Use of Real-Time Breath Analysis and Physiologically Based Pharmacokinetic Modeling to Evaluate Dermal Absorption of Aqueous Toluene in Human Volunteers

Karla D. Thrall,1, Karl K. Weitz and Angela D. Woodstock

Molecular Biosciences Department, Fundamental Science Division, Battelle, Pacific Northwest Laboratory, 902 Battelle Boulevard, Mail Stop P7-59, Richland, Washington 99352

Received January 18, 2002; accepted March 26, 2002


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Toluene is a ubiquitous chemical that is commonly used for its solvent properties in industry and manufacturing, and is a component of many paint products. Although human exposure to toluene is most likely to be through inhalation, toluene is also found in well and surface water. Therefore, an assessment of the dermal contribution to total toluene uptake is useful for understanding human exposures. To evaluate the significance of these exposures, the dermal absorption of toluene was assessed in human volunteers using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Human volunteers wearing swimsuits were submerged in warm tap water to neck level in a stainless steel hydrotherapy tub containing an initial concentration of approximately 500-µg/l toluene. Volunteers were provided purified breathing air to eliminate inhalation exposures, and exhaled breath was continually analyzed before, during, and post exposure to track the absorption and subsequent elimination of the compound in real time. A PBPK model was used to estimate the dermal permeability coefficient (Kp) to describe each set of exhaled breath data from 4–6 human volunteers. An average Kp value of 0.012 ± 0.007 cm/h was found to provide a good fit to all data sets. Volunteers also participated in a second study phase, in which the subject was allowed to breathe the room air during immersion, thus both dermal and inhalation exposures to toluene occurred. Exhaled breath analyses revealed that concurrent inhalation of volatilized toluene resulted in a transient increase in the peak exhaled-breath level by 100 ppb, or an approximate 50% increase over breath levels observed in dermal-only studies. For perspective, the total intake of toluene associated with oral consumption of 2 liters of water containing toluene at bath water concentrations were estimated to be more than 30 times greater than the dermal contribution due to bathing.

Key Words: dermal; toluene; human; aqueous; PBPK; breath analysis.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Toluene is a clear, colorless liquid commonly used for its solvent properties in industry and manufacturing, and is a component of paints, paint thinners, lacquers, adhesives, rubber, and fingernail polish (U.S. EPA, 1990). Toluene occurs naturally in crude oil, is a component of gasoline, and is found in cigarette smoke. In 1998, EPA reported that toluene was found in well or surface water at 99% of hazardous waste sites surveyed (ATSDR, 2000Go). Given the ubiquitous nature of toluene, both occupational and non-occupational exposures may occur via ingestion, inhalation, and dermal routes. For aqueous contaminants, traditional approaches for evaluating exposure in domestic tap water supplies assume that ingestion represents the major route (Weisel and Jo, 1996Go). However, an understanding of the dermal contribution to total uptake is useful for predicting realistic human exposures. For example, previous investigators have suggested that inhalation and dermal absorption of chloroform due to showering with chlorinated water contributes a measurable dose to the body burden (Andelman, 1985Go; McKone, 1987Go; Jo et al., 1990aGo,bGo; Maxwell et al., 1991Go).

From the 1950s through the 1970s, the rate of uptake of a chemical through the skin was generally estimated based on studies of humans (Paustenbach and Leung, 1993Go). More recently, evaluations of dermal uptake of a chemical are made using animal skin (in vivo or in vitro) or human skin in vitro. In vivo, the rate of uptake of a chemical through the skin has been estimated using radiolabeled compounds and tracking the radioactivity in blood and excreta following topical application (Paustenbach and Leung, 1993Go). While this method for determining percutaneous absorption provides an estimate of the total absorbed dose, it often fails to reveal information on the uptake, distribution, and clearance phases of dermal absorption kinetics. Further, since blood levels may be very low in these situations, this practice is often restricted by sensitivity limits of the assay or analysis. An additional drawback with this methodology is that the nature of the radioactivity, whether it represents the parent or metabolites, is often undefined; thus kinetic interpretation is limited.

Recent studies have illustrated that exhaled breath presents a useful alternative to radiotracer studies by providing a non-invasive methodology for assessing bioavailability of volatile compounds (Corley et al., 2000Go; Thrall et al., 2000Go). Breath measurements are particularly useful in studies where repeated sample collection in real time allows for the tracking of absorption and elimination trends. Since breath concentrations reflect blood concentrations, continual analysis of exhaled breath provides a unique opportunity to evaluate differences in the rapid exponential emptying of the blood compartment that occurs immediately following peak exposure. Furthermore, the non-invasive nature of breath analysis improves the participation rate in controlled human exposure and environmental or occupational biomonitoring studies.

For exhaled breath measurements to be useful, they must be evaluated using some form of a kinetic model. Physiologically based pharmacokinetic (PBPK) models are particularly useful for integrating a variety of data, including breath analysis, to determine the penetration rates of chemicals through the skin. A PBPK model is exceptionally well suited for assessing dermal exposures under non-steady state conditions, where the transdermal flux is a function of the permeability coefficient (Kp), the area exposed, and the changing concentration gradient across the skin (Jepson and McDougal, 1997Go). The integration of real-time, exhaled-breath measurements with a PBPK model to determine dermal absorption has been successfully used for a number of compounds, including methyl chloroform, trichloroethylene, and benzene (Thrall et al., 2000Go).

The objective of the study presented here was to evaluate the dermal absorption of aqueous toluene in humans under realistic exposure conditions using exhaled breath analysis and PBPK modeling. To compare the significance of dermal absorption with inhalation exposures, each human volunteer participated in 2 phases of study: one with dermal exposure only, and one with combined dermal and inhalation exposure. A PBPK model developed to describe dermal exposures to aqueous toluene in F344 male rats (Thrall and Woodstock, in press) was modified to describe each human volunteer and used to estimate permeability coefficient values.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Human subjects.
Human studies were conducted under approval from the Battelle Memorial Institutional Review Board (IRB) in accordance with the terms and conditions of Federal Regulation 45 CFR 46, under the authority of Multiple Project Assurance M1221. Demographic information, including sex, age, race, and body weight (bw) and height are given in Table 1Go. No volunteers reported chronic cardiovascular, hepatic, central nervous system, renal, hematological, gastrointestinal or dermatological problems. Volunteers provided and wore their own swimsuits during the study.


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TABLE 1 Demographic Information on Human Volunteers
 
Human exposure conditions.
Human dermal exposures were conducted by submersion in tap water to neck level in a 397 l stainless steel hydrotherapy tub (Whitehall Manufacturing, Industry, CA) containing an initial target concentration of 500 µg/l toluene. The target concentration of 500 µg/l was selected to stay below the toluene Washington State drinking water guidelines of 800 µg/l and the U.S. EPA drinking water guidelines of 1000 µg/l as a maximum contaminant level (U.S. EPA, 2000Go). The tub was connected to municipal hot and cold water supplies and water inflow was adjusted to fill the tub at approximately 38°C. The hydrotherapy tub was filled with approximately 70 gallons (265 l) of tap water; total water volume in the tub was measured during filling, using a calibrated electronic digital flow meter/accumulator (Great Plains Industries, Wichita, KS). To quantitate total exposure, triplicate water samples (5-ml) were collected from the hydrotherapy tub prior to the addition of toluene, immediately upon addition of the toluene to the bathwater, and at 5-min intervals throughout the exposure phase. These samples were analyzed by a gas chromatographic (GC) headspace method using a Hewlett-Packard model 5890 Series II (Avondale, PA). The GC used a hydrogen flame ionization detector (FID) with nitrogen as the carrier gas; the column was a DB-Wax, 30 m, 1.5 µm film thickness (Restek, Bellefonte, PA). The detector was operated at 275°C, the inlet at 180°C, and the final oven temperature was 180°C. Under these conditions, toluene had a retention time of approximately 0.6 min. Water temperature was measured using a digital thermometer (Fluka, Everett, WA) and recorded at the same time intervals as the collection of aliquots for analysis. Water temperature was relatively constant (within ±1°C) over the course of the exposures.

The study protocol entailed monitoring volunteer exhaled breath for background toluene levels for 5 min prior to entry into the tub. Throughout the study, the water was continually mixed using an 0.5 horsepower turbine agitator. During the exposure phase, each volunteer was submerged to neck level in the tub for 20–30 min, while continually providing exhaled breath for real-time analysis. A weighed quantity of neat toluene, prepared in a separate room, was added to the water near the agitator after the volunteer was comfortably situated in the hydrotherapy tub, and during the on-going analysis of exhaled breath. The addition of the toluene to the bathwater was designated as time zero for water analysis. At the end of the monitoring period, the volunteer left the tub, briefly towel dried, and provided 15–30 min of post-exposure exhaled breath for real-time analysis. Volunteers participated in 2 separate study phases, with at least 1 week between each phase. In the first phase, exposure to toluene was by the dermal route only; volunteers were provided purified breathing air (Certified Grade D) throughout the exposure period, while immersed in the hydrotherapy tub. In the second phase, the volunteer was allowed to breathe the room air during the exposure phase, and thus exposures were by both dermal and inhalation routes. Regardless of study phase, breathing air was provided to the volunteer to isolate any volatilized compound in the room during the 5-min pre-exposure background check and for the post-exposure period.

Real-time breath analysis system.
The exhaled-breath monitoring system utilized during the human studies consisted of a breath inlet device connected to a Discovery II (LGC Inc., Los Gatos, CA) ion-trap tandem-mass spectrometer (MS/MS) equipped with an atmospheric sampling glow-discharge ionization source (ASGDI) and has been described previously (Gordon et al., 1998Go). Volunteers wore a Hans Rudolph facemask (Kansas City, MO) containing two 1-way, non-rebreathing valves. Purified breathing air was supplied by a gas cylinder to a 15-l foil-lined buffer bag to provide air to the volunteer on demand. Expired air was passed through a large-diameter Teflon tube into a heated-glass mixing chamber (1.3-l volume). Breath samples entered the mixing chamber through a tube that bent off to one side, and exited the mixing chamber via a tube bent in the opposite direction, thus ensuring that samples were well mixed by turbulent flow. The ASGDI-MS/MS system continually drew air samples from the center of the mixing chamber at a calibrated rate of 12 l/h to provide a new data point every 4.6 s. Excess exhaled air was vented from the mixing chamber via a large-bore hole exit tube with negligible flow restriction.

Intensity data from the mass spectrometer were converted to concentration (ppb) using external gas standards prepared in Tedlar bags (Supelco Inc., Bellefonte, PA) and a calibration curve. A new calibration curve was generated on each day of operation. The ASGDI-MS/MS methodology had detection limits in the 2–10-ppb range for toluene and calibrations were linear to 57 ppm (57,000 ppb).

A pilot study was conducted to evaluate the volatilized loss of toluene from the hydrotherapy tub following addition of toluene to the water. In this case, the hydrotherapy tub was filled with water as described for the human volunteer studies. The breath inlet device was positioned over the water at the level roughly corresponding to the breathing zone of the volunteers and used to continually sample the toluene air concentration prior to, and for 30 min after, addition of the toluene to the water, as described previously.

Human blood to air partition coefficient.
Heparinized human blood (single donor) was obtained from Golden West Biologicals, Inc. (Temecula, CA). The human blood-to-air partition coefficient was determined using a vial equilibration method, as described by Sato and Nakajima (1979) and modified by Gargas et al. (1989). Briefly, 2 ml of heparinized blood was weighed into a 25-ml scintillation vial and sealed airtight with caps that were modified by drilling a 4-mm-diameter hole in the center and replacing the cap liner with a Teflon-coated rubber septum (Supelco Inc., Bellefonte, PA). Toluene (1 ml) was introduced into the sealed vial as a vapor from a Tedlar gas-sampling bag (Supelco Inc., Bellefonte, PA) containing an air concentration of 5000 ppm. Sealed vials were incubated at 37°C with shaking in a vortex evaporator for 1 and 3 h. At the end of the incubation period, the toluene headspace concentration was determined by gas chromatography (GC), using a Hewlett Packard model 5890 Series II system (Avondale, PA) as described above. Tissue to blood partition coefficients were determined by dividing tissue to air values determined in the F344 rat as reported by Gargas et al. (1989) by the human blood to air value determined here.

PBPK model.
The dermal toluene PBPK model (Fig. 1Go) has been used to successfully describe the dermal absorption of aqueous toluene in the rat (Thrall and Woodstock, in press). Anatomical compartments in the model were used to describe the distribution of toluene into the rapidly perfused, slowly perfused, fat, liver, and skin compartments. The skin compartment in the model represented exposed skin; non-exposed skin was lumped into the slowly perfused compartment. Total skin, with a volume of 10% of the bw, was assumed to receive 5% of the cardiac output. The exposed skin volume and blood-flow rate were calculated as described by Jepson and McDougal (1997). For toluene, metabolism has been shown to occur primarily in the liver, and to be independent of the route of exposure (Cohr and Stokholm, 1979Go). Although metabolism at the site of entry is considered possible, this loss was assumed to be insignificant in comparison to liver metabolism. Therefore, metabolism was described as occurring only in the liver. Model parameters specific to human physiology, partition coefficients, and metabolism were taken from the literature or determined here (Table 2Go). To simulate dermal exposures, the rate of change in the concentration of toluene in the skin compartment (Csk, µg/ml) was related to the rate of penetration through the skin (the flux) and the rate of delivery due to blood flow and arterial concentration (the perfusion) as described by Thrall et al. (2000). In the PBPK model, this is written as:

where Vsk is the volume of the skin exposed (ml), Kp is the permeability coefficient (cm/h), A is the exposed surface area (cm2), Qsk is the blood-flow rate to the exposed skin (ml/h), Ca is the arterial concentration (µg/ml), Csk is the skin concentration (µg/ml), Cliq is the liquid toluene concentration (µg/ml), Psk/b is the toluene skin-to-blood partition coefficient (unitless; calculated by dividing the toluene solubility ratio of skin:air by blood:air), and Psk/liq is the toluene skin-to-water partition coefficient (unitless; calculated by dividing the toluene solubility ratio of skin:air by water:air).



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FIG. 1. Schematic representation of the PBPK model used to describe dermal and inhalation absorption of toluene in human volunteers.

 

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TABLE 2 Toluene PBPK Model Parameters
 
The skin permeability coefficient (Kp) for each volunteer was estimated from this equation, based on the kinetics of absorption as described by the exhaled breath. A maximum-likelihood search algorithm in SimuSolv (version 3.0; Dow Chemical Co., Midland, MI) was used to vary the Kp coefficient until an optimal fit was achieved that described the time-course data. The percent variability explained for all optimized values was always >= 80%. The use of these routines has been described previously (Agin and Blau, 1982Go). Optimized permeability coefficients were determined for each individual volunteer (n = 6) and averaged; data is expressed as the average ± the SD.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The human blood-to-air partition coefficient is given in Table 2Go. The value of 13.9 ± 4.6 (average ± SD of 10 samples) determined here was within the range of values of 10 to 16 reported in the literature (Fiserova-Bergerova and Diaz, 1986Go; Jang and Droz, 1996Go; Sato and Nakajima, 1979Go).

Human subjects participated in 2 separate studies: a dermal-only exposure and a dermal-plus-inhalation exposure. Target water toluene initial concentrations were 500 µg/l, or approximately 1.9 mg/kg bw for a 70-kg volunteer in 265 l of water. The actual water concentrations were verified for each volunteer by GC analysis of water samples collected immediately following addition of the toluene to the hydrotherapy tub, and at 5-min intervals thereafter. Actual initial exposure concentrations were found to range from 455 to 550 µg/ml (Table 3Go). A comparison of the triplicate water samples collected at different locations within the tub at the same time interval indicated that the toluene was well mixed within the hydrotherapy tub (data not shown). Tap water samples collected prior to the addition of toluene had no measurable toluene present.


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TABLE 3 PBPK Model Results for Toluene Exposures
 
Analysis of air over the hydrotherapy tub, in the vicinity of the breathing zone of a volunteer, revealed that toluene was rapidly volatilized from the hydrotherapy tub following addition to the water (Fig. 2Go). This is consistent with the analysis of water samples collected over time, which showed an immediate peak and rapid decline in initial concentration following addition of the compound to the water. A first-order loss term (Kloss) was optimized from water analysis data and included in the PBPK model to describe the changing water-toluene concentrations. A single Kloss value of 3 h-1 was found to describe all sets of water analysis data. Figure 3Go illustrates a representative comparison of the measured toluene water concentrations (average ± SD of 3 samples per time interval) to the PBPK model estimation, assuming an initial water concentration of 546 µg/l.



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FIG. 2. Room air concentration of toluene (ppb) measured in the breathing zone of a volunteer under conditions identical to those employed during the human study. Arrow indicates the addition of toluene to the hydrotherapy tub.

 


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FIG. 3. Comparison of the PBPK model prediction (line) and measured water concentration (points) for a human volunteer (Subject A, Table 1Go) exposed to an initial toluene water concentration of 546.4 µg/l in a 265-liter hydrotherapy tub. The water concentration data is given as the average ± SD of 3 samples collected immediately following introduction of the toluene to the tub, and at 5-min intervals thereafter.

 
The toluene exhaled-breath profile, given as the average (points) and SD (shaded bars) from 6 volunteers exposed to aqueous toluene by the dermal route, is given in Figure 4Go. The exhaled-breath data clearly showed toluene to be rapidly absorbed, with peak concentrations achieved within seconds after the addition of toluene to the water. At initial exposure concentrations ranging from 455 to 550 µg/l, toluene was found in the exhaled breath at peak levels of approximately 100 to 200 ppb (Table 3Go). Upon exit from the hydrotherapy tub, the concentration of toluene in the exhaled breath rapidly declined. By 5 min post-exposure, there was essentially no detectable toluene in the expired air. No measurable toluene was detected in the 5-min pre-exposure, exhaled-breath samples in any of the volunteers.



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FIG. 4. Exhaled toluene breath concentration, given as the average (points) and SD (shaded bars) of 6 volunteers exposed to aqueous toluene by the dermal route. The arrow indicates the addition of toluene to the hydrotherapy tub.

 
The exhaled breath profile for each volunteer was analyzed using the toluene PBPK model described previously, with physiological parameters set for humans. The changing concentration of toluene in the water over the exposure period was represented as loss from the exposure system (Kloss). For simplicity, the PBPK model assumed that all subjects had the same bw-proportional blood flow, ventilation, and metabolism rates. A representative exhaled-breath profile is given in Figure 5Go. The optimized Kp for human dermal absorption of toluene in water was found to range from 0.003 to 0.020 cm/h. A single averaged Kp value of 0.012 ± 0.007 cm/h (average ± SD, n = 6) was found to adequately describe all the individual dermal-exposure data sets.



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FIG. 5. PBPK model prediction (line) and exhaled breath data (points) for a human volunteer (Subject A, Table 1Go) exposed by the dermal route to an initial toluene water concentration of 545.6 µg/l in a 265-l hydrotherapy tub. The arrows indicate the addition of toluene to the hydrotherapy tub and when the volunteer exited the tub.

 
Each volunteer, with the exception of one, participated in a second phase of study wherein exposure to toluene occurred by both dermal and inhalation routes. Figure 6Go shows the exhaled breath profile as the average (points) and SD (shaded bars) from 5 volunteers exposed to toluene by both inhalation and dermal routes. Peak exhaled breath levels for the combined exposure ranged from approximately 300 to 350 ppb (Table 3Go). A comparison between dermal-only and dermal plus inhalation exposures within each volunteer (excluding volunteer B), indicated that inhalation exposure contributed a brief increase in peak exhaled-breath levels of roughly 100 ppb, or an approximate 50% increase over breath levels observed from dermal-only exposures. The contribution due to inhalation rapidly falls off as the air concentration declines. This is exemplified in Figure 7Go, which illustrates the comparative exhaled breath profiles from a female volunteer (subject E) exposed to toluene by dermal (line) versus inhalation-plus-dermal exposure (points).



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FIG. 6. Exhaled toluene breath concentration, given as the average (points) and SD (shaded bars) of 5 volunteers exposed to toluene by both inhalation and dermal routes. The arrow indicates the addition of toluene to the hydrotherapy tub.

 


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FIG. 7. Comparison of the exhaled-breath toluene concentration for a human volunteer (Subject E, Table 1Go) exposed by the dermal route (line) versus the dermal plus inhalation route (points).

 
The PBPK model, including a changing air-exposure concentration over time, was used to simulate the dermal plus inhalation exposure, using the permeability coefficient for each volunteer determined in the first study phase. A representative exhaled-breath profile for a volunteer exposed to toluene by both the dermal and the inhalation routes is given in Figure 8Go. This figure demonstrates the ability of the PBPK model to adequately simulate the real-time, exhaled-breath data from this combined exposure.



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FIG. 8. PBPK model prediction (line) and exhaled breath data (points) for a human volunteer (Subject A, Table 1Go) exposed by both dermal and inhalation routes to an initial toluene water concentration of 546.4 µg/l in a 265-l hydrotherapy tub. The arrows indicate the addition of toluene to the hydrotherapy tub and when the volunteer exited the tub.

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In a recent study conducted using F344 male rats, the permeability coefficient (Kp) for dermal absorption of aqueous toluene, estimated using PBPK modeling and exhaled-breath analysis, was found to be 0.074 ± 0.005 cm/h (Thrall and Woodstock, in press). In comparison, the human value determined here of 0.012 ± 0.007 cm/h is roughly 6 times lower than that of the rat. The magnitude of this difference is consistent with the results of previous studies comparing rat and human toluene-vapor exposures, where the rat in vivo Kp of 0.72 cm/h (McDougal et al., 1990Go) is roughly 5 times greater than the in vivo human value of 0.14 cm/h for hand and forearm vapor exposures (Kezic et al., 2000Go). Numerous investigators have shown that the dermal absorption of a variety of compounds is greater in rats than in humans, although the extent of the differences appears to vary by compound (Bronaugh, 1998Go; Jepson and McDougal, 1997Go; McDougal et al., 1990Go; U.S. EPA, 1992Go).

Due to differences in exposure conditions, it is difficult to directly compare the permeability coefficients determined here to literature estimates. For example, Kezic et al. (2001) exposed human volunteers to neat toluene for brief (3-min) periods of time; however, permeability may have been altered due to reported skin irritation. The U.S. EPA (1992) human Kp value for aqueous toluene was estimated to be 1 cm/h based on flux data from Dutkiewicz and Tyras (1968). In the human studies described by Dutkiewicz and Tyras (1968), the amount of toluene absorbed was quantified by measuring the loss of the compound from the donor solution, and steady-state conditions were not verified. However, permeability may be overestimated by assuming that the rate of chemical loss from the exposure solution represents the average flux into the skin (Jepson and McDougal, 1997Go). In addition, erroneous estimates of percutaneous absorption may be determined when standard Fick’s-law calculations of dermal flux are used without verifying that steady state was achieved (Jepson and McDougal, 1997Go). Therefore, it is particularly important for dermal absorption estimation methods to be able to account for unsteady-state dermal absorption (Roy et al., 1996Go). A PBPK model is ideally suited for estimating the actual permeability when the exposure concentration changes with time.

In the studies reported here, each volunteer, with the exception of one, participated in two study phases: a dermal only, and an inhalation-plus-dermal exposure phase. Thus, these studies provided a unique opportunity to compare the relative impact of each route of exposure on total body burden. It has been suggested that, for a given concentration of some volatile organic compounds, the daily dose absorbed during a 20-min bath may be similar in magnitude to the daily inhalation dose from volatilization of these same organic compounds (Shatkin and Szejnwald-Brown, 1991). However, the data presented here suggests that inhalation exposure to volatilized toluene from tap water is transient and contributes little to the overall total body burden over the course of the 30-min bath. This was illustrated by the rapid loss of toluene in the room air following addition of toluene to the hydrotherapy tub (Fig. 2Go), and by comparison of the exhaled breath profiles for a single volunteer exposed to toluene by dermal versus dermal-plus-inhalation routes (Fig. 7Go).

To estimate the relative importance of dermal and oral exposures, the total amount of toluene absorbed through each route of exposure was calculated using the PBPK model. For a 70-kg male taking a 30-min bath in municipal water containing an initial concentration of 500 µg/l of toluene, the dermal route will result in an estimated 0.41 µg toluene absorbed/kg bw (ignoring the inhalation contribution). In comparison, the oral-equivalent dose, calculated by assuming 100% oral bioavailability and the consumption of 2 liters of water/70 kg bw will contribute 14 µg/kg when toluene is present at the same concentration. Therefore, for a total (dermal + oral) body burden of 14.41 µg toluene absorbed/kg bw, the dermal route contributes approximately 2.8% of the daily body burden. It bears mentioning that these calculations of the relative contribution by route of exposure to total body burdens are exposure-specific as well as chemical-specific. The role of dermal uptake may be of greater or lesser importance, depending upon the actual exposure scenario. For example, while these studies were conducted at a water temperature of approximately 38°C, Corley et al. (2000) observed significant increases in exhaled chloroform and thus skin Kp values in volunteers exposed in bathwater studies as water temperature was increased from 30 to 40°C. Further, a sensitivity analysis of the PBPK model parameters used here indicated that the estimation of the Kp value was most sensitive to the breathing rate. Perhaps future studies could employ human plethysmography to individually measure breathing rates and thus reduce the variability in estimated Kp values.

In summary, the experiments on volunteers reported here provided definitive data for assessing the absorption of aqueous toluene through human skin. These whole-body experiments using human volunteers avoided interspecies extrapolations and variations in absorption based on the site of application. Furthermore, the utilization of sensitive, real-time instrumentation allowed for controlled exposures to be conducted under a realistic bath-water scenario. Analysis of the resultant exhaled breath data, using a PBPK model, estimated an average whole-body Kp for dermal absorption of aqueous toluene of 0.012 ± 0.007 cm/h. This value is more than 80 times lower than the U.S. EPA estimate based on a prior study (U.S. EPA, 1992Go).


    ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the laboratory assistance of Jolen Soelberg and Hong Wu.


    NOTES
 
This work was supported by Grant Number 1-P42-ES10338–01 from the National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH), and with funds from the U.S. Environmental Protection Agency (EPA). The contents of this manuscript are solely the responsibility of the authors, and do not necessarily represent the official views of NIEHS, NIH, or EPA.

1 To whom correspondence should be addressed. Fax: (509) 376-9064. E-mail: karla.thrall{at}pnl.gov. Back


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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