflammation, and apoptosis. This review investigates the potential neurotoxic effects of prolonged 6-PPDQ exposure, the mechanisms underlying its cytotoxicity, and the associated health risks. We emphasize the need for future research, including precise exposure assessments, identification of individual differences, and development of risk assessments and intervention strategies. This article provides a comprehensive overview of 6-PPDQ's behavior, impact, and neurotoxicity in the environment, highlighting key areas and challenges for future research.

Detection of 6-PPD and 6-PPDQ in airborne particulates and assessment of their toxicity in lung cells

The extensive use of synthetic antioxidants, notably N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD), in rubber-related products, particularly in tire manufacturing, has induced concerns regarding their environmental impact and potential health hazards. Despite the identification of 6-PPD and its derivative, 6-PPD quinone (6-PPDQ), in various water samples and their lethal effects on certain aquatic species (e.g., coho salmon, rainbow trout and brook trout), the levels of airborne 6-PPD/6-PPDQ and their respiratory toxicity remain relatively unexplored. In this study, we aimed to evaluate the respiratory toxicity potential of 6-PPD and its derivatives, with a specific focus on detecting these compounds in airborne particulates and assessing their toxic effects on lung cells. Characterization of four airborne fine particulate (FP) samples revealed spherical morphologies with diameters ranging from 17.7 to 225.7 nm, displaying slight agglomeration and negative surface charge. methanol/acetonitrile extraction followed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) analysis confirmed the presence of both 6-PPD and 6-PPDQ on the surfaces of FPs, with significant variations (0.26-1.05 μg g-1) in loading capacity observed among the samples. Subsequent exposure of lung cells (THP-1, BEAS-2B, and A549) to 6-PPD and 6-PPDQ revealed dose-dependent declines in mitochondrial metabolic activity induced by 6-PPD, along with severe membrane damage, ATP depletion, and pro-inflammatory cytokine release. Conversely, 6-PPDQ exhibited negligible toxicity in all tested parameters. These findings underscore the potential health risks associated with airborne 6-PPD exposure and emphasize the importance of further research into the respiratory toxicity of 6-PPD derivatives.

Estrogenic, Genotoxic, and Antibacterial Effects of Chemicals from Cryogenically Milled Tire Tread

Tire and road wear particles (TRWP) contain complex mixtures of chemicals and release them to the environment, and potential toxic effects of these chemicals still need to be characterized. We used a standardized surrogate for TRWP, cryogenically milled tire tread (CMTT), to isolate and evaluate effects of tire-associated chemicals. We examined organic chemical mixtures extracted and leached from CMTT for the toxicity endpoints genotoxicity, estrogenicity, and inhibition of bacterial luminescence. The bioassays were performed after chromatographic separation on high-performance thin-layer chromatography (HPTLC) plates. Extracts of CMTT were active in all three HPTLC bioassays with two estrogenic zones, two genotoxic zones, and two zones inhibiting bacterial luminescence. Extracts of CMTT artificially aged with thermooxidation were equally bioactive in each HPTLC bioassay. Two types of aqueous leachates of unaged CMTT, simulating either digestion by fish or contact with sediment and water, contained estrogenic chemicals and inhibitors of bacterial luminescence with similar profiles to those of CMTT extracts. Of 11 tested tire-associated chemicals, two were estrogenic, three were genotoxic, and several inhibited bacterial luminescence. 1,3-Diphenylguanidine, transformation products of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, and benzothiazoles were especially implicated through comparison to HPTLC retention factors in the CMTT samples. Other bioactive bands in CMTT samples did not correspond to any target chemicals. Tire particles clearly contain and can leach complex mixtures of toxic chemicals to the environment. Although some known chemicals contribute to estrogenic, genotoxic, and antibacterial hazards, unidentified toxic chemicals are still present and deserve further investigation. Overall, our study expands the understanding of potential adverse effects from tire particles and helps improve the link between those effects and the responsible chemicals. Environ Toxicol Chem 2024;43:1962-1972. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Tire-derived contaminants 6PPD and 6PPD-Q: Analysis, sample handling, and reconnaissance of United States stream exposures

The environmental ubiquity of tire and road wear particles (TRWP) underscores the need to understand the occurrence, persistence, and environmental effects of tire-related chemicals in aquatic ecosystems. One such chemical is 6PPD-quinone (6PPD-Q), a transformation product of the tire antioxidant 6PPD. In urban stormwater runoff 6PPD-Q can exceed acute toxicity thresholds for several salmonid species and is being implicated in significant coho salmon losses in the Pacific Northwest. There is a critical need to understand the prevalence of 6PPD-Q across watersheds to identify habitats heavily affected by TRWPs. We conducted a reconnaissance of 6PPD and 6PPD-Q in surface waters across the United States from sites (N = 94) with varying land use (urban, agricultural, and forested) and streamflow to better understand stream exposures. A rapid, low-volume direct-inject, liquid chromatography mass spectrometry method was developed for the quantitation of 6PPD-Q and screening for 6PPD. Laboratory holding times, bottle material, headspace, and filter materials were investigated to inform best practices for 6PPD-Q sampling and analysis. Glass bottles with PTFE-lined caps minimized sorption and borosilicate glass fiber filters provided the highest recovery. 6PPD-Q was stable for at least 5 months in pure laboratory solutions and for 75 days at 5 °C with minimal headspace in the investigated surface water and stormwaters. Results also indicated samples can be frozen to extend holding times. 6PPD was not detected in any of the 526 analyzed samples and there were no detections of 6PPD-Q at agricultural or forested sites. 6PPD-Q was frequently detected in stormwater (57%, N = 90) and from urban impacted sites (45%, N = 276) with concentrations ranging from 0.002 to 0.29 μg/L. The highest concentrations, above the lethal level for coho salmon, occurred during stormwater runoff events. This highlights the importance of capturing episodic runoff events in urban areas near ecologically relevant habitat or nursery grounds for sensitive species.

Distinct Species-Specific and Toxigenic Metabolic Profiles for 6PPD and 6PPD Quinone by P450 Enzymes: Insights from In Vitro and In Silico Studies

The tire rubber antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone product (6PPDQ) are prevalent emerging contaminants, yet their biotransformation profiles remain poorly understood, hampering the assessment of environmental and health risks. This study investigated the phase-I metabolism of 6PPD and 6PPDQ across aquatic and mammalian species through in vitro liver microsome (LM) incubations and in silico simulations. A total of 40 metabolites from seven pathways were identified using the highly sensitive nano-electrospray ionization mass spectrometry. Notably, 6PPDQ was consistently detected as a 6PPD metabolite with an approximate 2% yield, highlighting biotransformation as a neglected indirect exposure pathway for 6PPDQ in organisms. 6PPDQ was calculated to form through a facile two-step phenyl hydroxylation of 6PPD, catalyzed by cytochrome P450 enzymes. Distinct species-specific metabolic kinetics were observed, with fish LM demonstrating retarded biotransformation rates for 6PPD and 6PPDQ compared to mammalian LM, suggesting the vulnerability of aquatic vertebrates to these contaminants. Intriguingly, two novel coupled metabolites were identified for 6PPD, which were predicted to exhibit elevated toxicity compared to 6PPDQ and result from C-N oxidative coupling by P450s. These unveiled metabolic profiles offer valuable insights for the risk assessment of 6PPD and 6PPDQ, which may inform future studies and regulatory actions.

Effects of environmental concentrations of 6PPD and its quinone metabolite on the growth and reproduction of freshwater cladoceran

The widespread occurrence and accumulation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite, 6PPD quinone (6PPD-Q), have been globally recognized as a critical environmental issue. However, knowledge on the adverse effects of 6PPD and 6PPD-Q on freshwater invertebrates is limited. This study investigated the effects of 6PPD and its oxidative byproduct, 6PPD-Q, on the growth and reproduction of Daphnia pulex. Through 21-day exposure experiments, we measured the uptake of 0.1, 1, and 10 μg/L 6PPD and 6PPD-Q by D. pulex and assessed the effects on growth and fecundity of D. pulex. While 6PPD and 6PPD-Q did not affect the mortality rate of D. pulex, 6PPD-Q exposure inhibited the growth of D. pulex, indicating potential ecological risks. In particular, the reproductive capacity of D. pulex remained unaffected across the tested concentrations of 6PPD and 6PPD-Q, suggesting specific toxicological pathways that warrant further investigation. This study underscored the importance of evaluating the sublethal effects of emerging contaminants such as 6PPD and 6PPD-Q on aquatic invertebrates, and highlighted the need for comprehensive risk assessments to better understand their environmental impacts.

High-Resolution Mass Spectrometry for Human Exposomics: Expanding Chemical Space Coverage

In the modern "omics" era, measurement of the human exposome is a critical missing link between genetic drivers and disease outcomes. High-resolution mass spectrometry (HRMS), routinely used in proteomics and metabolomics, has emerged as a leading technology to broadly profile chemical exposure agents and related biomolecules for accurate mass measurement, high sensitivity, rapid data acquisition, and increased resolution of chemical space. Non-targeted approaches are increasingly accessible, supporting a shift from conventional hypothesis-driven, quantitation-centric targeted analyses toward data-driven, hypothesis-generating chemical exposome-wide profiling. However, HRMS-based exposomics encounters unique challenges. New analytical and computational infrastructures are needed to expand the analysis coverage through streamlined, scalable, and harmonized workflows and data pipelines that permit longitudinal chemical exposome tracking, retrospective validation, and multi-omics integration for meaningful health-oriented inferences. In this article, we survey the literature on state-of-the-art HRMS-based technologies, review current analytical workflows and informatic pipelines, and provide an up-to-date reference on exposomic approaches for chemists, toxicologists, epidemiologists, care providers, and stakeholders in health sciences and medicine. We propose efforts to benchmark fit-for-purpose platforms for expanding coverage of chemical space, including gas/liquid chromatography-HRMS (GC-HRMS and LC-HRMS), and discuss opportunities, challenges, and strategies to advance the burgeoning field of the exposome.

Environmental fate of tire-rubber related pollutants 6PPD and 6PPD-Q: A Review

To enhance tire durability, the antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is used in rubber, but it converts into the toxic 6PPD quinone (6PPD-Q) when exposed to oxidants like ozone (O3), causing ecological concerns. This review synthesizes the existing data to assess the transformation, bioavailability, and potential hazards of two tire-derived pollutants 6PPD and 6PPD-Q. The comparative analysis of different thermal methods utilized in repurposing waste materials like tires and plastics into valuable products are analyzed. These methods shed light on the aspects of pyrolysis and catalytic conversion processes, providing valuable perspectives into optimizing the waste valorization and mitigating environmental impacts. Furthermore, we have examined the bioavailability and potential hazards of chemicals used in tire manufacturing, based on the literature included in this review. The bioavailability of these chemicals, particularly the transformation of 6PPD to 6PPD-Q, poses significant ecological risks. 6PPD-Q is highly bioavailable in aquatic environments, indicating its potential for widespread ecological harm. The persistence and mobility of 6PPD-Q in the environment, along with its toxicological effects, highlight the critical need for ongoing monitoring and the development of effective mitigation strategies to reduce its impact on both human health and ecosystem. Future research should focus on understanding the chronic effects of low-level exposure to these compounds on both terrestrial and aquatic ecosystems, as well as the potential for bioaccumulation in the food chain. Additionally, this review outlines the knowledge gaps, recommending further research into the toxicity of tire-derived pollutants in organisms and the health implications for humans and ecosystems.

Direct and indirect photodegradation in aquatic systems mitigates photosensitized toxicity in screening-level substance risk assessments of selected petrochemical structures

Photochemical processes are typically not incorporated in screening-level substance risk assessments due to the complexity of modeling sunlight co-exposures and resulting interactions on environmental fate and effects. However, for many substances, sunlight exerts a profound influence on environmental degradation rates and ecotoxicities. Recent modeling advances provide an improved technical basis for estimating the effect of sunlight in modulating both substance exposure and toxicity in the aquatic environment. Screening model simulations were performed for 25 petrochemical structures with varied uses and environmental fate properties. Model predictions were evaluated by comparing the ratios of predicted exposure concentrations with and without light to the corresponding ratios of toxicity thresholds under the same conditions. The relative ratios of exposure and hazard in light vs. dark were then used to evaluate how inclusion of light modulates substance risk analysis. Results indicated that inclusion of light reduced PECs by factors ranging from 1.1- to 63-fold as a result of photodegradation, while reducing PNECs by factors ranging from 1- to 49-fold due to photoenhanced toxicity caused by photosensitization. Consequently, the presence of light altered risk quotients by factors that ranged from 0.1- to 17-fold, since the predicted increase in substance hazard was mitigated by the reduction in exposure. For many structures, indirect photodegradation decreases environmental exposures independently of the direct photolysis pathway which is associated with enhanced phototoxicity. For most of the scenarios and chemicals in the present work, photosensitization appears to be mitigated by direct and indirect degradation from sunlight exposure.

Leaching hazards of tire wear particles in hydrothermal treatment of sludge: Exploring molecular composition, transformation mechanism, and ecological effects of tire wear particle-derived compounds

Tire wear particles (TWPs) are considered a significant contributor of microplastics (MPs) in the sludge during heavy rainfall events. Numerous studies have shown that hydrothermal treatment (HT) of sludge can accelerate the leaching of MP-derived compound into hydrothermal liquid, thus impairing the performance of subsequent anaerobic digestion and the quality of the hydrothermal liquid fertilizer. However, the leaching behavior of TWPs in the HT of sludge remains inadequately explored. This study examined the molecular composition of TWP-derived compounds and transformation pathways of representative tire-related additives under different hydrothermal temperatures using liquid chromatography-tandem mass spectrometry (LC-MS/MS) combined with mass difference analysis. The acute toxicity and phytotoxicity of TWP leachates were assessed using Vibrio qinghaiensis Q67 and rice hydroponics experiments. The results indicated that elevating the hydrothermal temperature not only amplified the leaching behavior of TWPs but also enhanced the chemical complexity of the TWP leachate. Utilizing both suspect and non-target screenings, a total of 144 compounds were identified as additives, including N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PDD), hexa(methoxymethyl)melamine (HMMM), dibutyl phthalate (DBP). These additives underwent various reactions, such as desaturation, acetylation, and other reactions, leading to the formation of different transformation products (TPs). Moreover, certain additives, including caprolactam and 2,2,6,6-tetramethyl-4-piperidinol, demonstrated the potential to form conjugate products with amino acids or Maillard products. Meanwhile, TWP-derived compounds showed significant acute toxicity and detrimental effects on plant growth. This study systematically investigated the environmental fate of TWPs and their derived compounds during the HT of sludge, offering novel insights into the intricate interactions between the micropollutants and dissolved organic matter (DOM) in sludge.

Navigating the environmental dynamics, toxicity to aquatic organisms and human associated risks of an emerging tire wear contaminant 6PPD quinone

N-1,3-Dimethylbutyl-N'-phenyl-p-quinone diamine (6PPDQ) is a derivative of 6PPD, a synthetic antioxidant used in tire manufacturing to control the degradation caused by oxidation and heat aging. Its discovery in 2020 has raised important environmental concern, particularly regarding its association with acute mortality in coho salmon, prompting surge in research on its occurrence, fate, and transport in aquatic ecosystems. Despite this attention, there remain notable gaps in grasping the knowledge, demanding an in depth overview. Thus, this review consolidates recent studies to offer a thorough investigation of 6PPDQ's environmental dynamics, pathways into aquatic ecosystems, toxicity to aquatic organisms, and human health implications. Various aquatic species exhibit differential susceptibility to 6PPDQ toxicity, manifesting in acute mortalities, disruption of metabolic pathways, oxidative stress, behavioral responses, and developmental abnormalities. Whereas, understanding the species-specific responses, molecular mechanisms, and broader ecological implications requires further investigation across disciplines such as ecotoxicology, molecular biology, and environmental chemistry. Integration of findings emphasizes the complexity of 6PPDQ toxicity and its potential risks to human health. However, urgent priorities should be given to the measures like long-term monitoring studies to evaluate the chronic effects on aquatic ecosystems and the establishment of standardized toxicity testing protocols to ensure the result comparability and reproducibility. This review serves as a vital resource for researchers, policymakers, and environmental professionals seeking appraisals into the impacts of 6PPDQ contamination on aquatic ecosystems and human health.

Spatiotemporal variation of 6PPD and 6PPDQ in dust and soil from e-waste recycling areas

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its derivative 6PPDQ have been detected in various environmental media, with harmful consequences for both ecosystems and biological health. However, the distribution of 6PPD and 6PPDQ in areas around e-waste recycling areas is currently unknown. We collected soil and dust samples from areas around a traditional e-waste recycling zone, an emerging recycling park, and a reference area. Higher levels of 6PPD were found in dust from residential areas around the traditional e-waste recycling zone compared to the reference area (median: 108.99 versus 33.57 ng/g, P < 0.01). Lower levels of 6PPDQ were detected in dust samples from around the emerging e-waste recycling parks compared to traditional e-waste recycling zones (median: 15.40 versus 46.37 ng/g, P < 0.05). The median concentrations of 6PPD and 6PPDQ were higher in the dust samples than in the soil samples (P < 0.001). The concentrations of 6PPD and 6PPDQ in the dust and soil varied seasonally, with the highest total concentrations occurring in the winter. Results from a multiple linear regression analysis indicate that 6PPDQ is negatively correlated with temperature and positively correlated with 6PPD, O3, and radiation. This study confirms that e-waste is a potential contributor to 6PPD and 6PPDQ. In residential areas, 6PPD and 6PPDQ are more likely to accumulate in dust than in soil. The emerging e-waste recycling parks have greatly improved the local 6PPDQ pollution situation. Further studies are necessary to understand the distribution of newly found substances in various settings.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Association between 6PPD-quinone exposure and BMI, influenza, and diarrhea in children

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) has received extensive attention due to its ubiquitous distribution and potential toxicity. However, the distribution characteristics of 6PPD-quinone in dust from e-waste recycling areas and the consequential health risks to children are unclear. A total of 183 dust samples were collected from roads (n = 40), homes (n = 91), and kindergartens (n = 52) in Guiyu (the e-waste-exposed group) and Haojiang (the reference group) from 2019 to 2021. The results show that the concentrations of 6PPD-quinone in kindergarten and house dust from the exposed group were significantly higher than those from the reference group (P < 0.001). These findings show that e-waste may be another potential source of 6PPD-quinone, in addition to rubber tires. The exposure risk of 6PPD-quinone in children was assessed using their daily intake. The daily intake of 925 kindergarten children was calculated using the concentration of 6PPD-quinone in kindergarten dust. The daily intake of 6PPD-quinone via ingestion was approximately five orders of magnitude higher than via inhalation. Children in the exposed group had a higher exposure risk to 6PPD-quinone than the reference group. A higher daily intake of 6PPD-quinone from kindergarten dust was associated with a lower BMI and a higher frequency of influenza and diarrhea in children. This study reports the distribution of 6PPD-quinone in an e-waste recycling town and explores the associated health risks to children.

Aromatic amine antioxidants (AAs) and p-phenylenediamines-quinones (PPD-Qs) in e-waste recycling industry park: Occupational exposure and liver X receptors (LXRs) disruption potential

Recently, evidence of aromatic amine antioxidants (AAs) existence in the dust of the electronic waste (e-waste) dismantling area has been exposed. However, there are limited studies investigating occupational exposure and toxicity associated with AAs and their transformation products (p-phenylenediamines-quinones, i.e., PPD-Qs). In this study, 115 dust and 42 hand wipe samples collected from an e-waste recycling industrial park in central China were analyzed for 19 AAs and 6 PPD-Qs. Notably, the median concentration of ∑6PPD-Qs (1,110 ng/g and 1,970 ng/m2) was significantly higher (p < 0.05, Mann-Whitney U test) than that of ∑6PPDs (147 ng/g and 34.0 ng/m2) in dust and hand wipes. Among the detected analytes, 4-phenylaminodiphenylamine quinone (DPPD-Q) (median: 781 ng/g) and 1,4-Bis(2-naphthylamino) benzene quinone (DNPD-Q) (median: 156 ng/g), were particularly prominent, which were first detected in the e-waste dismantling area. Occupational exposure assessments and nuclear receptor interference ability, conducted through estimated daily intake (EDI) and molecular docking analysis, respectively, indicated significant occupational exposure to PPD-Qs and suggested prioritized Liver X receptors (LXRs) disruption potential of PPDs and PPD-Qs. The study provides the first evidence of considerable levels of AAs and PPD-Qs in the e-waste-related hand wipe samples and underscores the importance of assessing occupational exposure and associated toxicity effects.

Insight into chemically reactive metabolites of aliphatic amine pollutants: A de novo prediction strategy and case study of sertraline

The uncommon metabolic pathways of organic pollutants are easily overlooked, potentially leading to idiosyncratic toxicity. Prediction of their biotransformation associated with the toxic effects is the very purpose that this work focuses, to develop a de novo method to mechanistically predict the reactive toxicity pathways of uncommon metabolites from start aliphatic amine molecules, which employed sertraline triggered by CYP450 enzymes as a model system, as there are growing concerns about the effects on human health posed by antidepressants in the aquatic environment. This de novo prediction strategy combines computational and experimental methods, involving DFT calculations upon sequential growth, in vitro and in vivo assays, dissecting chemically reactive mechanism relevant to toxicity, and rationalizing the fundamental factors. Significantly, desaturation and debenzylation-aromatization as the emerging metabolic pathways of sertraline have been elucidated, with the detection of DNA adducts of oxaziridine metabolite in mice, highlighting the potential reactive toxicity. Molecular orbital analysis supports the reactivity preference for toxicological-relevant C-N desaturation over N-hydroxylation of sertraline, possibly extended to several other aliphatic amines based on the Bell-Evans-Polanyi principle. It was further validated toward some other wide-concerned aliphatic amine pollutants involving atrazine, ε-caprolactam, 6PPD via in silico and in vitro assays, thereby constituting a complete path for de novo prediction from case study to general applications.

Bioretention Design Modifications Increase the Simulated Capture of Hydrophobic and Hydrophilic Trace Organic Compounds

Stormwater rapidly moves trace organic contaminants (TrOCs) from the built environment to the aquatic environment. Bioretention cells reduce loadings of some TrOCs, but they struggle with hydrophilic compounds. Herein, we assessed the potential to enhance TrOC removal via changes in bioretention system design by simulating the fate of seven high-priority stormwater TrOCs (e.g., PFOA, 6PPD-quinone, PAHs) with log KOC values between -1.5 and 6.74 in a bioretention cell. We evaluated eight design and management interventions for three illustrative use cases representing a highway, a residential area, and an airport. We suggest two metrics of performance: mass advected to the sewer network, which poses an acute risk to aquatic ecosystems, and total mass advected from the system, which poses a longer-term risk for persistent compounds. The optimized designs for each use case reduced effluent loadings of all but the most polar compound (PFOA) to <5% of influent mass. Our results suggest that having the largest possible system area allowed bioretention systems to provide benefits during larger events, which improved performance for all compounds. To improve performance for the most hydrophilic TrOCs, an amendment like biochar was necessary; field-scale research is needed to confirm this result. Our results showed that changing the design of bioretention systems can allow them to effectively capture TrOCs with a wide range of physicochemical properties, protecting human health and aquatic species from chemical impacts.

Polyethylene nanoparticles at environmentally relevant concentrations enhances neurotoxicity and accumulation of 6-PPD quinone in Caenorhabditis elegans

The exposure risk of 6-PPD quinone (6-PPDQ) has aroused increasing concern. In the natural environment, 6-PPDQ could interact with other pollutants, posing more severe environmental problems and toxicity to organisms. We here examined the effect of polyethylene nanoplastic (PE-NP) on 6-PPDQ neurotoxicity and the underling mechanisms in Caenorhabditis elegans. In nematodes, PE-NP (1 and 10 μg/L) decreased locomotion behavior, but did not affect development of D-type neurons. Exposure to PE-NP (1 and 10 μg/L) strengthened neurotoxicity of 6-PPDQ (10 μg/L) on the aspect of locomotion and neurodegeneration induction of D-type motor neurons. Exposure to PE-NPs (10 μg/L) caused increase in expressions of mec-4, asp-3, and asp-4 governing neurodegeneration in 10 μg/L 6-PPDQ exposed nematodes. Moreover, exposure to PE-NP (10 μg/L) increased expression of some neuronal genes (daf-7, dbl-1, jnk-1, and mpk-1) in 6-PPDQ exposed nematodes, and RNAi of these genes resulted in susceptibility to neurotoxicity of PE-NP and 6-PPDQ. 6-PPDQ could be adsorbed by PE-NPs, and resuspension of PE-NP and 6-PPDQ after adsorption equilibrium exhibited similar neurotoxicity to co-exposure of PE-NP and 6-PPDQ. In addition, exposure to PE-NP (1 and 10 μg/L) increased 6-PPDQ accumulation in body of nematodes and increased defecation cycle length in 6-PPDQ exposed nematodes. Therefore, 6-PPDQ could be adsorbed on nanoplastics (such as PE-NPs) and enhance both neurotoxicity and accumulation of 6-PPDQ in organisms.

Ubiquitous occurrence of p-Phenylenediamine (PPD) antioxidants and PPD-quinones in fresh atmospheric snow and their amplification effects on associated aqueous contamination

p-Phenylenediamine (PPD) antioxidants are heavily used for protection of commercial rubber products (e.g., vehicle tire), resulting in their widespread contamination in ecosystem. PPD-quinones (PPDQs), the toxic quinone derivatives of PPDs, are also discovered as novel environmental pollutants. However, the contamination characteristics of PPDs/PPDQs in fresh atmospheric snow (without deposition on the Earth surface) have seldom been studied. This work first reports the broad distributions of PPDs and PPDQs in fresh atmospheric snow collected from seven Chinese urban areas. Individual median values of detected concentrations were in the ranges of 0.4 to 260 pg g-1 (PPDs) and 0.7 to 104 pg g-1 (PPDQs). The concentration deviation by long-term deposition on the ground was eliminated. In most sampling regions, wearing of vehicle rubber tires was possibly responsible for spatial-dependent PPDs' pollution level variations, and high concentrations of PPDs promoted PPDQs' formation in snow from atmosphere. Yet, excessive O3 may further oxidize and reduce PPDQs in atmospheric fresh snow from Zhengzhou, which is different from previous research. Furthermore, snowfall was noticed might amplify concentrations of three PPDs and PPDQs in an inland lake, which possibly worsen corresponding pollution in water system. Current study elucidates the potential impacts of snow-bound PPDs/PPDQs on ecosystems should not be underestimated.

Transformation of 6PPDQ during disinfection: Kinetics, products, and eco-toxicity assessment

N-phenyl-N'-(1,3-dimethyl butyl)-p-phenylenediamine-quinone (6PPDQ) currently arouses broad concerns because of its acute lethality to coho salmon and rainbow trout at environmentally relevant concentrations and the wide occurrence in runoff-impacted water. Investigation on the fate and transformation of 6PPDQ in various treatment processes is necessary for its risk assessment and control. Here, we explored the transformation of 6PPDQ during disinfection with its precursor 6PPD as a reference, focusing on kinetics, products, and toxicity variation. 6PPDQ readily reacted with hypochlorite and chlorine dioxide with second-order rate constants of 2580 ± 143 M-1 s-1 and 614 ± 52 M-1 s-1 (pH 7.0 and 25 °C), which are slightly lower than the reactions of 6PPD. We tentatively identified thirteen transformation products for 6PPDQ and eight for 6PPD in reaction with the two disinfectants. It seems that the quinone ring of 6PPDQ and the p-phenylenediamine moiety of 6PPD are reactive sites. The transformation of these compounds probably proceeds through Cl-substitution, ring cleavage, hydroxylation, and amine oxidation and hydrolysis. Tests with zebrafish embryos revealed that the transformation products of 6PPDQ could have higher eco-toxicity than the parent compound, while the toxicity of the 6PPD products remained nearly unchanged. The increased toxicity of 6PPDQ during disinfection highlights the necessity to substantially reduce its content before the disinfection of runoff-impacted water.

Understanding the importance of atmospheric transformation in assessing the hazards of liquid crystal monomers

Liquid crystal monomers (LCMs), a group of synthetic chemicals released from liquid crystal devices such as televisions and smartphones, have recently been recognized as emerging contaminants due to their widespread occurrence in the environment and potential negative impacts on human health. Airborne LCMs can undergo atmospheric oxidation reactions to form various transformation products. Despite the certainty of atmospheric transformation chemistry, the knowledge about the hazard properties of transformation products remains largely unknown. Here, we perform an in silico model-based evaluation of the persistence, bioaccumulation potential, mobility, and toxicity of two representative LCMs, namely, 1-ethyl-4-(4-(4-propylcyclohexyl)phenyl)benzene and 4''-ethyl-2'-fluoro-4-propyl-1,1':4',1''-terphenyl, and their transformation products. We found that, among the investigated transformation products, 38% have overall persistence greater than the minimum of 331 days among the persistent organic pollutants regulated by the Stockholm Convention, 62% meet the bioaccumulation threshold of 1000 L kg-1 used by the United States Environmental Protection Agency, 44% are classified "mobile" according to the criterion used by the German Environmental Agency, and 58% have the potential to induce unacceptable toxic effects in aquatic organisms. Furthermore, we identified several transformation products with increased persistence, bioaccumulation potential, and mobility compared to their parent compounds. These findings not only offer insights for prioritizing LCM transformation products for future risk assessment, but also underscore the significance of considering atmospheric transformation in the evaluation of environmental risks posed by emerging contaminants, including LCMs.