Mass or total surface area with aerosol size distribution as exposure metrics for inflammatory, cytotoxic and oxidative lung responses in rats exposed to titanium dioxide nanoparticles.

There is currently no consensus on the best exposure metric(s) for expressing nanoparticle (NP) dose. Although surface area has been extensively studied for inflammatory responses, it has not been as thoroughly validated for cytotoxicity or oxidative stress effects. Since inhaled NPs deposit and interact with lung cells based on agglomerate size, we hypothesize that mass concentration combined with aerosol size distribution is suitable for NP risk assessment. The objective of this study was to evaluate different exposure metrics for inhaled 5 nm titanium dioxide aerosols composed of small (SA < 100 nm) or large (LA > 100 nm) agglomerates at 2, 7, and 20 mg/m3 on rat lung inflammatory, cytotoxicity, and oxidative stress responses. We found a significant positive correlation ( r = 0.98, p < 0.01) with the inflammatory reaction, measured by the number of neutrophils and the mass concentration when considering all six (SA + LA) aerosols. This correlation was similar ( r = 0.87) for total surface area. Regarding cytotoxicity and oxidative stress responses, measured by lactate dehydrogenase and 8-isoprostane, respectively, and mass or total surface area as an exposure metric, we observed significant positive correlations only with SA aerosols for both the mass concentration and size distribution ( r > 0.91, p < 0.01), as well as for the total surface area ( r > 0.97, p < 0.01). These data show that mass or total surface area concentrations alone are insufficient to adequately predict oxidant and cytotoxic pulmonary effects. Overall, our study indicates that considering NP size distribution along with mass or total surface area concentrations contributes to a more mechanistic discrimination of pulmonary responses to NP exposure.

Evaluation of occupational exposure: comparison of biological and environmental variabilities using physiologically based toxicokinetic modeling.

PURPOSE: Few studies compare the variabilities that characterize environmental (EM) and biological monitoring (BM) data. Indeed, comparing their respective variabilities can help to identify the best strategy for evaluating occupational exposure. The objective of this study is to quantify the biological variability associated with 18 bio-indicators currently used in work environments. METHOD: Intra-individual (BV(intra)), inter-individual (BV(inter)), and total biological variability (BV(total)) were quantified using validated physiologically based toxicokinetic (PBTK) models coupled with Monte Carlo simulations. Two environmental exposure profiles with different levels of variability were considered (GSD of 1.5 and 2.0). RESULTS: PBTK models coupled with Monte Carlo simulations were successfully used to predict the biological variability of biological exposure indicators. The predicted values follow a lognormal distribution, characterized by GSD ranging from 1.1 to 2.3. Our results show that there is a link between biological variability and the half-life of bio-indicators, since BV(intra) and BV(total) both decrease as the biological indicator half-lives increase. BV(intra) is always lower than the variability in the air concentrations. On an individual basis, this means that the variability associated with the measurement of biological indicators is always lower than the variability characterizing airborne levels of contaminants. For a group of workers, BM is less variable than EM for bio-indicators with half-lives longer than 10-15 h. CONCLUSION: The variability data obtained in the present study can be useful in the development of BM strategies for exposure assessment and can be used to calculate the number of samples required for guiding industrial hygienists or medical doctors in decision-making.

Effects of inhaled nano-TiO2 aerosols showing two distinct agglomeration states on rat lungs.

Nano-aerosols composed of large agglomerates (LA) (>100nm) are more likely to promote pulmonary clearance via macrophages phagocytosis. Small agglomerates (SA) (<100nm) seem to escape this first defense mechanism and are more likely to interact directly with biological material. These different mechanisms can influence pulmonary toxicity. This hypothesis was evaluated by comparing the relative pulmonary toxicity induced by aerosolized nano-TiO(2) showing two different agglomeration states: SA (<100nm) and LA (>100nm) at mass concentrations of 2 or 7mg/m(3). Groups of Fisher 344 male rats were nose-only exposed for 6h. The median number aerodynamic diameters were 30 and 185nm at 2mg/m(3), and 31 and 194nm at 7mg/m(3). We found in rat's bronchoalveolar lavage fluids (BALF) a significant 2.1-fold increase in the number of neutrophils (p<0.05) in the group exposed to the 7mg/m(3) LA nano-aerosol suggesting a mild inflammatory response. Rats exposed to the 7mg/m(3) SA nano-aerosol showed a 1.8-fold increase in LDH activity and 8-isoprostane concentration in BALF, providing evidence for cytotoxic and oxidative stress effects. Our results indicate that biological responses to nanoparticles (NP) might depend on the dimension and concentration of NP agglomerates.

Impact of emerging pollutants on pulmonary inflammation in asthmatic rats: ethanol vapors and agglomerated TiO2 nanoparticles.

CONTEXT: Titanium dioxide nanoparticles (nano-TiO(2)) and ethanol vapors are air contaminants with increasing importance. The presence of a pathological pulmonary condition, such as asthma, may increase lung susceptibility to such contaminants. OBJECTIVE: This study aimed to investigate if exposure to inhaled ethanol vapors or nano-TiO(2) can modulate the rat pulmonary inflammatory response resulting from an allergic asthmatic reaction. MATERIALS AND METHODS: Brown Norway rats were sensitized (sc) and challenged (15 min inhalation, 14 days later) with chicken egg ovalbumin (OVA). Leukocytes were counted in bronchoalveolar lavages (BAL) performed at 6, 24, 36, 48 and 72 h following the challenge and either after ethanol exposures (3000 ppm, 6 h/day, daily) or at 48 h (peak inflammation) for nano-TiO(2) exposures (9.35 mg/m(3) aerosol for 6 and 42 h after the OVA challenge). For the nano-TiO(2) exposures, plasma and BAL cytokines were measured and lung histological analyzes were performed. RESULTS: Exposure to ethanol did not significantly affect BAL leukocytes after OVA challenge. Exposure to nano-TiO(2) significantly decreased BAL leukocytes compared to OVA-challenged controls. Plasma and BAL IL-4, IL-6, and INF-γ levels were also decreased in the nano-TiO(2) group. DISCUSSION: While ethanol vapors do not modify the pulmonary inflammation in rats during an asthmatic response, a surprising protective effect for agglomerated nano-TiO(2) was observed. A putative mechanistic basis involving a decrease in the Th2 response caused by OVA is proposed. CONCLUSION: Allergic pulmonary inflammation is not up-regulated by inhalation of the pollutants ethanol and nano-TiO(2). On the contrary, nano-TiO(2) decreases lung inflammation in asthmatic rats.

Physiologically based modeling of p-tert-octylphenol kinetics following intravenous, oral or subcutaneous exposure in male and female Sprague-Dawley rats.

The objective of this study was to develop a physiologically based pharmacokinetic (PBPK) model for p-tert-octylphenol (OP) for understanding the qualitative and quantitative determinants of its kinetics in Sprague-Dawley rats. Compartments of the PBPK model included the liver, richly perfused tissues, poorly perfused tissues, reproductive tissues, adipose tissue and subcutaneous space, in which OP uptake was described as a blood flow- or a membrane diffusion-limited process. The PBPK model successfully simulated previously published data on blood and tissue OP concentrations in Sprague-Dawley rats following oral, intravenous (i.v.) or subcutaneous (s.c.) routes. The model predicted that OP concentrations would reach 6.8, 13.8 and 27.9 ng ml(-1) (male) and 7.2, 14.7 and 31.4 ng ml(-1) (female), 4 h after a single i.v. dose of 2, 4 and 8 mg kg(-1), respectively. The model also predicted that OP concentrations would reach 53.3, 134.8 and 271.2 ng ml(-1) (male) and 87.4, 221.4 and 449.7 ng ml(-1) (female) 4 h after a single oral dose (50, 125 and 250 mg kg(-1)) and that, 4 h after a single s.c. dose (125 mg kg(-1)), OP concentrations would reach 111.3 ng ml(-1) (male) and 121.6 ng ml(-1). A marked sex difference was seen in blood and tissue OP concentrations. This was reflected in the model by a gender-specific maximal velocity of metabolism (V(max)) that was higher (1.77 x) in male than in female rats. Further studies are required to elucidate the mechanism underlying the gender differences and to evaluate whether that is also observed in humans.

Impact of biological and environmental variabilities on biological monitoring--an approach using toxicokinetic models.

Biological monitoring of occupational exposure is characterized by important variability, due both to variability in the environment and to biological differences between workers. A quantitative description and understanding of this variability is important for a dependable application of biological monitoring. This work describes this variability, using a toxicokinetic model, for a large range of chemicals for which reference biological reference values exist. A toxicokinetic compartmental model describing both the parent compound and its metabolites was used. For each chemical, compartments were given physiological meaning. Models were elaborated based on physiological, physicochemical, and biochemical data when available, and on half-lives and central compartment concentrations when not available. Fourteen chemicals were studied (arsenic, cadmium, carbon monoxide, chromium, cobalt, ethylbenzene, ethyleneglycol monomethylether, fluorides, lead, mercury, methyl isobutyl ketone, penthachlorophenol, phenol, and toluene), representing 20 biological indicators. Occupational exposures were simulated using Monte Carlo techniques with realistic distributions of both individual physiological parameters and exposure conditions. Resulting biological indicator levels were then analyzed to identify the contribution of environmental and biological variability to total variability. Comparison of predicted biological indicator levels with biological exposure limits showed a high correlation with the model for 19 out of 20 indicators. Variability associated with changes in exposure levels (GSD of 1.5 and 2.0) is shown to be mainly influenced by the kinetics of the biological indicator. Thus, with regard to variability, we can conclude that, for the 14 chemicals modeled, biological monitoring would be preferable to air monitoring. For short half-lives (less than 7 hr), this is very similar to the environmental variability. However, for longer half-lives, estimated variability decreased.

Toxicokinetics of p-tert-octylphenol in male and female Sprague-Dawley rats after intravenous, oral, or subcutaneous exposures.

This study was undertaken to characterize the toxicokinetics of p-tert-octylphenol (OP), a weak estrogenic compound, in male and female rats. Male and female Sprague-Dawley rats were given a single dose of OP either by oral gavage (50, 125 or 250 mg/kg), by intravenous (iv) injection (2, 4, or 8 mg/kg), or by subcutaneous (sc) injection (125 mg/kg). In a repeated dosing experiment, rats were given OP (oral) daily (25, 50, or 125 mg/kg) for 35 d (female) or 60 d (male). Blood and tissue samples were collected and analyzed for OP content using gas chromatography with detection by mass spectrometry. Blood OP concentrations were generally higher in female than male rats following a single oral or sc administration but were similar following a single iv injection. Tissue OP concentrations were also higher in female than male rats following oral exposure, consistent with the faster metabolism of OP observed in male rat liver microsomes. After subchronic administration, blood OP concentrations were higher at the end of exposure for female (33 d) (2.26-fold, not significant) and male (57 d) (3.47-fold) rats than single dosing but there was no change in the tissue OP concentrations. Gender differences in tissue OP concentrations may contribute, in part, to gender differences in the toxicity of OP in rats. The fact that OP was found in all reproductive tissues confirms its potential for direct endocrine-like effects.

Influence of ALDH2 polymorphism on ethanol kinetics and pulmonary effects in male and female rats exposed to ethanol vapors.

Ethanol is being added in various proportions to fuel in order to reduce greenhouse gas emissions. This is likely to result in involuntary exposure to ethanol vapors. Whether or not such exposure might cause health effects is still unknown. Acetaldehyde, an important metabolite of ethanol detoxified by aldehyde dehydrogenase (ALDH2) is more toxic that ethanol. This study assessed the impact of genetic ALDH2 polymorphism in male and female Sprague-Dawley rats on ethanol kinetics and pulmonary effects following sub-chronic exposure to ethanol vapors. Homozygote rats ALDH2(Q)/2(Q) (fast ALDH2 activity) and ALDH2(R)/2(R) (ALDH2 deficiency) were exposed to 1000 or 3000 ppm, 6 h/day, 5 days/week for 13 weeks. Blood ethanol concentrations (BEC) were measured at various post-exposure times. Cellularity in bronchoalveolar lavages (BAL) and lung histological evaluation were performed at week 13. Results showed that BEC in males were systematically lower than in females, e.g. BEC in ALDH2(Q)/2(Q) males (2 min, 1,000 ppm, day 1) was significantly (p < 0.05) lower (66.8 +/- 10.7 microM) compared to females (87.6 +/- 15.3 microM). BEC for ALDH2(Q)/2(Q) rats were different from ALDH2(R)/2(R) only for males exposed for more than 64 days. Repeated exposures resulted in a significant decrease of BEC, e.g. for ALDH2(Q)/2(Q) males (3,000 ppm) BEC on day 1 and day 85 were 324.6 +/- 102.6 microM and 187.5 +/- 32.1 microM, respectively. BAL and histological evaluation revealed no pulmonary toxicity for all groups. Overall, results showed that 3,000 ppm of ethanol vapors represents no observed adverse effect level (NOAEL) for pulmonary toxicity in the rat.

Determination of p-tert-octylphenol in blood and tissues by gas chromatography coupled with mass spectrometry.

A sensitive and reproducible procedure using gas chromatography coupled with mass spectrometry is described for the determination of p-tert-octylphenol (OP), a persistent degradation product of alkylphenol ethoxylates that binds to the estrogen receptor in blood and tissues. The first step involved the extraction of blood (200 microL) or tissue homogenate (400 microL) with methyl tert-butyl ether, including p-tert-butylphenol (BP) as internal standard. After extraction, the sample was evaporated to dryness with a gentle stream of nitrogen at 45 degrees C, and OP and BP were derivatized with an acetylation reaction involving acetic anhydride and catalyzed by pyridine. Samples were then analyzed by a gas chromatograph equipped with a mass spectrometer (single ion monitoring) with a Varian VF-5ms capillary column. The limit of detection and the limit of quantification of the method in blood were 4.6 and 15.5 ng/mL, respectively. The linearity and reproducibility of the method were acceptable, with coefficients of variation of approximately 10% for blood and ranging between 9% and 27% for tissues. This method was applied to the determination of unchanged OP in blood and tissues obtained from Sprague-Dawley rats after oral and IV OP administration.

Physiologically based modeling of the inhalation pharmacokinetics of ethylbenzene in B6C3F1 mice.

A physiologically based pharmacokinetic (PBPK) model was developed for inhaled ethylbenzene (EB) in B6C3F1 mice. The mouse physiological parameters were obtained from the literature, but the blood:air and tissue:air partition coefficients were determined by vial equilibration technique. The maximal velocity for hepatic metabolism (Vmax) obtained from a previously published rat study was increased by a factor of approximately 3 to account for enzyme induction during repeated exposures. The Michaelis affinity constant (Km) for hepatic metabolism of EB, obtained from a previously published rat PBPK modeling study, was kept unchanged during single and repeated exposure scenarios. Hepatic metabolism alone could not adequately describe the clearance of EB from mouse blood. Additional metabolism was assumed to be localized in the lung. The parameters for pulmonary metabolism were obtained by optimization of PBPK model fits to kinetic data collected following exposures to 75-1000 ppm. The PBPK model successfully predicted all available blood and tissue concentration data in mice exposed to 75 or 750 ppm EB. Overall, the results indicate that the rate of EB clearance is markedly higher in B6C3F1 mice than rats or humans and exceeds the hepatic metabolism capacity. Available biochemical evidence is consistent with a significant role for pulmonary metabolism; however, the extent to which the extrahepatic metabolism is localized in the lung is unclear. Overall, the PBPK model developed for the mouse adequately simulated the blood and tissue kinetics of EB by accounting for its high rate of clearance.

Inhalation pharmacokinetics of ethylbenzene in B6C3F1 mice.

The objective of the present study was to characterize the inhalation pharmacokinetics of ethylbenzene (EB) in male and female B6C3F1 mice following single and repeated exposures. Initially, groups of 28 male and female mice were exposed for 4 h to 75, 200, 500, or 1000 ppm in order to determine potential non-linearity in the kinetics of EB. Then, groups of male and female mice were exposed for 6 h to 75 ppm and 750 ppm (corresponding to the NTP exposures) for 1 or 7 consecutive days, to evaluate whether EB kinetics was altered during repeated exposures, The maximal blood concentration (Cmax; mean+/-SD, n=4) observed in female mice at the end of a 4-h exposure to 75, 200, 500, and 1000 ppm was 0.53+/-0.18, 2.26+/-0.38, 19.17+/-2.74, and 82.36+/-16.66 mg/L, respectively. The areas under the concentration vs. time curve (AUCs) following 4-h exposure to 75, 200, 500, and 1000 ppm were 88.5, 414.0, 3612.2, and 19,104.1 mg/L/min, respectively, in female mice, and 116.7, 425.7, 3148.3, and 16,039.1 mg/L/min in male mice. The comparison of Cmax and the kinetic profile of EB in mice exposed to 75 ppm suggests that they are similar between 1-day and 7-day exposures. However, at 750 ppm, the rate of EB elimination would appear to be greater after repeated exposures than single exposure, the pattern being evident in both male and female mice. Overall, the single and repeated exposure pharmacokinetic data collected in the present study suggest that EB kinetics is saturable at exposure concentrations exceeding 500 ppm (and therefore at 750 ppm used in the NTP mouse cancer bioassay) but is in the linear range at the lower concentration used in the bioassay (75 ppm). These data suggest that consideration of the nature and magnitude of non-linear kinetics and induction of metabolism during repeated exposures is essential for the conduct of a scientifically sound analysis of EB cancer dose-response data collected in B6C3F1 mice.

Physiologically based modeling of n-hexane kinetics in humans following inhalation exposure at rest and under physical exertion: impact on free 2,5-hexanedione in urine and on n-hexane in alveolar air.

We used a modified physiologically based pharmacokinetic (PBPK) to describe/predict n-hexane (HEX) alveolar air concentrations and free 2,5-HD urinary concentrations in humans exposed to n-HEX by inhalation during a typical workweek. The effect of an increase in workload intensity on these two exposure indicators was assessed and, using Monte Carlo simulation, the impact of biological variability was investigated. The model predicted HEX alveolar air concentrations at rest of 19.0 ppm (25 ppm exposure) and 38.7 ppm (50 ppm exposure) at the end of the last working day (day 5), while free 2,5-HD urinary concentrations of 3.4 micromol/L (25 ppm) and 6.3 micromol/L (50 ppm) were predicted for the same period (last 4.5 hours of Day 5). Monte Carlo simulations showed that the range of values expected to occur in a group of 1000 individuals exposed to 50 ppm of HEX (95% confidence interval) for free 2,5-HD (1.7-14.7 micromol/L) is much higher compared with alveolar air HEX (33.4-46 ppm). Simulations of exposure at 50 ppm with different workloads predicted that an increase in workload intensity would not greatly affect both indicators studied. However, the alveolar air HEX concentration is more sensitive to modifications of workload intensity and time of sampling, after the end of exposure, compared with 2,5-HD. The PBPK model successfully described the HEX alveolar air concentrations and free 2,5-HD urinary concentrations measured in human volunteers and is the first, to our knowledge, to describe the excretion kinetics of free 2,5-HD in humans over a 5-day period.

Comparison of unchanged n-hexane in alveolar air and 2,5-hexanedione in urine for the biological monitoring of n-hexane exposure in human volunteers.

INTRODUCTION AND AIM: Biological monitoring of n-hexane (HEX) is based on the measurement of urinary 2,5-hexanedione (2,5-HD). In 2001, the American Conference of Governmental Industrial Hygienists modified the biological exposure index (BEI) for HEX and suggested measuring free urinary 2,5-HD (without hydrolysis) (3.5 micromol/l) instead of total 2,5-HD (acid hydrolysis). This BEI value was derived from four field studies that involved worker exposures to variable concentrations of HEX and other solvents. This study was undertaken to characterize, for 5 consecutive days, the relationship between HEX exposure (25 ppm and 50 ppm) and (1). 2,5-HD urinary excretion and (2). HEX in alveolar air. METHODS: Five volunteers (three women, two men) were exposed to HEX in an exposure chamber for 2 non-consecutive weeks (7 h/day). They were exposed to 50 ppm HEX, during the first week and to 25 ppm during the second week. Alveolar air and urine samples were collected at different intervals before, during and after the exposures. The concentration of unchanged HEX in alveolar air and the concentration of urinary 2,5-HD under three analytical conditions (with acid, or enzymatic hydrolysis and without hydrolysis) were measured. RESULTS: The results show that the mean concentrations of HEX in alveolar air were 18 ppm (25 ppm) and 37 ppm (50 ppm), which indicates that approximately 73% of inspired HEX was expired unchanged in alveolar air by the volunteers. The mean (+/- SD) concentrations of urinary 2,5-HD for the last 4 h of exposure at the end of the week (day 5) following exposure to 50 ppm HEX were 30.4 micromol/l (+/-7.8 micromol/l) (acid hydrolysis); 5.8 micromol/l (+/-1.0 micromol/l) (enzymatic hydrolysis); 6.2 micromol/l (+/-0.9 micro mol/l) (without hydrolysis). Following the volunteers' exposure to 25 ppm HEX, the urinary excretion concentrations were 15.2 micromol/l +/- 1.9 micromol/l, 3.1 micromol/l +/- 0.7 micromol/l and 3.7 micromol/l +/- 0.5 micromol/l, respectively. CONCLUSION: Both free urinary 2,5-HD and HEX in alveolar air measurements could be used for the biological monitoring of HEX. Between these two indicators, HEX in alveolar air is less variable than 2,5-HD in urine, but the sampling time is more critical. Therefore, biological monitoring of HEX based on the measurement of free urinary 2,5-HD is preferable to HEX in alveolar air. Additionally, we believe that the 2,5-HD values reported in this study better reflect the actual levels of exposure to HEX alone than what has been previously reported in studies that involved co-exposure to other solvents, and that the current BEI value for HEX is most likely more protective than what has been believed up until now.

Effects of peak concentrations on the neurotoxicity of styrene in volunteers.

The manufacture of fibreglass reinforced plastic products may give rise to substantial peak exposures to styrene. Such exposure patterns need further consideration in terms of styrene neurotoxicity. The aim of this study was to evaluate the neurotoxic effects of short-term peak exposures in volunteers, at levels respecting the Quebec occupational exposure limits (8 hours time weighed average of 213 mg/m3 and 15 min average of 426 mg/ m3). The volunteers had not been previously exposed to styrene and they had no documented exposure to known neurotoxicants during the study. Twenty-four volunteers were exposed to five exposure scenarios during 6 hours: a, stable exposure to 106 mg/m3; b, variable exposure with a mean concentration of 106 mg/m3 with four 15 min peaks mounting up to 213 mg/m3; c, stable exposure to 213 mg/m3; d, variable exposure with a mean concentration of 213 mg/m3 and four peaks of 426 mg/m3 and e, two stable exposures to 5 mg/m3 (control). Before and after each exposure scenario, volunteers were submitted to a battery of sensory tests (visual and olfactory), neuropsychological tests (reaction time, attention, memory, psychomotor function), and self-evaluation questionnaires (mood and symptoms) in a test-retest design. The results show that the different exposure scenarios involving peak exposures did not influence either the performance to any test or subjective signs and symptoms. However, due caution must be exercised in extrapolation of the current results to occupational exposure since only acute exposures were tested and volunteers were at rest during exposure, which resulted in lower doses than those experienced by physically active workers.

Adjustment of permissible exposure values to unusual work schedules.

Research activities sought development of a method to adjust exposure limits for 694 substances for unusual work schedules. A consensus was established on the basic toxicological principle for adjustment; criteria for adjustment were selected by a panel of scientists coordinated by a committee of international experts and supported by toxicokinetic modeling; and a group of toxicologists attributed primary health effects and related adjustment category to each substance. A consensus among scientists and employers' and workers' representatives was established on the protocol of the application, in the field, of the adjusted exposure limits. The guiding toxicological principle for adjusting exposure standards to unusual work schedules is to guarantee an equivalent degree of protection for workers with unusual schedules as for workers with a conventional schedule of 8 hours per day, 5 days per week. The process of the adjustment is inspired from the Occupational Safety and Health Administration logic for attribution of primary health effects and adjustment categories ranging from no adjustment to daily or weekly adjustments. The adjusted exposure limits are calculated according to Haber's rule. Decisions on attribution of adjustment categories for the following toxicological effects were reached: respiratory sensitizers (asthma); skin sensitizers; tissue irritants versus tissue toxicants; methemoglobinenia-causing agents; cholinesterase inhibitors; and reproductive system toxicants and teratogens. A simple procedure is presented to facilitate the calculation, application, and interpretation of the adjusted exposure limits.

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.

Evaluation of the health risk associated with exposure to chloroform in indoor swimming pools.

The exposure of swimmers to chloroform (CHCl3) was investigated in indoor swimming pools of the Quebec City region along with the associated carcinogenic risk. Six training sessions involving 52 competition swimmers (11 to 20 yr old) were conducted in 3 different pools, while 12 adult leisure swimmers attended 5 sessions, each held in a different pool. For each session, water and ambient air CHCl3 concentrations were measured and CHCl3 levels in alveolar air samples (CHCl3 ALV) collected from swimmers prior to entering the swimming pool premises and after 15, 35, and 60 min of swimming. Mean water concentrations varied from 18 microg/L to 80 microg/L, while those in air ranged from 78 microg/m3 to 329 microg/m3. Multiple linear regression analyses revealed that CHCl3 ALV values in competition swimmers were strongly correlated to ambient air and water levels, and to a lesser degree to the intensity of training. Only ambient air concentration was positively correlated to CHCl3 ALV in the leisure group. Concentrations of CHCl3 metabolites bound to hepatic and renal macromolecules, estimated using a physiologically based pharmacokinetic (PBPK) model, were 1.6 and 1.9 times higher for the competition swimmers than for the leisure swimmers, respectively. The highest hepatic concentration predicted in competition swimmers, 0.22 microg CHCl3 equivalents/kg of tissue, was at least 10,000 times lower than the smallest no observed effect level for liver tumors in animals. Data indicate that the safety margin is therefore very large, for competitive swimmers as well as for leisure swimmers.

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.

Evaluation of the pharmacokinetic interactions between orally administered trihalomethanes in the rat.

The blood kinetics of trihalomethanes has recently been reported to differ between an oral administration of any single trihalomethane (0.25 mmol/kg) [THMs: chloroform, bromoform, bromodichloromethane (BDCM), dibromochloromethane (DBCM)] and a combined administration of 0.25 mmol/kg of each of the 4 THMs. The significant increase in blood concentrations of THMs could be a consequence of pharmacokinetic interactions between two or more of the THMs present simultaneously. The objective of the present study was to characterize the blood kinetics of THMs following oral administration singly or as binary mixtures in order to assess the relative contribution of each THM to the kinetic interferences observed with the quaternary mixture. A single dose of each THM (0.5 mmol/kg) alone or of a binary mixture containing 0.5 mmol/kg of each THM was administered by gavage to male Sprague-Dawley rats. The venous blood concentrations of unchanged THMs were measured for up to 720 min postadministration by headspace gas chromatography. Results showed that, compared to single administration, each binary mixture caused a significant increase in the blood concentrations of both THMs present and this effect increased with time. The impact, however, was not similar for each mixture, especially during the first hour following administration of the compounds (bromoform and DBCM). Among the four THMs, bromoform and DBCM kinetics appeared to be more sensitive to the mixture effect and to exert the greatest impact on the kinetics of the second THM present in the mixture. Simulation exercises conducted with physiologically based toxicokinetic models suggest metabolic inhibition as the possible mechanism of the interaction between THMs. In conclusion, the results of this study show that, at the dose level investigated, every binary combination of THMs, when orally administered, resulted in a significant modulation of their pharmacokinetics and suggest that this is probably the consequence of a mutual metabolic inhibition between the THMs.

o-cresol: a good indicator of exposure to low levels of toluene.

This study evaluates the suitability of using urinary excretion of o-cresol (o-CR) as a biological marker of occupational exposure to various concentrations of toluene (TOL). Thirty-eight individuals from three plants involved in the manufacture of paints or inks agreed to participate in the environmental and biological monitoring evaluations, which lasted one to two days. In all, 62 measurements of environmental TOL and urinary o-CR and hippuric acid (HA) levels were made. The eight-hour TOL exposure (time-weighted average [TWA]) ranged from 0 to 111 ppm, depending on plant and job title. TOL exposure was well correlated to post-shift urinary o-CR (r = 0.89) and HA (r = 0.67) levels. At low exposure levels (below 50 ppm), however, o-CR shows a stronger correlation (r = 0.71) than HA (r = 0.24). Based on our results, occupational exposure to 50 ppm of TOL would result in end-of-shift urinary o-CR concentration of 0.72 mumol/mmol creatinine (0.69 mg/L, assuming a urinary creatinine concentration of 1 g/L). This value is of the same order of magnitude as the level proposed by the American Conference of Governmental Industrial Hygienists (ACGIH) in 1998 for exposure to 50 ppm of TOL, namely 0.5 mg/L. Our results suggest that the level of urinary o-CR is a more sensitive index of exposure to low concentrations of TOL than is the urinary concentration of HA.