Occurrence of p-phenylenediamine antioxidants in human urine

Associations of human exposure to 6PPD and 6PPDQ with colorectal cancer: A mixture analysis

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidation product, 6PPD-quinone (6PPDQ), are widely present in the environment. Toxicological studies have demonstrated that they can induce adverse health effects on the intestinal system. However, epidemiological studies examining the association between human 6PPD and 6PPDQ exposure and colorectal cancer (CRC) risk remain scarce. In this study, human urinary 6PPD and 6PPDQ concentrations were analyzed in 329 controls and 367 CRC cases from Quzhou, China. A combination of analyses, including unconditional logistic regression, Bayesian kernel machine regression (BKMR), and restricted cubic spline analysis, was employed to evaluate associations between urinary 6PPD and 6PPDQ levels and CRC risk, adjusting for demographic and lifestyle variables. The median concentration of 6PPDQ in CRC cases (0.94 vs 0.14 μg/g creatinine) was significantly higher than that in controls (Mann-Whitney U test, p = 0.001), while the median concentration of 6PPD showed no significant (p = 0.061) difference between the two groups (0.31 vs 0.38 μg/g creatinine). Higher urinary 6PPDQ concentrations were significantly associated with increased CRC risk, especially among participants with third (adjusted OR = 2.79, 95 % CI: 1.76-4.47; p for trend <0.001) and fourth (adjusted OR = 7.13, 95 % CI: 4.31-12.0; p for trend <0.001) quartiles of exposure. Additionally, the joint effects of 6PPD and 6PPDQ exposure, assessed using the BKMR model, indicated a positive association with CRC risk, suggesting a cumulative risk from co-exposure. This study provides the first epidemiological evidence linking human 6PPDQ exposure to CRC risk, highlighting its potential role in colorectal carcinogenesis.

Occurrence, fate and chiral signatures of p-phenylenediamines and their quinones in wastewater treatment plants, China

p-Phenylenediamines (PPDs) and their derived quinones (PPD-Qs) are emerging pollutants. Information on their occurrence, fate and chiral signatures in the wastewater treatment plant (WWTP) and its receiving waters is scarce. This study explored the distribution, removal efficiency, mass balance, environmental emission and enantiomeric fractions of these pollutants in two WWTPs in Guangzhou, China. It also examined the impacts of WWTP effluents on the distribution of these contaminants in the receiving rivers. The concentrations of Σ(PPDs+PPD-Qs) ranged from 3.95 to 20.4 ng/L in influent, 0.69-3.94 ng/L in effluent, and 2.14-19.8 ng/g dry weight in sludge, respectively. PPDs (81.6 %) and PPD-Qs (78.6 %) were effectively removed in the WWTPs primarily through biotransformation, but sludge adsorption and separation also contributed to the removal. One of the WWTPs could increase the levels of these pollutants in the downstream of receiving river, suggesting that WWTP effluents are significant vectors of PPDs and PPD-Qs to the aquatic environment. The results also highlight different nonracemic chiral signatures of 6PPD and 6PPD-Q between the two WWTPs and the receiving waters, which merit further investigation for mechanism. Future studies should elucidate the environmental risks that PPDs and PPD-Qs may pose to receiving rivers and accurately assess such effects at the enantiomeric level.

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.

Correlation between 6PPD-Q and immune along with metabolic dysregulation induced liver lesions in outdoor workers

Outdoor workers who are exposed to traffic-derived pollutants often suffer from a range of diseases, with liver disease being particularly notable. Recently, a rubber stabilizing additive antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its transformed-quinone product 6PPD-quinone (6PPD-Q) attracted attention. However, their implication for human health remains inadequately elucidated. In this study, outdoor and indoor workers were recruited to analyze 6PPD and 6PPD-Q distribution in their serum and urine. Simultaneously, blood cell counts, liver function, renal function, blood glucose level, and lipid profile were evaluated by 23 physiological parameters. For the first time, we found that the concentrations of 6PPD (0.54 - 1.66 μg L-1) and 6PPD-Q (0.58 - 4.04 μg L-1) in outdoor group serum were two- and three-fold in the indoor group, respectively. Compared with indoor workers, 18 biochemical parameters, notably total bilirubin and indirect bilirubin, were elevated in outdoor workers (p < 0.05). A computed tomography scan showed liver lesions in 60% of the outdoor group, whereas only 30% of the indoor group. The statistical analysis exhibited that significant positive correlations exist between the serum 6PPD-Q and immune cell counts, total bilirubin, indirect bilirubin, and triglycerides in human beings (p < 0.05). The logistic regression implied that for each 1 μg L-1 increase of 6PPD-Q in serum, the risk of human liver lesions increased by 2.31 times. Our results suggest that outdoor exposure is associated with increased concentrations of 6PPD-Q in serum, which could potentially influence glucose and lipid metabolism, immune cell regulation, and liver health.

6PPD induces apoptosis and autophagy in SH-SY5Y cells via ROS-mediated PI3K/AKT/mTOR pathway: In vitro and in silico approaches

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), an extensively employed antioxidant in rubber materials, is considered as an emerging contaminant. 6PPD was proven to have potential neurotoxicity, which poses risks to human health. However, the research on its neurotoxicity is still limited. This work explored the neurotoxicity of 6PPD to SH-SY5Y cells and in-depth mechanisms with a combination of in vitro and in silico approaches. Our results indicated that 6PPD could reduce cell viability and cause oxidative damage by increasing reactive oxygen species (ROS) accumulation and altering the levels of glutathione (GSH), catalase (CAT), superoxide dismutase (SOD), and malondialdehyde (MDA). 6PPD induced neuronal apoptosis of mitochondrial pathway and autophagy dysfunction, as characterized by the increased expressions of Cleaved caspase-3, Bax/Bcl-2, Beclin-1, LC3-II/I, and P62. 6PPD downregulated the expression of PI3K, p-AKT, and p-mTOR, while the PI3K inhibitor suppressed PI3K/AKT/mTOR pathway and promoted both apoptosis and autophagy, indicating that PI3K/AKT/mTOR pathway was involved in 6PPD-induced apoptosis and autophagy. The inhibition of this pathway was attributed to ROS accumulation in SH-SY5Y cells. Molecular docking analysis further revealed that 6PPD exhibits strong binding affinity to PI3K, AKT, and mTOR protein molecules, which could effectively interfere with downstream signaling pathways. These findings enrich the understanding of 6PPD-induced neurotoxicity and contribute to the evaluation of ecological risks associated with 6PPD.

Assessment of tire-derived additives and their metabolites into fruit, root and leafy vegetables and evaluation of dietary intake in Swiss adults

Tire wear particles, released at an estimated 6 million tons annually worldwide, introduce various chemical substances into agricultural environments through atmospheric deposition, road runoff, and reclaimed wastewater. These tire-derived compounds are known to impact ecosystem health. This study investigates the transfer of such additives and their metabolites into vegetables, assessing human dietary intake. Using UPLC-MS/MS, eleven tire-related compounds were analyzed in 100 vegetable samples from nine Swiss retailers, including leafy (lettuce, cabbage, spinach), root (onion, potato, carrot), and fruit (tomato, bell pepper, zucchini, pumpkin) vegetables. Contamination was detected in all vegetable varieties. 31 % of the 100 samples contained benzothiazole (BTH), 1,3-diphenylguanidine (DPG), 6-PPD, or 1,3-dicyclohexylurea (DCU) at levels exceeding the limit of quantification (LOQ) whereas blank values remained below LOD. DPG was most frequently detected (18 %, n = 100), followed by 6-PPD (15 %, n = 100), DCU (10 %, n = 100), and BTH (3 %, n = 100). Spinach comprised 78 % of DPG-positive leafy samples. Daily intakes of 6-PPDQ, DCU, 6-PPD, and DPG from vegetables were estimated at 0-18.7, 0-57.7, 0-42.3, and 0-42.4 ng/person/day, respectively. While current toxicological data suggest no immediate health concerns, significant knowledge gaps remain regarding long-term toxicity. This study offers critical insights into the presence of tire-derived substances in agriculture and underscores the need for further research to better assess environmental and human health risks.

Enhanced residual risk of abamectin induced by 6PPD: in water, soil, and vegetables

Abamectin, one of the most widely used pesticides globally, is known for its effectiveness in protecting crops and animal health. However, the residual risk of abamectin in agricultural products and the environment may be exacerbated by other pollutants, posing greater potential hazards. One such emerging environmental pollutant is N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a common tire antioxidant found in various environments, including agricultural products and human urine. Our study is the first to reveal the co-existence of 6PPD and abamectin in water and soil, and it demonstrated that 6PPD significantly enhances the residual persistence of abamectin in environmental media and vegetables. Specifically, 6PPD extended the half-life of abamectin by 79% in pak choi and by 70% in cabbage. Additionally, 6PPD increased the photolysis half-life of abamectin by 191% in water and by 50% on soil surfaces. Furthermore, 6PPD also prolonged the photolysis half-life of four other macrolides in water. This study reveals the mechanism through which 6PPD extends the half-life of abamectin: by scavenging free radicals and inhibiting hydroxylation and oxidation, thus slowing its degradation. And this paper highlights that 6PPD significantly exacerbates the environmental risks and food safety issue associated with abamectin. Moreover, it provides valuable insights for studying the safe use of pesticides in complex environments with multiple contaminants.

Prediction of p-phenylenediamine antioxidant concentrations in human urine using machine learning models

p-phenylenediamine antioxidants (PPDs) are extensively used in rubber manufacturing for their potent antioxidative properties, but PPDs and 2-anilino-5-[(4-methylpentan-2yl)amino]cyclohexa-2,5-diene-1,4-dione (6PPDQ) pose potential environmental and health risks. Existing biomonitoring methods for assessing human exposure to PPDs are labor-intensive, costly, and provide limited data. Thus, there is a critical need to develop predictive models for evaluating PPDs and 6PPDQ exposure levels to facilitate health risk assessments. In this study, machine learning (ML) models were developed to predict the concentration of three PPDs and 6PPDQ in human urine samples. A total of 759 participants from three cities in Zhejiang Province, China, provided urine samples, which were analyzed for PPDs and 6PPDQ concentrations using liquid chromatography-tandem mass spectrometry. Eight ML models were employed to predict PPDs and 6PPDQ concentrations based on demographic and environmental exposure factors such as age, gender, body mass index (BMI), and occupation. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) was the most frequently detected PPD (mean 3.03 ng/mL, range < LOD-18.65 ng/mL), followed by 6PPDQ (mean 2.76 ng/mL, range < LOD-20.85 ng/mL) and N-phenyl-N'-cyclohexyl-p-phenylenediamine (mean 2.04 ng/mL, range < LOD-10.22 ng/mL). Random forest model demonstrated the highest accuracy in predicting PPDs and 6PPDQ concentrations in human urine among the ML models evaluated. Through the application of these models, age, BMI, and occupation emerged as significant predictors of urinary PPDs and 6PPDQ concentrations. This research significantly contributes by using ML models to enhance exposure assessment accuracy and efficiency, providing a novel framework for future studies on environmental health risks related to PPDs and 6PPDQ.

Human urinary occurrence of thiourea vulcanization accelerators and their human exposure

Thiourea vulcanization accelerators (TVAs) have been detected in various household dust samples, indicating their widespread human exposure. Until now, the occurrence of TVAs in human urine, a suitable matrix for assessing human exposure, has remained unknown. The present study comprehensively examined eight kinds of TVAs in urine samples (n = 277) from participants living in Taizhou, China. A total of eight TVAs were found in these human urine samples, exhibiting the detection frequencies ranging from 13% to 91%, highlighting widespread exposure among the general population. N, N'-ethylenethiourea (ETU; 1.7 ng mL-1, min-max range, < limits of detection (LOD)-13 ng mL-1) exhibited the highest mean human urinary concentration, followed by N, N'-diethylthiourea (DETU; 0.51 ng mL-1, < LOD-3.1 ng mL-1) and N, N'-diphenylthiourea (DPTU; 0.37 ng mL-1, < LOD-0.69 ng mL-1). Gender-specific analysis demonstrated no significant (p > 0.05) distinctions in urinary concentrations of ETU, DETU, and DPTU between males and females. Additionally, a significantly (p < 0.05) negative correlation was observed between urinary ETU levels and age, with younger individuals showing relatively higher urinary concentrations. Human daily exposure (DE) values of ETU, DETU, and DPTU were estimated, primarily using their determined urinary concentrations and the fraction of these TVAs that were excreted in human urine. The mean DE values of ETU, DETU, and DPTU were 99 ng/kg bw/day, 46 ng/kg bw/day, and 19 ng/kg bw/day, respectively. To our knowledge, the present study comprehensively characterized the human urinary occurrence of TVAs for the first time. These obtained data are important for advancing the knowledge on human exposure to these TVAs.

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.

Tire rubber-derived contaminants 6PPD and 6PPD-quinone reduce attachment and outgrowth of trophoblast spheroids onto endometrial epithelial cells

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a synthetic additive widely used in the rubber industry, and its oxidized product 6PPD-quinone (6PPDQ), have garnered widespread attention as an emerging hazardous chemicals owing to their potential detrimental effects on aquatic ecosystem and human health. The effects of 6PPD and 6PPDq on the female reproductive tract, especially embryo implantation, remain unknown and were investigated in this study. We used the spheroid attachment and outgrowth models of BeWo trophoblastic spheroids and Ishikawa cells as surrogates for the human blastocyst and endometrial epithelium, respectively. Treatment with the chemicals for up to 48 h decreased the viability of the cells in a dose- and cell line-dependent manner (20-100 μM 6PPD and 10-100 μM 6PPDQ for both the cell lines). At a noncytotoxic concentration, exposure of Ishikawa cells to 1 and 10 μM 6PPD reduced the attachment of BeWo spheroids and further inhibited their invasion and outgrowth on the endometrial epithelial monolayer. A similar result was observed in 1 μM 6PPDQ-exposed groups. Gene expression profiling of 6PPD- and 6PPDQ-exposed endometrial epithelial cells revealed that both 6PPD and 6PPDQ differentially regulated a panel of transcript markers toward overall downregulation of receptivity and invasion. The study provides the first proof of the adverse effects of 6PPD and 6PPDQ on human endometrial receptivity and trophoblast invasion during the window of implantation, warranting the need for further in vivo and clinical studies.

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.

Rubber additives and relevant oxidation products in groundwater in a central China region: Levels, influencing factors and exposure

This study investigated the presence of rubber additives and relevant oxidation products (RAROPs) in groundwater in central China's aboveground river region. Seven RAROPs were detected, and their levels in shallow groundwater showed a mild decreasing trend from the area near the Yellow River (Avg: 8.49 ng L-1) to the area on the far bank of the Yellow River (Avg: 5.01 ng L-1). In contrast, deep groundwater's RAROPs contents showed a dramatic decrease to only 0.26 ng L-1. The dominant contaminant was found to be N-(1, 3-dimethylbutyl) -N'-phenyl -p-phenylenediamine (6PPD). The vicinity of the garages and car parks was often characterized as contamination hotspots. Correlation analyses further indicated that aquaculture was likely to be a potential pathway for shallow groundwater contaminant inputs. The amount of RAROPs intake by humans through groundwater is nearly 30 times different due to the imbalanced development between urban and rural areas. Children were the most vulnerable to RAROPs. Therefore, human activities (transportation, waste tire storage, water resource allocation and utilization patterns, diversion of Yellow River water to aquaculture ponds) may exacerbate RAROPs pollution in groundwater by leaching contaminants through the surface soil. These results are important for developing appropriate utilization and protection strategies for groundwater resources in developing countries.

Presence of 1, 3-diphenylguanidine and its derivatives in human urine and their human exposure

Recent studies have demonstrated the widespread presence of 1,3-diphenylguanidine (DPG) and its derivatives in environmental matrices. While, the amount of human exposure to these rubber additives remains unclear. In this study, we collected human urine samples from healthy general adults (n = 221) living in Quzhou, China, and analyzed these samples for DPG and its five derivatives. DPG, 1,6-bis(cyano-guanidino)hexane (HCG), 1,3-di-o-tolylguanidine (DTG) and exhibited detection frequencies exceeding 50% in collected human urine. Presence of HCG, 1-(o-tolyl)biguanide (detection frequency 17%), and 1-(4-cyanophenyl)guanidine (6.0%) in human urine was also demonstrated for the first time. The highest mean human urinary concentration was found for DPG (0.89 ng/mL, < LOD-4.7 ng/mL), followed by DTG (0.57 ng/mL, < LOD-3.1 ng/mL) and HCG (0.34 ng/mL, < LOD-1.8 ng/mL). Male participants had consistently higher average human urinary levels of DPG, DTG, and HCG than female subjects, but none of these differences were significant (p > 0.10). DPG and DTQ consistently showed a decline in the human urinary concentrations as age of the participant increased. DPG (mean 170 ng/kg bw/day, median 137 ng/kg bw/day) had the highest human daily exposure amount, followed by DTG (106 ng/kg bw/day, 91 ng/kg bw/day) and HCG (58 ng/kg bw/day, 38 ng/kg bw/day). The study enhances our understanding of human exposure to these rubber additives, which is crucial for assessing their potential health risks.

2-Mercaptobenzothiazole-derived vulcanization accelerators in urine samples from Chinese adults

Studies have discovered wide presence of 2-mercaptobenzothiazole (2-MBT) and 2-MBT-derived vulcanization accelerators (MVAs) in household dust samples, suggesting that these chemicals may have been pervasive in the environment. However, despite the potential for human exposure, the presence of MVAs in human urine, a common matrix used for assessing exposure to environmental chemicals, has not been thoroughly investigated. The current study comprehensively analyzed 11 kinds of MVAs in urine samples from the recruited general population (n = 197) living in Taizhou city, China. Five kinds of MVAs were detectable in >50 % of human urine samples. This indicates the widespread exposure to these vulcanization accelerators among the general population. The predominant target analytes in human urine were 2-MBT and 2,2'-dithiobisbenzothiazole (MBTS), with the mean urinary concentrations of 2.7 ng/mL (range Collapse

Key Words

• 2-MBT
• Human exposuren
• MBTS
• Urinary concentration
• Vulcanization accelerators

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MESH Headings

• Humans
• Benzothiazoles/urine
• China
• Adult
• Male
• Female
• Middle Aged
• Environmental Exposure/analysis
• Environmental Monitoring
• Environmental Pollutants/urine
• East Asian People

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Grants Collapse

Affiliation(s)

Xiaoyu Wu Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou, Zhejiang 318000, PR China
Yingying Zhu School of Life Sciences, Taizhou University, Taizhou, Zhejiang 318000, 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 310032, PR China
Juxiu Huang Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou, Zhejiang 318000, 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 310032, PR China
Lisha Zhou Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou, Zhejiang 318000, PR China.

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Untargeted metabolomic analysis reveals a time-course hepatic metabolism disorder induced by short-term 6PPD exposure in rats

The tire antioxidant 6PPD has garnered extensive attention due to its widespread presence in the environment and the harmful effects of its transformation products on aquatic organisms. 6PPD has been detected in airborne dust, and it can enter mammals through inhalation exposure. While the toxic effects of 6PPD exposure have been reported in mammals, its effects on hepatic metabolism still remain poorly understood. Here, we collected the serum and liver samples at 1, 6, and 72 h following a single oral exposure of 100 mg/kg body weight (bw) 6PPD, respectively. We also investigated changes in serum and hepatic physiological indicators and metabolites, correspondingly. Results indicated that single time oral exposure a high dose of 6PPD did not significantly affect the physiological indexes of rats within a short time frame. However, untargeted metabolomics analysis of the metabolites in the liver at 1, 6, and 72 h revealed that the number of differential expression metabolites gradually increased over time and the most affected substances were lipids and lipid-like molecules. Interestingly, the KEGG pathway enrichment analysis indicated that 6PPD disrupted the riboflavin metabolism, leading to a significant decrease in FMN levels at all time points. In addition, the hepatic glucose metabolism was significantly affected at 6 and 72 h after oral administration. Taken together, short-term exposure to 6PPD disturbed lipid and riboflavin metabolism and gradually affected glucose metabolism in the liver of rats. These findings revealed the impacts of 6PPD on the hepatic metabolism in animals, and also offered some important insights into its toxicology and health risk.

p-phenylenediamines (PPDs) and PPD-quinones (PPD-Qs) in human urine and breast milk samples: Urgent need for focus on PPD-Qs and the establishment of health threshold criteria

PPDs and their oxidation products, PPD-Qs, are emerging environmental contaminants arising from the addition and oxidation of rubber products. Although numerous studies have been conducted to elucidate their risks, the primary focus has been on 6PPD and 6PPD-Q, with limited attention given to other PPDs and especially other PPD-Qs. This study comprehensively examines the occurrences of frequently used PPDs and their degradation products, PPD-Qs, in human urine and breast milk samples. The average concentrations of ΣPPDs and ΣPPD-Qs in urine were 27 ± 78 ng/mL and 16 ± 12 ng/mL, respectively. IPPD and DNPD were the predominant PPDs, while DPPD-Q, CPPD-Q, and IPPD-Q were the predominant PPD-Qs. Notably, the concentrations of 6PPD, CPPD, and DPPD were significantly lower than their oxidized quinone products. Weak or no correlations were observed between most PPDs and their corresponding PPD-Qs, suggesting that PPD-Qs in the human body are primarily derived from direct environmental intake rather than in vivo conversion of PPDs. PPDs and PPD-Qs were widely detected in breast milk, exhibiting concentrations and patterns similar to those found in urine, with comparable major pollutants. Estimated daily intakes of PPDs + PPD-Qs for infants were several μg/(kg·day), with the 95th percentile intake approaching 10 μg/(kg·day).

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.

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.

Characterizing the Metabolism of Tire Rubber-Derived p-Phenylenediamine Quinones to Identify Potential Exposure Biomarkers in Humans

There is growing evidence of the frequent detection of tire rubber-derived contaminants p-phenylenediamine-derived quinones (PPD-Qs) (e.g., highly toxic N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q)) in the environment and biota and the adverse impact on organisms. Hence, a better understanding of their biotransformation/metabolism in humans is essential. However, relevant data are lacking owing to recent discoveries. Herein, the biotransformation patterns of 6PPD-Q and other five commonly detected PPD-Qs were characterized via combined in vitro assay and maternal cord blood screening monitoring. Rapid metabolism was found for each PPD-Q incubated with human liver S9 fraction and microsomes, resulting in the formation of abundant phase I and phase II metabolites. The subsequent screening for potential PPD-Q metabolites in blood samples showed the presence of suspect metabolites. Three detected metabolites were confirmed by matching the mass spectra and retention times of in vitro metabolites. N-Dealkylated, carboxy, carbonyl, and reductive metabolites and glucose, cysteine, and methionine conjugates were observed for the first time. The semiquantitative concentrations of metabolites were higher than those of the parent PPD-Qs, and several metabolites such as carboxy products were proposed as candidate biomarkers of PPD-Q exposure to humans. 6PPD-Q and N,N'-diphenyl-p-phenylenediamine quinone were detected in maternal and/or cord whole blood samples for the first time. This study holds great importance in elucidating the potential risks and health effects of PPD-Qs.

Emerging N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and 6PPD quinone in paired human plasma and urine from Tianjin, China: Preliminary assessment with demographic factors

With increasing concerns about N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and 6PPD-quinone (6PPD-Q), relevant environmental investigations and toxicological research have sprung up in recent years. However, limited information could be found for human body burden assessment. This work collected and analyzed 200 samples consisting of paired urine and plasma samples from participants (50 male and 50 female) in Tianjin, China. Low detection frequencies (DF, <15 %) were found except for urinary 6PPD-Q (86 %), which suggested the poor residue tendency of 6PPD and 6PPD-Q in blood. The low DFs also lead to no substantial association between two chemicals. Data analysis based on urinary 6PPD-Q showed a significant difference between males and females (p < 0.05). No significant correlation was found for other demographic factors (Body Mass Index (BMI), age, drinking, and smoking). The mean values of daily excretion (ng/kg bw/day) calculated using urinary 6PPD-Q for females and males were 7.381 ng/kg bw/day (female) and 3.360 ng/kg bw/day (male), and apparently female suffered higher daily exposure. Further analysis with daily excretion and ALT (alanine aminotransferase)/TSH (thyroid stimulating hormone)/ blood cell analysis indicators found a potential correlation with 6PPD-Q daily excretion and liver/immune functions. Considering this preliminary assessment, systematic research targeting the potential organs at relevant concentrations is required.

Occurrence of bisphenol analogues and their conjugated metabolites in foodstuff

Bisphenol analogues (BPs) are prevalent in diverse foodstuff samples worldwide. However, the occurrence of conjugated bisphenol A (BPA) and bisphenol S (BPS) metabolites in foodstuff remains poorly understood. This study analyzed eight BPs, and four conjugated BPA and BPS metabolites, in three animal-derived foodstuff and five plant-derived foodstuff samples from China. Results showed that fish foodstuff (9.7 ng/g ww) contained the highest mean concentration of BPA, followed by rice (5.1 ng/g ww) and beans foodstuff (3.6 ng/g ww). BPA-sulfate had higher mean concentrations than BPA-glucuronide in different foodstuff categories, except that in eggs foodstuff (p < 0.05). Compared with other foodstuff items, fish (3.4 ng/g ww) and vegetable (1.6 ng/g ww) foodstuff samples exhibited comparatively higher mean concentrations of BPS. Mean concentrations of BPS-sulfate were consistently higher than BPS-glucuronide in vegetables, meats, and fish foodstuff (p < 0.05). BPA contributed the major total dietary intake (DI) of BPs, with the mean DI of 435 ng/kg bw/day for women and 374 ng/kg bw/day for men, respectively. To our knowledge, this study is the first to investigate the occurrence of conjugated BPA and BPS metabolites in foodstuff, which enhances our comprehension of the origins of these conjugated metabolites in the human body.

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.

Comparison of intestinal toxicity in enhancing intestinal permeability and in causing ROS production of six PPD quinones in Caenorhabditis elegans

As the derivatives of p-phenylenediamines (PPDs), PPD quinones (PPDQs) have received increasing attention due to their possible exposure risk. We compared the intestinal toxicity of six PPDQs (6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ and IPPDQ) in Caenorhabditis elegans. In the range of 0.01-10 μg/L, only 77PDQ (10 μg/L) moderately induced the lethality. All the examined PPDQs at 0.01-10 μg/L did not affect intestinal morphology. Different from this, exposure to 6-PPDQ (1-10 μg/L), 77PDQ (0.1-10 μg/L), CPPDQ (1-10 μg/L), DPPDQ (1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (10 μg/L) enhanced intestinal permeability to different degrees. Meanwhile, exposure to 6-PPDQ (0.1-10 μg/L), 77PDQ (0.01-10 μg/L), CPPDQ (0.1-10 μg/L), DPPDQ (0.1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (1-10 μg/L) resulted in intestinal reactive oxygen species (ROS) production and activation of both SOD-3::GFP and GST-4::GFP. In 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ exposed nematodes, the ROS production was strengthened by RNAi of genes (acs-22, erm-1, hmp-2, and pkc-3) governing functional state of intestinal barrier. Additionally, expressions of acs-22, erm-1, hmp-2, and pkc-3 were negatively correlated with intestinal ROS production in nematodes exposed to 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ. Therefore, exposure to different PPDQs differentially induced the intestinal toxicity on nematodes. Our data highlighted potential exposure risk of PPDQs at low concentrations to organisms by inducing intestinal toxicity.

A Review of N-(1,3-Dimethylbutyl)-N'-phenyl-p-Phenylenediamine (6PPD) and Its Derivative 6PPD-Quinone in the Environment

As an antioxidant and antiozonant, N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is predominantly used in the rubber industry to prevent degradation. However, 6PPD can be ozonated to generate a highly toxic transformation product called N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone), which is toxic to aquatic and terrestrial organisms. Thus, 6PPD and 6PPD-quinone, two emerging contaminants, have attracted extensive attention recently. This review discussed the levels and distribution of 6PPD and 6PPD-quinone in the environment and investigated their toxic effects on a series of organisms. 6PPD and 6PPD-quinone have been widely found in air, water, and dust, while data on soil, sediment, and biota are scarce. 6PPD-quinone can cause teratogenic, developmental, reproductive, neuronal, and genetic toxicity for organisms, at environmentally relevant concentrations. Future research should pay more attention to the bioaccumulation, biomagnification, transformation, and toxic mechanisms of 6PPD and 6PPD-quinone.

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.