Effects of inhaled combined Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX): Toward an environmental exposure model

Combined environmental exposures to the volatile organic compounds (VOCs) Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) pose clear risks to public health. Research into these risks is under-studied even as BTEX levels in the atmosphere are predicted to rise. This review focuses on the available literature using single- and combined-BTEX component inhaled solvent exposures in animal models, necessarily also drawing on findings from models of inhalant abuse and occupational exposures. Health effects of these exposures are discussed for multiple organ systems, but with particular attention on neurobehavioral outcomes such as locomotor activity, impulsivity, learning, and psychopharmacological responses. It is clear that animal models have significant differences in the concentrations, durations and patterns of exposure. Experimental evidence of the deleterious health and neurobehavioral consequences of exposures to the individual components of BTEX were found, but these effects were typically assessed using concentrations and exposure patterns not characteristic of environmental exposure. Future studies with animal models designed appropriately to explore combined BTEX will be necessary and advantageous to discovering health outcomes and more subtle neurobehavioral impacts of long-term environmental exposures.

Nonionic surfactant enhanced biodegradation of m-xylene by mixed bacteria and its application in biotrickling filter

In this study, m-xylene biodegradation was examined in bacteria-water mixed solution and biotrickling filter (BTF) systems amended with the nonionic surfactant Tween 80. The mixed bacteria were obtained from the activated sludge of a coking plant through a multisubstrate acclimatization process. High-throughput sequencing analysis revealed that Rhodanobacter sp. was the dominant species among the mixed bacteria. In the bacteria-water mixed solution, the bacterial density increased with increasing Tween 80 concentration. Hence, Tween 80 could be utilized as substrate by the mixed bacteria. Tween 80, with concentrations of 50-100 mg L^-1, could enhance the bioavailability of m-xylene and consequently improve the degradation efficiency of m-xylene. However, further increasing the initial concentration of Tween 80 would decrease the degradation efficiency of m-xylene. At concentrations exceeding 100 mg L^-1, Tween 80 was preferentially degraded by the mixed bacteria over m-xylene. In BTF systems, when the m-xylene inlet concentration was 1200 mg m^-3 and the empty bed residence time was 20 sec, the removal efficiency and elimination capacity of BTF1 with Tween 80 addition were at most 20% and 24% higher than those of BTF2 without Tween 80 addition. Overall, the integrated application of the mixed bacteria and surfactant was demonstrated to be a highly effective strategy for m-xylene waste gas treatment.

IMPLICATIONS

The integrated application of mixed bacteria and surfactant was demonstrated to be a promising approach for the highly efficient removal of m-xylene. Surfactant can activate mixed bacteria to degrade m-xylene by increasing its bioavailability. Besides, surfactant can be utilized as carbon source by the mixed bacteria so that the growth of mixed bacteria can be promoted. It is expected that the integrated application of both technologies will become more common in future chemical industry.

Assessing human variability in kinetics for exposures to multiple environmental chemicals: a physiologically based pharmacokinetic modeling case study with dichloromethane, benzene, toluene, ethylbenzene, and m-xylene

The objective of this study was to compare the magnitude of interindividual variability in internal dose for inhalation exposure to single versus multiple chemicals. Physiologically based pharmacokinetic models for adults (AD), neonates (NEO), toddlers (TODD), and pregnant women (PW) were used to simulate inhalation exposure to "low" (RfC-like) or "high" (AEGL-like) air concentrations of benzene (Bz) or dichloromethane (DCM), along with various levels of toluene alone or toluene with ethylbenzene and xylene. Monte Carlo simulations were performed and distributions of relevant internal dose metrics of either Bz or DCM were computed. Area under the blood concentration of parent compound versus time curve (AUC)-based variability in AD, TODD, and PW rose for Bz when concomitant "low" exposure to mixtures of increasing complexities occurred (coefficient of variation (CV) = 16-24%, vs. 12-15% for Bz alone), but remained unchanged considering DCM. Conversely, AUC-based CV in NEO fell (15 to 5% for Bz; 12 to 6% for DCM). Comparable trends were observed considering production of metabolites (AMET), except for NEO's CYP2E1-mediated metabolites of Bz, where an increased CV was observed (20 to 71%). For "high" exposure scenarios, Cmax-based variability of Bz and DCM remained unchanged in AD and PW, but decreased in NEO (CV= 11-16% to 2-6%) and TODD (CV= 12-13% to 7-9%). Conversely, AMET-based variability for both substrates rose in every subpopulation. This study analyzed for the first time the impact of multiple exposures on interindividual variability in toxicokinetics. Evidence indicates that this impact depends upon chemical concentrations and biochemical properties, as well as the subpopulation and internal dose metrics considered.

Risk assessment of volatile organic compounds benzene, toluene, ethylbenzene, and xylene (BTEX) in consumer products

Exposure and risk assessment was performed by evaluating levels of volatile organic compounds (VOC) benzene, toluene, ethylbenzene, and xylene (BTEX) in 207 consumer products. The products were categorized into 30 different items, consisting of products of different brands. Samples were analyzed for BTEX by headspace-gas chromatography/mass spectrometry (headspace-GC/MS) with limit of detection (LOD) of 1 ppm. BTEX were detected in 59 consumer products from 18 item types. Benzene was detected in whiteout (ranging from not detected [ND] to 3170 ppm), glue (1486 ppm), oil-based ballpoint pens (47 ppm), and permanent (marking) pens (2 ppm). Toluene was detected in a leather cleaning product (6071 ppm), glue (5078 ppm), whiteout (1130 ppm), self-adhesive wallpaper (15-1012 ppm), shoe polish (806 ppm), permanent pen (609 ppm), wig adhesive (372 ppm), tapes (2-360 ppm), oil-based ballpoint pen (201 ppm), duplex wallpaper (12-52 ppm), shoes (27 ppm), and air freshener (13 ppm). High levels of ethylbenzene were detected in permanent pen (ND-345,065 ppm), shoe polish (ND-277,928 ppm), leather cleaner (42,223 ppm), whiteout (ND-2,770 ppm), and glue (ND-792 ppm). Xylene was detected in permanent pen (ND-285,132 ppm), shoe polish (ND-87,298 ppm), leather cleaner (12,266 ppm), glue (ND-3,124 ppm), and whiteout (ND-1,400 ppm). Exposure assessment showed that the exposure to ethylbenzene from permanent pens ranged from 0 to 3.11 mg/kg/d (men) and 0 to 3.75 mg/kg/d (women), while for xylene, the exposure ranges were 0-2.57 mg/kg/d and 0-3.1 mg/kg/d in men and women, respectively. The exposure of women to benzene from whiteout ranged from 0 to 0.00059 mg/kg/d. Hazard index (HI), defined as a ratio of exposure to reference dose (RfD), for ethylbenzene was 31.1 (3.11 mg/kg/d/0.1 mg/kg/d) and for xylene (2.57 mg/kg/d/0.2 mg/kg/d) was 12.85, exceeding 1 for both compounds. Cancer risk for benzene was calculated to be 3.2 × 10(-5) based on (0.00059 mg/kg/d × 0.055 mg/kg-d(-1), cancer potency factor), assuming that 100% of detected levels in some products such as permanent pens and whiteouts were exposed in a worst-case scenario. These data suggest that exposure to VOC via some consumer products exceeded the safe limits and needs to be reduced.

Pilot study of aromatic hydrocarbon adsorption characteristics of disposable filtering facepiece respirators that contain activated carbon

Disposable filtering facepiece respirators (FFRs) used by health care workers are not designed to reduce the inhalation of volatile organic compounds (VOCs). Smoke-generating surgical procedures release VOCs and have been associated with the following complaints: foul smell, headaches, nausea, irritated throat and lungs, and asthma. Organic vapor FFRs that contain activated carbon are used by industrial workers to provide odor relief. These respirators remove irritating odors but are not marketed as respirators that provide respiratory protection against a gas or vapor. This study investigated the aromatic hydrocarbon adsorption capabilities of nuisance organic vapor (OV) FFRs. Three OV FFR models were tested to determine the 10% breakthrough time of three aromatic hydrocarbons at ambient room temperature and relative humidity. All respirator models were exposed to each vapor separately in three duplicate tests (n = 27). The respirator was sealed with silicone to an AVON-ISI headform that was placed in a chamber and exposed to VOC-laden air (20 ppm, 37 L/min). Periodically, gas samples were directed to an SRI gas chromatograph (Model 8610C) for analysis. All respirators performed similarly. The average 10% breakthrough values for all tests were at least 64 min, 96 min, and 110 min for benzene, toluene, and xylene, respectively. Respirators were tested with challenge concentrations at nuisance levels (20 ppm) and did not exceed 10% breakthrough values for at least 61 min. While the results of this pilot study hold promise, there is a need for further investigation and validation to determine the effectiveness of nuisance FFRs in mitigating organic vapors such as benzene, toluene, and xylene.

A mechanistic modeling framework for predicting metabolic interactions in complex mixtures

BACKGROUND

Computational modeling of the absorption, distribution, metabolism, and excretion of chemicals is now theoretically able to describe metabolic interactions in realistic mixtures of tens to hundreds of substances. That framework awaits validation.

OBJECTIVES

Our objectives were to a) evaluate the conditions of application of such a framework, b) confront the predictions of a physiologically integrated model of benzene, toluene, ethylbenzene, and m-xylene (BTEX) interactions with observed kinetics data on these substances in mixtures and, c) assess whether improving the mechanistic description has the potential to lead to better predictions of interactions.

METHODS

We developed three joint models of BTEX toxicokinetics and metabolism and calibrated them using Markov chain Monte Carlo simulations and single-substance exposure data. We then checked their predictive capabilities for metabolic interactions by comparison with mixture kinetic data.

RESULTS

The simplest joint model (BTEX interacting competitively for cytochrome P450 2E1 access) gives qualitatively correct and quantitatively acceptable predictions (with at most 50% deviations from the data). More complex models with two pathways or back-competition with metabolites have the potential to further improve predictions for BTEX mixtures.

CONCLUSIONS

A systems biology approach to large-scale prediction of metabolic interactions is advantageous on several counts and technically feasible. However, ways to obtain the required parameters need to be further explored.

Occurrence and accumulation features of polycyclic aromatic hydrocarbons and synthetic musk compounds in finless porpoises (Neophocaena phocaenoides) from Korean coastal waters

Reports of the occurrence and accumulation patterns of polycyclic aromatic hydrocarbons (PAHs) and synthetic musk compounds (SMCs) in marine mammals are scarce. In this study, the concentrations and accumulation profiles of PAHs and SMCs were determined in blubber from finless porpoises in Korean coastal waters. Total concentrations of PAHs and SMCs ranged from 6.0 to 432 (mean: 160) ng/g lipid weight and from 17 to 144 (mean: 52) ng/g lipid weight, respectively. Residue levels of PAHs were lower than those reported from other studies, while residue levels of SMCs were relatively higher than those reported in other studies. Naphthalene was the most abundant PAH and HHCB was the dominant SMC observed in finless porpoises. The concentrations of PAHs and SMCs were not correlated with each other, but were significantly correlated within the same chemical groups. No correlations were found between body size and residue levels of PAHs and SMCs.

Influence of sodium benzoate on the metabolism ofo-xylene in the rat

The main metabolites of o-xylene in urine are o-methylhippuric acid, o-toluic acid, o-toluic acid glucuronide, 3,4-dimethylphenol, 3,4-dimethylphenol conjugates and o-xylylmercapturic acid. The urinary excretion of o-toluic acid, o-toluic acid conjugates and o-xylene were increased by the prior administration of sodium benzoate. Conversely, the amounts of o-methylhippuric acid, 3,4-dimethylphenol conjugates and o-xylylmercapturic acid decreased by sodium benzoate pretreatment. In addition, the urinary excretion of o-methylhippuric acid was delayed by the pretreatment. The percentages of urinary excretion of the o-xylene metabolites were substantially changed by the pretreatment with sodium benzoate. These results therefore highlight a potential interaction of an air pollutant with a food additive, an interaction that remains to be established in man.

A PBPK Modeling Assessment of the Competitive Metabolic Interactions of JP-8 Vapor with Two Constituents,m-Xylene and Ethylbenzene

Jet Propellant 8 (JP-8) is a kerosene-based jet fuel used in the military and is composed of hundreds of hydrocarbons. A PBPK model was developed to assess the metabolic interactions of JP-8 vapor on two prominent constituents of JP-8 vapor, m-xylene (XYL) and ethylbenzene (EBZ). A limited number of rats were exposed to JP-8 vapor in a Leach chamber for 4 h to 380, 1100, or 2700 mg/m3 (total hydrocarbon). Several individual hydrocarbons were monitored in the chamber atmosphere, including XYL, EBZ, and the total hydrocarbon concentration. Blood and liver were harvested and analyzed by a novel headspace SPME/GC-MS method that allowed for identification of individual hydrocarbons and low limits of detection. The PBPK model was able to describe the metabolic interactions between XYL, EBZ, and a lumped aromatic fraction of JP-8 vapor estimated to be 18 to 25% of the fuel vapor. Competitive inhibition of XYL and EBZ metabolism was observed for JP-8 vapor inhalation exposures of 1100 and 2700 mg/m3. Future inhalation studies with jet fuel include aerosol exposures and expansion of the PBPK models to include other hydrocarbons such as n-alkanes and upper respiratory tract dosimetry of aerosol droplets.

Purification capability of tobacco transformed with enzymes from a methylotrophic bacterium for formaldehyde

Plants have the ability to remediate environmental pollution. Especially, they have a high purification capability for airpollution. We have measured the purification characteristics of foliage plants for indoor airpollutants--for example, formaldehyde (HCHO), toluene, and xylene--using a tin oxide gas sensor. HCHO is an important intermediate for biological fixation of C1 compounds in methylotrophs. The ribulose monophosphate pathway of HCHO fixation is inherent in many methylotrophic bacteria, which can grow on Cl compounds. Two genes for the key enzymes, HPS and PHI, from the methylotrophic bacterium Mycobacterium gastri MB19 were introduced into tobacco. In this article, the HCHO-removal characteristic of the transformant was examined by using the gas sensor in order to evaluate quantitatively. The purification characteristics of the transformant for toluene, xylene, and styrene were also measured. The results confirmed an increase of 20% in the HCHO-removal capability. The differences of the purification capabilities for toluene, xylene, and styrene were not recognized.

Excretion of unchanged volatile organic compounds (toluene, ethylbenzene, xylene and mesitylene) in urine as result of experimental human volunteer exposure

OBJECTIVES

To investigate elimination of unchanged volatile organic compounds (VOC's) through urine and the use of respective data for occupational exposure assessment, six volunteers were exposed under controlled conditions to toluene (TOL), ethylbenzene (EB), xylene (XYL) and mesitylene (MES) at concentrations ranging from 20 to 200 mg/m(3). The study was to elicit the toxicokinetic data and compare the precision of VOC's exposure assessment based on determining unchanged compounds in blood, urine and their metabolites in urine.

METHODS

During and after exposure blood and urine samples were analysed by gas chromatography using the headspace and SPME headspace technique

RESULTS

The kinetics of VOC's elimination in urine complied with an open two-compartment model. The (half-time) T (1/2 )values varied from 0.45 to 0.88 h for phase I and from 6.7 to 19.2 h for phase II. The precision of the method for unchanged VOC's was similar to that based on unchanged compounds in blood and better than their main metabolites in urine.

CONCLUSION

The obtained result indicate that determining unchanged VOC's in urine can be used as an exposure test even in the ranges of VOC's in the air that are much lower than the current TWA for occupational exposure.

Modeling VOC-odor exposure risk in livestock buildings

This paper describes a novel idea of linking models of exposure, internal dosimetry, and health effects. Risk assessment approach that integrates predicted odor caused by volatile organic compounds (VOC-odor) of toluene/xylene concentrations in human tissues leads to predict exposure risks in livestock buildings. First, VOC transport model was developed to calculate airborne toluene/xylene concentrations. Based on a physiologically based pharmacokinetic (PBPK) model, concentrations within five compartments representing lung, liver, fat, slowly perfused tissues, and rapidly perfused tissues could be quantified. By using a pharmacodynamic (PD) Hill model, we can optimally fit data from rat and human experiments to reconstruct dose-response relationships for accounting human health effects from nose poke and eye irritation. Results demonstrated that peak tissue concentration occurring at 5-10h in that fat contains the highest concentration than other tissues at around 4ppm of toluene and 1.8ppm of xylene. The EC(10) values are 114 and 232ppm, whereas expected risks are estimated to be 0.71% and 0.26% of human exposure to toluene and xylene, respectively. Risk analyses indicate that inhalation exposure in livestock buildings poses no significant threat to human health under the present environmental conditions. This method provides a rigorous and effective approach to relate target tissue concentration to human nose poke or eye irritation. We suggest that our probabilistic framework and methods be taken seriously because they produce general conclusions that are more robust and could offer a risk-management framework for discussion of future establishment of limits for respiratory exposure to VOC-odor.

Levels, tissue distribution, and age-related accumulation of synthetic musk fragrances in Chinese sturgeon (Acipenser sinensis): comparison to organochlorines

Chinese sturgeon (Acipenser sinensis) was listed as a Grade I protected animal in China in 1989, and the observed intersexual phenomenon and sex ratio deviation have suggested that chemicals have posed a risk as environment pollutants. This study analyzed seven musk fragrances in liver, muscle, heart, gonad, stomach, intestines, adipose, gill, pancreas, kidney, gallbladder, and roe from 13 female Chinese sturgeons, and the toxicokinetic behavior of musks were studied and compared with some organochlorines. Of the seven musks, HHCB, AHTN, and musk xylene were detected, and the highest concentrations were observed in adipose tissue: from 33.7 to 62.1 ng/g wet weight (ww), from 1.0 to 5.4 ng/g ww, and from 1.1 to 13 ng/g ww, respectively. Similar to the tissue distribution of DDTs and HCB, musks were detected frequently in high lipid content tissues such as roe, adipose, and liver, suggesting that tissue distribution of musks is controlled by the affinity to lipids. The concentration ratios based on lipid weight between roe and adipose were estimated to be 0.47 for HHCB, 0.58 for AHTN, and 0.51 for musk xylene, and those for the total DDTs and HCB were 0.27 and 0.61, which were relatively low compared with mammals. Relatively high concentrations of p,p'-DDE (68.4-449 ng/g ww) were detected in 10 of total 11 samples, which would cause the feminization of Chinese sturgeons during embryonic development. It was found that lipid-corrected concentrations of HHCB, AHTN, p,p'-DDE, and p,p'-DDD increased with age in female sturgeon, of which the trends were similar to those in fishes and different from those in mammals.

The role of toxicokinetics in xylene-induced ototoxicity in the rat and guinea pig

In the rat, some aromatic solvents cause a high level of ototoxicity that is characterized by damage to outer hair cells in the cochlea, which results in irreversible hearing loss. However, there is a vast difference in their potency. Among the three isomers of xylene, only para-xylene has been shown to be ototoxic in the rat. Moreover, all the species do not show the same susceptibility to ototoxic solvents, the rat being the most susceptible and the guinea pig seeming resistant to this ototoxic effect. The objective of the study was to determine whether toxicokinetic factors could explain the differences in ototoxicity observed among the three isomers of xylene in the rat and the species-dependent ototoxicity in the rat and the guinea pig. Blood and brain concentrations of each isomer were monitored in Sprague-Dawley rats treated orally by gastric intubation with a single dose or a 10 day-repeated treatment of 8.47 mmol/kg (an ototoxic dosage for para-xylene) of each isomer. Moreover, histology of the cochlea was carried out and the toxicokinetics of meta-xylene was monitored in rats treated with a single dose or a 10 day-repeated treatment of 16.94 mmol/kg meta-xylene, a non-ototoxic isomer. Similarly, histology of the cochlea was carried out and the toxicokinetics of para-xylene was followed in guinea pigs treated by gavage with a single dose or a 10 day-repeated treatment of 8.47 mmol/kg para-xylene. Finally, the blood and brain concentrations of para-xylene were measured in both the rats and the guinea pigs after a 4-h exposure to 1800 ppm of para-xylene. Among the three isomers studied, para-xylene yielded the highest blood and brain concentrations in the acutely and repeatedly exposed rats. When given a high dosage of meta-xylene (16.94 mmol/kg), the rats showed blood and brain concentrations of meta-xylene in the same order as those obtained with 8.47 mmol/kg para-xylene, but no outer hair cell loss was observed. No outer hair cell loss was observed in the guinea pigs treated with para-xylene. Whatever the exposure pattern, the blood and brain concentrations of para-xylene in the rats were 3.1-9.5 times higher than those measured in the guinea pigs. These results indicate that toxicokinetic factors cannot explain the differences in ototoxicity observed with the three isomers in the rat. However, they suggest that the differences in susceptibility to para-xylene observed between the rats and the guinea pigs might be due to toxicokinetic factors.

Ototoxicity of the three xylene isomers in the rat

Numerous experiments have shown that the aromatic solvents can affect the auditory system in the rat, the cochlea being targeted first. Solvents differ in cochleotoxic potency: for example, styrene is more ototoxic than toluene or xylenes. The goal of this study was to determine the relative ototoxicity of the three isomers of xylene (o-, m- or p-xylene). Moreover, by dosing with the two urinary metabolites of xylene, methylhippuric (MHAs) and mercapturic acids (MBAs), this study points toward a causal relationship between the cochleotoxic effects and potential reactive intermediates arising from the biotransformation of the parent molecules. Separate groups of rats were exposed by inhalation to one isomer following this schedule: 1800 ppm, 6 h/d, 5 d/wk for 3 wk. Auditory thresholds were determined with brainstem-auditory evoked potentials. Morphological analysis of the organ of Corti was performed by counting both sensory and spiral ganglion cells. Among the three isomers, only p-xylene was cochleotoxic. A 39-dB permanent threshold shift was obtained over the tested frequencies range from 8 to 20 kHz. Whereas outer hair cells were largely injured, no significant morphological change was observed within spiral ganglia. The concentrations of urinary p-, o- or m-MHA were greater (p-MHA: 33.2 g/g; o-MHA: 7.8 g/g; m-MHA: 20.4 g/g) than those obtained for MBAs (p-MBA: 0.04 g/g; o-MBA: 6.2 g/g; m-MBA: 0.03 g/g). Besides, there is a large difference between o-MBA (6.2 g/g) and p-MBA (0.04 g/g). As a result, since the cysteine conjugates are not determinant in the ototoxic process of xylenes, the location of the methyl groups around the benzene nucleus could play a key role.

Removal of p-Xylene with Pseudomonas sp. NBM21 in biofilter

As a p-xylene (p-Xyl)-degrading microorganism, Pseudomonas sp. NBM21 was isolated from an activated sludge of a wastewater treatment plant. NBM21 degraded p-Xyl, m-xylene, benzene and toluene, but not o-xylene, ethylbenzene (Eb) and styrene. NBM21 was inoculated to a biofilter with Biosol as a packing material and p-Xyl removal was operated for 105 d under sterile and nonsterile conditions. The maximum elimination capacities for p-Xyl at higher than 90% removal efficiency were 160 g/m3/h and 150 g/m3/h under nonsterile and sterile conditions, respectively. A high load of Eb adversely affected to the removal of xylene.

Penetration of benzene, toluene and xylenes contained in gasolines through human abdominal skin in vitro

Few studies are available in literature on the risk for humans from skin exposure to gasolines. This work is focused on the in vitro skin penetration of benzene (carcinogenic substance), toluene and xylenes. We examined three commercial gasolines using the Franz diffusion cells and human abdominal full thickness skin. Gasoline composition was determined using a multi-dimensional gas chromatographic (MDGC) technique. Aromatic compounds into the receptor fluid, consisting of saline solution were quantitated by a gas chromatography technique equipped with a flame ionization detector (GC-FID) and coupled with a headspace-solid phase micro extraction system (HS-SPME). Among the three substances, benzene showed the highest average apparent permeability coefficient (K(p)=43.8x10(-5)cmh(-1)) compared to toluene (K(p)=6.48x10(-5)cmh(-1)) and xylenes (K(p)=0.84x10(-5)cmh(-1)). This value could be explained by the lower boiling point and higher water solubility of benzene. Lag times were about 1h for benzene and 2h for toluene and xylenes. Averaged total recoveries in the receptor fluid were 0.43% of dose for benzene, 0.06% for toluene and 0.008% for xylenes. A statistical significative difference (Student's t-test, P<0.05) between the fluxes calculated for the three gasolines are noted only for xylene and for toluene between gasolines #1 (richer in aromatic compounds) and #3. The obtained apparent permeability coefficient are useful for determining the permeability of these aromatics components from gasolines of a different composition. Hands exposure risk, calculated using RfD and RfC as defined by US EPA, is critical for benzene. The risk of skin permeation of gasoline, and, in particular, of benzene, should be better evaluated for those workers who have a large potential for exposure. Adequate personal protective equipment should be used in the high exposure jobs, mainly for hands and forearms.

Measurements of exposure concentrations of benzene, toluene and xylene, and amounts of respiratory uptake

With respect to benzene, toluene, and o-, m- and p-xylene contained in indoor air, this study determined the amounts of their uptake through the human respiratory system using the difference between concentrations in inhalation and exhalation, and examined their relationship to concentrations in blood and urine measured before and after exposure. At relatively high concentrations, respiratory absorption of these compounds tended to increase rapidly in the early stage of exposure but decrease after several hours. It was also confirmed that concentrations of these compounds in both blood and urine increased during the first 3 hours of exposure. These results suggested that measurements of concentrations in inhalation and exhalation may provide a simple method for estimating the extent of respiratory exposure to these substances.

A physiologically based toxicokinetic model of inhalation exposure to xylenes in Caucasian men

Widespread exposure to the volatile aromatic hydrocarbons, ortho-, meta-, and para-xylene occurs in many industries including the manufacture of plastics, pharmaceuticals, and synthetic fibers. This paper describes the development of a physiologically based toxicokinetic model using biomonitoring data to quantify the kinetics of ortho-, meta-, and para-xylenes. Serial blood concentrations of deuterium-labeled xylene isomers were obtained over 4 days after 37 controlled, 2h inhalation exposures to different concentrations of the isomers. Peak toxicant concentrations in blood occurred in all subjects at the termination of exposure. Systemic clearance averaged 116 L/h+/-34 L/h, 117 L/h+/-23 L/h, and 129 L/h+/-33 L/h for ortho-, para-, and meta-xylene, respectively. The half-life of each toxicant in the terminal phase (>90 h post-exposure) was fit by the model, yielding values of 30.3+/-10.2 h for para-xylene, 33.0+/-11.7 h for meta-xylene and 38.5+/-18.2 h for ortho-xylene. Significant isomeric differences were found (p<0.05) for toxicant half-life, clearance and extrahepatic metabolism. Inter-individual variability seen in this study suggests that airborne concentration guidelines may not protect all workers. A Biological Exposure Index is preferred for this purpose since it is integrative and reflective of inter-individual kinetic variability.

Evaluation of potential toxicity from co-exposure to three CNS depressants (toluene, ethylbenzene, and xylene) under resting and working conditions using PBPK modeling

Under OSHA and American Conference of Governmental Industrial Hygienists (ACGIH) guidelines, the mixture formula (unity calculation) provides a method for evaluating exposures to mixtures of chemicals that cause similar toxicities. According to the formula, if exposures are reduced in proportion to the number of chemicals and their respective exposure limits, the overall exposure is acceptable. This approach assumes that responses are additive, which is not the case when pharmacokinetic interactions occur. To determine the validity of the additivity assumption, we performed unity calculations for a variety of exposures to toluene, ethylbenzene, and/or xylene using the concentration of each chemical in blood in the calculation instead of the inhaled concentration. The blood concentrations were predicted using a validated physiologically based pharmacokinetic (PBPK) model to allow exploration of a variety of exposure scenarios. In addition, the Occupational Safety and Health Administration and ACGIH occupational exposure limits were largely based on studies of humans or animals that were resting during exposure. The PBPK model was also used to determine the increased concentration of chemicals in the blood when employees were exercising or performing manual work. At rest, a modest overexposure occurs due to pharmacokinetic interactions when exposure is equal to levels where a unity calculation is 1.0 based on threshold limit values (TLVs). Under work load, however, internal exposure was 87%higher than provided by the TLVs. When exposures were controlled by a unity calculation based on permissible exposure limits (PELs), internal exposure was 2.9 and 4.6 times the exposures at the TLVs at rest and workload, respectively. If exposure was equal to PELs outright, internal exposure was 12.5 and 16 times the exposure at the TLVs at rest and workload, respectively. These analyses indicate the importance of (1) selecting appropriate exposure limits, (2) performing unity calculations, and (3) considering the effect of work load on internal doses, and they illustrate the utility of PBPK modeling in occupational health risk assessment.

Development of a physiologically based pharmacokinetic model for volatile fractions of gasoline using chemical lumping analysis

Physiologically based pharmacokinetic (PBPK) models have often been used to describe the absorption, distribution, metabolism, and excretion of chemicals in animals but have been limited to single chemicals and simple mixtures due to the numerous parameters required in the models. To overcome the barrier to modeling more complex mixtures, we used a chemical lumping approach, used in the past in chemical engineering but not in pharmacokinetic modeling, in a rat PBPK model for gasoline hydrocarbons. Our previous gasoline model consisted of five individual components (benzene, toluene, ethylbenzene, xylene, and hexane) and a lumped chemical that included all remaining components of whole gasoline. Despite being comprised of hundreds of components, the lumped component could be described using a single set of chemical parameters that depended on the blend of gasoline. In the present study, we extend this approach to evaporative fractions of gasoline. The PBPK model described the pharmacokinetics of all of the volatility-weighted fractions of gasoline when differences in partitioning and metabolism between fractions were taken into account. Adjusting the ventilation rate parameter to account for respiratory depression at high exposures also allowed a much improved description of the data. At high exposure levels, gasoline components competitively inhibit each other's metabolism, and the model successfully accounted for binary interactions of this type, including between the lumped component and the five other chemicals. The model serves as a first example of how the engineering concept of chemical lumping can be used in pharmacokinetics.

Percutaneous absorption of m-xylene vapour in volunteers during pre-steady and steady state

Percutaneous absorption of m-xylene (XYL) was determined in volunteers exposed to 29.4 microg cm(-3) XYL vapour on the forearm and hand for 20, 45, 120 and 180 min. The internal exposure was assessed by measuring the concentration of XYL in exhaled air. The systemic kinetics were determined using a reference exposure by inhalation. The dermal permeation rate and the cumulative absorption of XYL as a function of time were calculated using mathematical deconvolution. From these relationships, the average flux into the skin throughout the exposure (J(skin, average)) and the maximal flux into the blood (J(blood, max)) were derived. Both fluxes were dependent on the duration of exposure, approaching each other at longer exposure durations. The values of J(skin, average), adjusted to a concentration of 1 microg cm(-3), were 0.091 microg cm(-2) h(-1) during 20-min exposure falling to 0.072, 0.066 and 0.061 microg cm(-2) h(-1) for 45, 120 and 180 min, respectively. The values of J(blood, max) showed an opposite trend, gradually increasing from 0.034 microg cm(-2) h(-1) at an exposure duration of 20 min to 0.042, 0.059 and 0.063 microg cm(-2) h(-1) for 45, 120 and 180 min of exposure durations, respectively.

Detection of Benzene, Toluene, Ethyl Benzene, and Xylenes (BTEX) Using Toluene Dioxygenase-Peroxidase Coupling Reactions

We have developed a simple, whole-cell bioassay for the detection of bioavailable benzene, toluene, ethyl benzene, and xylenes (BTEX) and similar compounds. A genetically engineered E. coli strain expressing toluene dioxygenase (TDO) and toluene dihydrodiol dehydrogenase (TodD) was constructed, enabling the conversion of BTEX into their respective catechols, which were quickly converted into colored products by a horseradish peroxidase (HRP)-coupled reaction. The intensity of the color formation was correlated to concentrations of the BTEX compounds. Under the optimized conditions, a detection limit (defined as three times the standard deviation of the response obtained from the blank) of 10, 10, 20, and 50 microM was observed for benzene, toluene, ethyl benzene, and xylene, respectively. The bioassay was selective toward BTEX-related compounds with no interference observed with commonly used pesticides, herbicides, and organic solvent. The bioassay was very stable with little change in response over a 10-week period. The excellent stability suggests that the reported bioassay may be suitable for field monitoring of BTEX to identify and track contaminated water and follow the bioremediation progress.

Sex differences in the toxicokinetics of inhaled solvent vaporsin humans 1. m-Xylene

The aim of this study was to evaluate possible sex differences in the inhalation toxicokinetics of m-xylene vapor. Seventeen healthy volunteers (nine women and eight men) were exposed to m-xylene (200 mg/m3) and to clean air (control exposure) on different occasions during 2 h of light physical exercise (50 W). The chosen level corresponds to the occupational exposure limit (8-h time weighted average) in Sweden. m-Xylene was monitored up to 24 h after exposure in exhaled air, blood, saliva, and urine by headspace gas chromatography. m-Methylhippuric acid (a metabolite of m-xylene) was analyzed in urine by high-performance liquid chromatography. Body fat and lean body mass (LBM) were estimated from sex-specific equations using bioelectrical impedance, body weight, height, and age. Genotypes and/or phenotypes of cytochromes P450 2E1 and 1A1, glutathione transferases M1 and P1, and epoxide hydrolase were determined. The toxicokinetic profile in blood was analyzed using a two-compartment population model. The area under the concentration-time curve (AUC) of m-xylene in exhaled air postexposure was larger in women than in men. In addition, the excretion via exhaled air was significantly higher in women when correcting for body weight or LBM. In contrast, the men had a significantly higher volume of distribution, excretion of m-methylhippuric acid in urine, and AUC of m-xylene in urine. The toxicokinetic analyses revealed no differences between subjects of different metabolic genotypes or phenotypes. In conclusion, the study indicates small sex differences in the inhalation toxicokinetics of m-xylene, which can be explained by body build.

Evaluation of the dermal bioavailability of aqueous xylene in F344 rats and human volunteers

Xylene is a clear, colorless liquid used as a solvent in the printing, rubber, and leather industries and is commonly found in paint thinners, paints, varnishes, and adhesives. Although humans are most likely to be exposed to xylene via inhalation, xylene is also found in well and surface water. Therefore, an assessment of the dermal contribution to total xylene uptake is useful for understanding human exposures. To evaluate the significance of these exposures, the dermal absorption of o-xylene was assessed in F344 male rats and human volunteers using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Animals were exposed to o-xylene dermally. Immediately following the initiation of exposure, individual animals were placed in a glass off-gassing chamber and exhaled breath was monitored. Human volunteers participating in the study placed both legs into a stainless steel hydrotherapy tub containing an initial concentration of approximately 500 microg/L o-xylene. Exhaled breath was continually analyzed from each volunteer before, during, and after exposure to track absorption and subsequent elimination of the compound in real time. In both animal and human studies, a PBPK model was used to estimate the dermal permeability coefficient (K(p)) to describe each set of exhaled breath data. Rat skin was found to be approximately 12 times more permeable to aqueous o-xylene than human skin. The estimated human and rat aqueous o-xylene K(p) values were 0.005 +/- 0.001 cm/h and 0.058 +/- 0.009 cm/h, respectively.

In vivo percutaneous absorption, skin barrier perturbation, and irritation from JP-8 jet fuel components

JP-8 jet fuel has been reported to cause systemic and dermal toxicities in animal models and humans. There is a great potential for human exposure to JP-8. In this study, we determined percutaneous absorption and dermal toxicity of three components of JP-8 (i.e., xylene, heptane, and hexadecane) in vivo in weanling pigs. In vivo percutaneous absorption results suggest a greater absorption of hexadecane (0.43%) than xylene (0.17%) or heptane (0.14%) of the applied dose after 30 min exposure. Transepidermal water loss (TEWL) provides a robust method for assessing damage to the stratum corneum. Heptane showed greater increase in TEWL than the other two chemicals. No significant (p < 0.05) increase in temperature was observed at the chemically treated site than the control site. Heptane showed greater TEWL values and erythema score than other two chemicals (xylene and hexadecane). We did not observe any skin reactions or edema from these chemicals. Erythema was completely resolved after 24 h of the patch removal in case of xylene and hexadecane.

Hippuric acid and methyl hippuric acid in rat hair: possible monitoring of xylene and toluene exposure

Thinner is mainly composed of toluene and xylenes, and we studied the incorporation of the main metabolites of toluene and xylenes, hippuric acid (HA) and o-, m-, and p-methyl hippuric acids (o-, m-, p-MHA), in dark agouti rats' hair. Rat black hair was shaved before any exposure with an electric shaver designed for animals. Studies were performed in vivo with exposures of 30 min per day at three different concentrations (100, 300, and 1000 ppm) of toluene and o-, m-, and p-xylene for a total of 10 times over 2 weeks. Newly grown hair was tweezed out from the root with tweezers at seventh of the last exposure. Hair samples were then washed, extracted, derivatized, and analyzed by gas chromatography-mass spectrometry (GC-MS). HA and o-, m-, and p-MHA were not detected (ND) in the unexposed rat hair. After exposure, the metabolite concentration in the hair changed depending on the exposure concentration. Mean concentrations ranged from ND to 7.6 ng/mg, from ND to 13.8 ng/mg, from ND to 10.1 ng/mg, and from ND to 9.2 ng/ml hair for HA, o-, m-, and p-MHA, respectively. These results indicate that the metabolites concentrations in hair are effective indices of thinner exposure.

Improved method for in vitro assessment of dermal toxicity for volatile organic chemicals

Cell culture methods are being developed to assess the dermal toxicity (irritancy and corrosion) of chemicals. These in vitro methods are being validated to categorize chemicals as irritating or non-irritating to humans. Currently, these cell culture tests are useful to assist in the ranking of chemicals for irritancy, but they are not useful for quantitative risk assessment for two reasons. First, for volatile chemicals the amount of chemical in the media that the cells are exposed to may decrease with exposure time. Also, effective concentrations such as EC(50) and IC(50) are reported as the concentrations in the media not the skin tissue/cells. We have developed an in vitro approach for dermal toxicity testing of volatile chemicals that avoids these problems. Using sealed vials lacking a headspace, dermal equivalents (dermal fibroblasts in a collagen matrix) were exposed to culture medium containing a test chemical (m-xylene) and compared to a traditional open well culture system. We found that about 90% of the m-xylene was lost from the open well plates and the viability was 4-6 times greater than in the closed system. Partition coefficients were measured and used to estimate the m-xylene concentration in the fibroblasts. The EC(50) for m-xylene in the dermal equivalents was 833.13+/-35.33 microg m-xylene per gram of fibroblasts. This method will provide an effective approach to relate target cell chemical concentration to cellular responses. Based on this method, a biologically-based mathematical model could be used to determine an equivalent external dose for a specific toxic end point.

Relationship between environmental exposure to toluene, xylene and ethylbenzene and the expired breath concentrations for gasoline service workers

This study evaluated the relationship between the breath concentrations of, and personal exposure to, toluene, xylene and ethylbenzene of thirty workers from ten gasoline stations. Personal exposure air samples and workplace samples were collected simultaneously. Each subject provided a sample of exhaled breath after his or her personal exposure air was sampled. Twenty-five personal air, 17 workplace and 30 breath samples were collected in this study. Results indicated that breath concentrations of toluene and xylene were significantly correlated with personal monitoring concentrations. Furthermore, multiple regression analysis showed that exhaled toluene levels were highly influenced by personal toluene concentrations and the amount of personal gasoline sold (r2 = 0.762), while exhaled xylene levels depended on wind speed and personal xylene exposure concentrations (r2 = 0.665). Exhaled ethylbenzene levels were too low to present a relationship between concentrations and personal exposure levels. The exhaled toluene, xylene and ethylbenzene concentrations ranged from 4.3 to 41.8, 0.9 to 13.9, and 0.2 to 6.5 ppb, and the corresponding personal monitoring concentrations ranged from 60.3 to 572.3, 16.4 to 156.6, and 10.7 to 136.6, respectively. The average number of symptoms per person, according to neurotoxic questionnaire 16 (abbreviated as Q16) was 4.1 and six workers showed over six symptoms in Q16. This study suggests that exhaled toluene and xylene levels are suitable for use as biological exposure indices even at the ppb-level of exposure. Gasoline service workers are exposed to high levels of volatile organic compounds (VOCs) and the potential threats to their neurological systems deserve further investigation.

Dermal absorption and disposition of musk ambrette, musk ketone and musk xylene in human subjects

Musk ambrette, musk ketone and musk xylene have a long history of use as fragrance ingredients, although musk ambrette is no longer used in fragrances. As part of the review of the safety of these uses, it is important to consider the systemic exposure that results from these uses. Since the primary route of exposure to fragrances is on the skin, dermal doses of carbon-14 labelled musk ambrette, musk ketone and musk xylene were applied to the backs (100 cm2) of healthy human volunteers (two to three subjects) at a nominal dose level of 10-20 microg/cm2 and excess material removed at 6 h. Means of 2.0% musk ambrette, 0.5% musk ketone and 0.3% musk xylene were absorbed based on the amounts excreted in urine and faeces during 5 days. Most of the material was excreted in the urine with less than 10% of the amount excreted being found in faeces. No radioactivity was detected in any plasma sample, consistent with low absorption, and no radioactivity was detected (<0.02% dose) in skin strips taken at 120 h. Analysis of urine samples indicated that all three compounds were excreted mainly as single glucuronide conjugates. The aglycones were chromatographically different, but of similar polarity, to the major rat metabolites excreted in bile also as glucuronides.

Synthetic musks in the environment. Part 1: Species-dependent bioaccumulation of polycyclic and nitro musk fragrances in freshwater fish and mussels

Bioaccumulation of polycyclic musks (HHCB, AHTN) and nitro musks (musk xylene, musk ketone, and their amino metabolites) in aquatic biota was investigated by analyzing 18 fish samples (rudd, tench, crucian carp, eel) and 1 pooled zebra mussel sample from the pond of a municipal sewage treatment plant. Furthermore, water samples taken at the effluent of the sewage plant as well as water samples and two series of semipermeable membrane devices (SPMDs) from the pond were included. This comprehensive data set allowed the determination of species-dependent bioaccumulation factors on a lipid basis (BAF(L)), e.g., for HHCB the BAF(L) in tench were more than 20 times higher than in eel. The BAF(L) for HHCB and AHTN in biota were lower than the partition coefficients K(SPMD/W) obtained from SPMD samples, which are assumed to represent model bioconcentration values. This stresses that metabolism of these compounds in fish must not be neglected.

Analysis of solvent central nervous system toxicity and ethanol interactions using a human population physiologically based kinetic and dynamic model

The effect of acute ethanol-mediated inhibition of m-xylene metabolism on central nervous system (CNS) depression in the human worker population was investigated using physiologically based pharmacokinetic (PBPK) models and probabilistic random (Monte Carlo) sampling. PBPK models of inhaled m-xylene and orally ingested ethanol were developed and combined by a competitive enzyme (CYP2E1) inhibition model. Human interindividual variability was modeled by combining estimated statistical distributions of model parameters with the deterministic PBPK models and multiple random or Monte Carlo simulations. A simple threshold pharmacodynamic model was obtained by simulating m-xylene kinetics in human studies where CNS effects were observed and assigning the peak venous blood m-xylene concentration (C(V,max)) as the dose surrogate of toxicity. Probabilistic estimates of an individual experiencing CNS disturbances given exposure to the current UK occupational exposure standard (100 ppm time-weighted average over 8 h), with and without ethanol ingestion, were obtained. The probability of experiencing CNS effects given this scenario increases markedly and nonlinearly with ethanol dose. As CYP2E1-mediated metabolism of other occupationally relevant organic compounds may be inhibited by ethanol, simulation studies of this type should have an increasingly significant role in the chemical toxicity risk assessment.

In vitro permeability and binding of hydrocarbons in pig ear and human abdominal skin

Human skin has continual exposure to chemicals due to various occupational activities. Chemicals that get on skin have the potential to be absorbed. Hence, the potential human health hazards of a chemical must include an estimate for percutaneous absorption. An inexpensive, easy, and adequate model for the quantitative measurement of skin penetration of chemicals from JP-8 is absent. Cutaneous penetration studies in vitro through human skin are severely limited due to the lack of availability of the human skin. In this study, we have shown that pig ear skin can be used as a model for risk assessment from the percutaneous absorption of chemicals. We determined flux and permeability coefficient (Kp) of three chemicals--heptane, hexadecane, and xylene--from their permeation profile through porcine and human skin. Binding of these chemicals to porcine stratum corneum (SC) and human SC were also determined. Factors of difference (FOD) in the permeability of pig and human skin were 1.71, 1.28, and 1.16, respectively, for heptane, hexadecane, and xylene. FOD in binding of heptane, hexadecane, and xylene to pig and human SC were found to be 1.04, 0.76, and 1.31, respectively. Since, FOD for permeability and binding parameters were less than 2, hence, we conclude that pig ear skin can be used as model for humans for risk assessment from percutaneous absorption of chemicals.

A PBPK modeling-based approach to account for interactions in the health risk assessment of chemical mixtures

The objectives of the present study were: (1) to develop a risk assessment methodology for chemical mixtures that accounts for pharmacokinetic interactions among components, and (2) to apply this methodology to assess the health risk associated with occupational inhalation exposure to airborne mixtures of dichloromethane, benzene, toluene, ethylbenzene, and m-xylene. The basis of the proposed risk assessment methodology relates to the characterization of the change in tissue dose metrics (e.g., area under the concentration-time curve for parent chemical in tissues [AUCtissue], maximal concentration of parent chemical or metabolite [Cmax], quantity metabolized over a period of time) in humans, during mixed exposures using PBPK models. For systemic toxicants, an interaction-based hazard index was calculated using data on tissue dose of mixture constituents. Initially, the AUCtarget tissue (AUCtt) corresponding to guideline values (e.g., threshold limit value [TLV]) of individual chemicals were obtained. Then, the AUCtt for each chemical during mixed exposure was obtained using a mixture PBPK model that accounted for the binary and higher order interactions occurring within the mixture. An interaction-based hazard index was then calculated for each toxic effect by summing the ratio of AUCtt obtained during mixed exposure (predefined mixture) and single exposure (TLV). For the carcinogenic constituents of the mixture, an interaction-based response additivity approach was applied. This method consisted of adding the cancer risk for each constituent, calculated as the product of q*tissue dose and AUCtt. The AUCtt during mixture exposures was obtained using an interaction-based PBPK model. The approaches developed in the present study permit, for the first time, the consideration of the impact of multichemical pharmacokinetic interactions at a quantitative level in mixture risk assessments.

A dermal model for spray painters. Part II: estimating the deposition and uptake of solvents

The contribution of dermal exposure to the total body burden of a given chemical is difficult to assess. However, it is possible that as regulatory pressures lead to reductions in inhalation exposure, the proportion of uptake via the dermal route will increase. This study brings together recent work in the field of dermal exposure to provide a model to estimate both exposure and uptake of solvents through the skin. Using spray painters as an example, the process of modelling exposure is described from identifying the determinants of exposure through to calculating the flux of solvent through the stratum corneum and thus the total dermal uptake. Results from a range of exposure scenarios are presented and areas requiring further research and validation are highlighted. The model should allow the estimation of combined dermal and inhalation exposure to solvents in both current work situations and retrospective epidemiological studies. Furthermore, it is envisaged that the model may provide the basis for generic assessment of dermal exposure and uptake.

Biological monitoring of workers exposed to ethylbenzene and co-exposed to xylene

OBJECTIVE

Ethylbenzene is an important constituent of widely used solvent mixtures in industry. The objective of the present study was to provide information about biological monitoring of occupational exposure to ethylbenzene, and to review the biological limit values corresponding to the threshold limit value of ethylbenzene.

METHODS

A total of 20 male workers who had been exposed to a mixture of ethylbenzene and xylene, through painting and solvent mixing with commercial xylene in a metal industry, were recruited into this study. Environmental and biological monitoring were performed during an entire week. The urinary metabolites monitored were mandelic acid for ethylbenzene and methylhippuric acid for xylene. Correlations were analyzed between urinary metabolites and environmental exposure for ethylbenzene and xylene. The interaction effects of a binary exposure to ethylbenzene and xylene were also investigated using a physiologically based pharmacokinetic (PBPK) model.

RESULTS

The average environmental concentration of organic solvents was 12.77 ppm for xylene, and 3.42 ppm for ethylbenzene. A significant correlation (R2 = 0.503) was found between environmental xylene and urinary methylhippuric acid. Urinary level of methylhippuric acid corresponding to 100 ppm of xylene was 1.96 g/g creatinine in the worker study, whereas it was calculated as 1.55 g/g creatinine by the PBPK model. Urinary level of mandelic acid corresponding to 100 ppm of ethylbenzene was found to be 0.7 g/g creatinine. PBPK results showed that the metabolism of ethylbenzene was highly depressed by co-exposure to high concentrations of xylene leading to a non-linear behavior.

CONCLUSIONS

At low exposures, both methylhippuric acid and mandelic acid can be used as indicators of commercial xylene exposures. However at higher concentrations mandelic acid cannot be recommended as a biological indicator due to the saturation of mandelic acid produced by the co-exposure to xylene.

Development of a physiologically based pharmacokinetic model of organic solvent in rats

A physiologically based pharmacokinetic model of the transfer of organic solvents in rat bodies was developed. The model has six compartments, i.e. lungs, vessel-rich tissue, muscles, fat tissue, tail, and liver, each being interconnected by the blood flow system. The transfer of organic solvents was expressed by simultaneous differential equations, which were then solved numerically by a personal computer using a simple spreadsheet program. m -xylene was used to represent organic solvents. The physiological parameters for rats (alveolar ventilation, cardiac output, tissue volume, tissue blood flow, etc.) and physicochemical or biochemical properties (blood/air partition coefficient, tissue/blood partition coefficients, metabolic constants, etc.) of m -xylene were based on the data obtained from the literature and our experiments. The partition coefficient of m -xylene for the tail and the blood flow and the volume of the rat tail were experimentally determined with adult rats. The results of simulation of rat exposure to m -xylene (50 and 500 ppm for 6 h) were essentially in good agreement with the experimental data on rats, i.e. the parent compound (m -xylene) concentration in the tail blood and the cumulative excretion of the metabolites in the urine were consistent.

Physiologically based modeling of the maximal effect of metabolic interactions on the kinetics of components of complex chemical mixtures

The objective of this study was to predict and validate the theoretically possible, maximal impact of metabolic interactions on the blood concentration profile of each component in mixtures of volatile organic chemicals (VOCs) [dichloromethane (DCM), benzene (BEN), trichloroethylene (TCE), toluene (TOL), tetrachloroethylene (PER), ethylbenzene (EBZ), styrene (STY), as well as para, ortho-, and meta-xylene (p-XYL, o-XYL, m-XYL)] in the rat. The methodology consisted of: (1) obtaining the validated, physiologically based toxicokinetic (PBTK) model for each of the mixture components from the literature, (2) substituting the Michaelis-Menten description of metabolism with an equation based on the hepatic extraction ratio (E) for simulating the maximal impact of metabolic interactions (i.e., by setting E to 0 or 1 for simulating maximal inhibition or induction, respectively), and (3) validating the PBTK model simulations by comparing the predicted boundaries of venous blood concentrations with the experimental data obtained following exposure to various mixtures of VOCs. All experimental venous blood concentration data for 9 of the 10 chemicals investigated in the present study (PER excepted) fell within the boundaries of the maximal impact of metabolic inhibition and induction predicted by the PBTK model. The modeling approach validated in this study represents a potentially useful tool for screening/identifying the chemicals for which metabolic interactions are likely to be important in the context of mixed exposures and mixture risk assessment.

Validation of a physiological modeling framework for simulating the toxicokinetics of chemicals in mixtures

The objective of this study was to investigate the usefulness of a physiologically based toxicokinetic (PBTK) modeling framework for simulating the kinetics of chemicals in mixtures of varying complexities and composition. The approach involved the simulation of the kinetics of components in two situations: (i) when one of the mixture components was substituted with another (i.e., benzene in the benzene (B)-toluene (T)-ethyl benzene (E)-m-xylene (X) mixture was substituted with dichloromethane (D)), and (ii) when another chemical was added to the existing four-chemical mixture model (i.e., when D was added to the existing BTEX mixture model). In both cases, differing compositions of mixtures were used to obtain simulations and to generate experimental data on kinetics for validation purposes. Since the quantitative and qualitative mechanisms of interaction among B, T, E, and X have already been established, the mechanisms of binary interactions between D and the BTEX components (e.g., competitive, noncompetitive, or uncompetitive metabolic inhibition) were investigated in the present study. The analysis of rat blood kinetic data (4-h inhalation exposures, 50-200 ppm each) to all binary combinations (D-B, D-T, D-E, and D-X) investigated in the present study was suggestive of competitive metabolic inhibition as the plausible interaction mechanism. By incorporating the newly estimated values of metabolic inhibition constant (K(i)) for each of these binary combinations within the five-chemical PBTK model (i.e., for the DBTEX mixture), the model adequately predicted the venous blood kinetics of chemicals in rats following a 4-h inhalation exposure to various mixtures (mixture 1:100 ppm of D and 50 ppm each of T, E, and X; mixture 2: 100 ppm each of D, T, E, and X; mixture 3: 100 ppm of D and 50 ppm each of B, T, E, and X; mixture 4: 100 ppm each of D, B, T, E, and X). The results of the present study suggest that the PBTK model framework is useful for conducting extrapolations of the kinetics of chemicals from one mixture to another differing in complexity and composition, based on mechanistic considerations of interactions elucidated at the binary level.

Adsorption of BTEX from aqueous solution by macroreticular resins

Theoretical and experimental investigations were conducted on the adsorption of benzene, toluene, ethylbenzene and xylene (BTEX) by macroreticular resins. A mass transfer model based on the squared-driving force principle is presented for describing the BTEX transfer between the aqueous and solid phases. Also proposed is a theoretical model for describing the BTEX breakthrough curves of the adsorption column. While the mass transfer model involves only an overall mass transfer coefficient, the column adsorption model has two model parameters. Those parameters are conveniently estimated using the observed mass transfer and breakthrough data. The predictions using the proposed models were found to compare well with the experimental data of batch and column BTEX adsorption tests.

Toxicity of nitromusks in early lifestages of South African clawed frog (Xenopus laevis) and zebrafish (Danio rerio)

Musk xylene (MX), musk ketone (MK) and musk moskene (MM) are synthetic nitro-containing fragrances. Due to their inherent lipophilicity and environmental persistence, they are frequently detected in environmental samples and especially in aquatic ecosystems. Despite this, the current environmental toxicity database of nitromusks is limited. Although nitromusks have been shown to accumulate in aquatic organisms, little is known about their potential developmental effects in the respective aquatic species. To investigate the developmental toxicity of these compounds to amphibians and fish, early lifestages of xenopus (Xenopus laevis) and zebrafish (Danio rerio) were exposed to three nitromusks for 96 h to examine the developmental effects of these compounds in the two species. Nitromusk body concentration measurements were carried out in parallel for correlation with potential developmental effects. No increased mortality, malformation or growth inhibition was observed in either species following 96-h exposure to 400 microg/l MX, MK and MM. However, an approximately 20% reduced viability was observed in xenopus larvae when exposed to 400 microg/l MX, MK and MM for 11 days. Xenopus and zebrafish exposed to 10, 153, 871 and 1637 microg/l 14C-MX for 96 h resulted in whole-body concentrations of 0.7 +/- 0.1, 11.1 +/- 1.1, 38.7 +/- 1.9 and 76.3 +/- 18.3 microg/g, and 4.3 +/- 0.6, 73.3 +/- 11.8, 440.0 +/- 72.7 and 664.0 +/- 47.7 microg/g wet body weight, respectively. Exposure of xenopus larvae to 400 microg/l MX, MK and MM for 11 days, resulted in whole body concentrations (extrapolated from gas chromatographic determinations) of 4700 +/- 5000, 1300 + 300 and 4600 + 4800 microg/g wet weight for MX, MK and MM, respectively. The latter toxicity results, in conjunction with the fact that the concentrations used for the above experiments were between 400- and 10000-fold higher than those detected in the environment, suggest that environmental concentrations of nitromusks are not hazardous for early lifestages of fish and amphibians.

Dermal absorption and disposition of musk ambrette, musk ketone and musk xylene in rats

Dermal doses of carbon-14 labelled musk ambrette (MA), musk ketone (MK) or musk xylene (MX) to male Sprague-Dawley CD rats were applied at a nominal dose level of 0.5 mg/kg (11 microg/cm2 of skin) and excess material removed at 6 h. Means of about 40, 31 and 19% of the applied doses of MA, MK and MX, respectively, were absorbed. Most of the absorbed material was excreted within 5 days with only 1-2% of the applied dose remaining in the animal at this time. Tissue concentrations of radiolabel were similar for all three compounds with peak concentrations occurring at 6-8 h. In general, fat and liver contained the highest concentrations at around 0.2 microg nitromusk equivalents/g but concentrations in fat declined fairly rapidly to around 0.005 microg equiv./g at 120 h. Most of the absorbed dose was eliminated in bile mainly in the form of polar conjugated metabolites. Structural characterisation of the major aglycones for MA and MX indicated that they were hydroxylated analogues formed by oxidation of the ring methyl. Repeated daily dosing for 14 days resulted in little bioaccumulation for musk xylene and accumulation of about three-fold for musk ketone.

The effects of an alpha hydroxy acid (glycolic acid) on hairless guinea pig skin permeability

The barrier integrity of hairless guinea pig skin after treatment with an alpha hydroxy acid was assessed through in vivo topical application of an oil-in-water emulsion containing 5 or 10% glycolic acid at pH 3.0. The control was a commercial moisturizing lotion, pH 7.8. A dosing regimen for the glycolic acid formulations that was tolerated by the hairless guinea pigs and significantly decreased stratum corneum turnover time was determined using the dansyl chloride staining technique. Once-daily dosing of hairless guinea pig skin for 3 weeks with the glycolic acid formulations resulted in approximately a 36-39% decrease in stratum corneum turnover time compared with the control lotion. After this treatment, hairless guinea pigs were sacrificed for the in vitro measurement of the percutaneous absorption of [14C]hydroquinone and [14C]musk xylol. No significant differences in the 24-hour absorption of either test compound were found for skin treated with the control lotion or the glycolic acid formulations. There were also no significant differences found in the absorption of [3H]water through skin from the different treatment groups. Although no increase in skin penetration occurred after treatment with the glycolic acid formulations, histology revealed approximately a twofold increase in epidermal thickness. Also the number of nucleated cell layers nearly doubled in skin treated with 5% and 10% glycolic acid compared with the control lotion and untreated skin. These studies demonstrate that substantial changes in the structure of hairless guinea pig epidermis can occur without significant effect on skin permeability of two model compounds.

Biotransformation and toxicokinetics of musk xylene in humans

Musk xylene (1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene, MX) is widely used as a fragrance ingredient in detergents and toiletries and is an environmental contaminant. High concentrations of MX have been found in fish, and humans are constantly exposed to MX as a result of its stability in the environment. We investigated the biotransformation and toxicokinetics of MX in humans. A single dose of 0.3 mg/kg body wt of 15N-labeled MX (15N-MX) was given to six volunteers (three male and three female) by the oral route and to another six volunteers (three males and three females) by the dermal route. Urine was collected for 96 h after exposure. Blood samples were taken at intervals for up to 140 days after administration. The metabolite 1-tert-butyl-3,5-dimethyl-15N-4-amino-2,6-dinitrobenzene in urine and 15N-MX in plasma were quantified by gas chromatography/electron-capture mass spectrometry (GC-MS/NCI). Peak plasma concentrations of 15N-MX after oral administration were 36-262 and 1.6-5.5 ng/ml plasma after dermal administration. The toxicokinetics of 15N-MX in plasma can be described by a two-compartment kinetic model with an initial rapid decrease, due to the distribution from the blood into a second compartment (likely fat tissue) and a terminal elimination phase with an average half-life of 70 days for both routes of administration. The amount of 1-tert-butyl-3,5-dimethyl-15N-4-amino-2,6-dinitrobenzene (15N-4-A-MX) in recovered urine represented 0.1-0.5% of the oral applied dose of 15N-MX, respectively, 0.02-0.16% of dermal dose. After a short time of invasion the concentrations of 15N-4-A-MX in urine reached a maximum 18-24 h after administration. The further elimination of the metabolite occurred by first-order kinetics with an average elimination half-life of 11.8 h. After the single oral or dermal dose of 15N-MX, 15N-4-A-MX was not detected in hemoglobin. However, hemoglobin samples contained 1-tert-butyl-3, 5-dimethyl-4-amino-2,6-dinitrobenzene (4-A-MX) (11.4-18.9 fmol/mg Hb), likely derived from chronic environmental exposures.

Haemoglobin binding of a musk xylene metabolite in man

1. Musk xylene (1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene) is used as a fragrance component in toiletries, detergents and skin care products. Musk xylene is widely distributed in the environment and has been identified as a persistent contaminant in fish and in mothers' milk. Experimental data in man indicate a slow elimination of musk xylene and a potential for accumulation. Nitroarenes may be biotransformed to the respective amines. Some aromatic amines are known to be tumorigenic in animals and in man. Quantitation of the binding of those aromatic amines to haemoglobin has been proposed as a biomarker of internal exposure. 2. To determine bioavailability, metabolic reduction and haemoglobin binding of musk xylene in man, we investigated the presence of musk xylene metabolites bound to haemoglobin in blood samples from rat and from 10 human volunteers not knowingly exposed to musk xylene. 3. Haemoglobin from the blood samples was isolated, and bound metabolites were liberated as amines by alkaline hydrolysis. In haemoglobin samples from all individuals, 1-tert-butyl-3,5-dimethyl-4-amino-2,6-dinitrobenzene and, after chemical derivatization, the corresponding N-perfluoropropyl amide were identified by GC/MS using electron-impact and electron-capture mass spectrometry. 4. The amounts of 1-tert-butyl-3,5-dimethyl-4-amino-2,6-dinitrobenzene bound to haemoglobin in the human blood samples ranged from 13 to 46 fmol/mg haemoglobin. 5. These data demonstrate that musk xylene is bioavailable in man. The use of haemoglobin binding as a biomarker for nitromusk exposure in the general population warrants further studies.

Estimation of the dermal absorption of m-xylene vapor in humans using breath sampling and physiologically based pharmacokinetic analysis

A physiologically-based pharmacokinetic model, containing a skin compartment, was derived and used to simulate experimentally determined exposure to m-xylene, using human volunteers exposed under controlled conditions. Biological monitoring was conducted by sampling, in exhaled alveolar air and blood, m-xylene and urinary methyl hippuric acid concentrations. The dermal absorption of m-xylene vapor was successfully and conveniently studied using a breath sampling technique, and the contribution to m-xylene body burden from the dermal route of exposure was estimated to be 1.8%. The model was used to investigate the protection afforded by an air-fed, half-face mask. By iteratively changing the dermal exposure concentration, it was possible to predict the ambient concentration that was required to deliver the observed urinary excretion of methylhippuric acid, during and following inhalation exposure to 50 ppm m-xylene vapor. This latter extrapolation demonstrates how physiologically-based pharmacokinetic modeling can be applied in a practical and occupationally relevant way, and permitted a further step not possible with biological monitoring alone. The ability of the model to extrapolate an ambient exposure concentration was dependent upon human metabolism data, thereby demonstrating the mechanistic toxicological basis of model output. The methyl hydroxylation of m-xylene is catalyzed by the hepatic mixed function oxidase enzyme, cytochrome P450 2E1 and is active in the occupationally relevant, (<100 ppm) exposure range of m-xylene. The use of a scaled-up in vitro maximum rate of metabolism (Vmaxc) in the model also demonstrates the increasingly valuable potential utility of biokinetic data determined using alternative, non-animal methods in human chemical-risk assessment.

Possible preferential metabolism of xylene isomers following occupational exposure to mixed xylenes

OBJECTIVES

Solvent exposures commonly involve mixtures of substances or mixtures of isomers of a single solvent. These may be metabolised through common pathways, resulting in the potential for metabolic interactions. These may then lead to accumulation of solvent or metabolic intermediates, some of which may be toxic. This paper describes a pilot study conducted to determine the correlation between airborne xylene isomers and the appearance of methylhippuric acid (MHA) isomers in urine of workers exposed mainly to xylene. The project also aimed to determine whether there is preferential metabolism of any isomer by comparison of the ratios of airborne isomers with the ratios of metabolite isomers appearing in urine.

SUBJECTS AND METHODS

A total of 12 workers (11 male, 1 female) were recruited into this study, with 2 of the participants providing samples on more than one occasion. Workers included flooring contractors (5), printers (2), chemical manufacturers (2), histology technicians (2) and one householder using a xylene-based varnish. Subjects were aged between 24 and 48 years (37.6+/-2.0 years; mean +/- SEM). After giving informed consent, workers provided a prework and postwork urine sample on a midweek work day. Samples were stored frozen prior to analysis. Breathing-zone air samples were collected using personal air samplers at 50 ml/min. Solvents were trapped on activated-charcoal sampling tubes. Subjects wore pumps for 18-304 (178+/-24) min on the same day on which urine samples were collected.

RESULTS

Xylene exposures ranged from 1.6 to over 7000 ppm. In all, 7 of 16 measurements exceeded the Australian TWA standard of 80 ppm. Two of the flooring contractors wore respiratory protective equipment (RPE) and the two histopathology technicians used workplace ventilation systems. Total urinary MHA output ranged from 10 to 8000 mmol/mol creatinine, with 6 of 16 samples exceeding the modified biological exposure index of 702 mmol/mol. Correlations between airborne concentrations of individual xylene isomers and their corresponding MHA isomers were poor but improved when workers using RPE were excluded from the analysis. Gradients of the regression lines (millimoles of MHA per mole of creatinine per parts per million of xylene) were 3.2 for o-isomers, 7.0 for p-isomers, and 14.4 for m-isomers. Comparisons of isomer ratios of xylene in air were made with the corresponding ratio of MHA isomers in urine. These revealed higher ratios of m-MHA to other MHA isomers than those of m-xylene to the other xylene isomers. The MHA isomer ratios were expected to be the same as the airborne xylene isomer ratios if there were no preferential elimination of any isomer. m-MHA appeared in urine in a greater proportion than would be predicted from the proportion of m-xylene detected in air. The time course of the appearance of MHA isomers in urine also suggests that interactions were taking place, with m-MHA appearing in high proportion in urine following several days of repeated heavy xylene exposure. On a single moderate exposure, m-MHA appeared initially in high proportion in the first few hours but was undetectable in urine after 18 h. p-MHA was detectable for up to 6 h after exposure, and o-MHA remained detectable after 18 h.

CONCLUSIONS

This study suggests that excretion of m-MHA in urine is favoured over that of the other isomers following exposure to mixed xylenes. This is independent of airborne xylene isomer composition and suggests that the metabolism of m-xylene occurs preferentially to that of the other isomers. It is not clear at which step in the metabolism of xylene this preference occurs, although other work indicates that the initial oxidation of xylene to methylbenzyl alcohol by cytochrome P450 2E1 occurs at the same rate for each isomer. These findings suggest that there is potential for metabolic interactions between xylene isomers and that these may be the basis for xylene toxicity.

ATSDR evaluation of health effects of chemicals. V. Xylenes: health effects, toxicokinetics, human exposure, and environmental fate

Xylenes, or dimethylbenzenes, are among the highest-volume chemicals in production. Common uses are for gasoline blending, as a solvent or component in a wide variety of products from paints to printing ink, and in the production of phthalates and polyester. They are often encountered as a mixture of the three dimethyl isomers, together with ethylbenzene. As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that are of greatest concern for public health purposes. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of the bulk of this profile (ATSDR, 1995) into the mainstream scientific literature. An extensive listing of known human and animal health effects, organized by route, duration, and end point, is presented. Toxicological information on toxicokinetics, biomarkers, interactions, sensitive subpopulations, reducing toxicity after exposure, and relevance to public health is also included. Environmental information encompasses physical properties, production and use, environmental fate, levels seen in the environment, analytical methods, and a listing of regulations. ATSDR, as mandated by CERCLA (or Superfund), prepares these profiles to inform and assist the public.

Musk xylene: analysis, occurrence, kinetics, and toxicology

1,3-Dimethyl-2,4,6-trinitro-5-tert.-butylbenzene (musk xylene, MX), a synthetic musk, is often used in fragrances and soaps to substitute the natural musk. MX belongs to the common group of nitromusk compounds. The main environmental intake of MX occurs after sewage introduction. The consumption of fish and drinking water as well as the use of body care and perfumed household products could lead to an ingestion of this substance in humans. Although the acute oral and dermal toxicity of MX is low, some hint for the carcinogenic potential of MX was found in one animal experiment. These findings and the high potential of MX as environmental contaminant, it is stable against biological and chemical degradation and it is highly lipophil, raised considerable attention in the field of environmental medicine. Biological monitoring and the toxicology of MX, which previously has been described to occur in human milk, human fat tissue, as well as human blood samples, are of central interest. The aim of this article is to summarize the data on the analysis, occurrence, kinetics, and toxicology of MX. As there is a lack of knowledge on human toxicity and human carcinogenicity of MX, a final evaluation of the toxicological data with regard to public health is still impossible. Nevertheless, in view of the published data about MX, there is no evidence for any substantial human risk at the moment.