Effects of experimental conditions on absorption of glycol ethers through human skin in vitro

Effect of Receptor Solution in Studies of In Vitro Permeation Test (IVPT)

In Vitro Permeation Test (IVPT) is commonly used to evaluate skin penetration of chemicals and performance of dermatological products. For a permeant with low aqueous solubility, an additive that is expected not to alter the skin barrier can be used in the receptor solution to improve permeant solubility. The objective of this study was to (a) evaluate the effects of these additives in IVPT receptor solution on skin permeability of model permeants and skin electrical resistance and (b) determine the solubility of the permeants in these receptor solutions. Bovine serum albumin (BSA), 2-hydroxypropyl-beta-cyclodextrin (HPCD), ethanol, nonionic surfactant Brij-98, and propylene glycol were the additives, and phosphate buffered saline (PBS) was the control. Steady-state skin permeability coefficients and resistances were determined. The receptor solutions examined in this study did not cause a significant increase in skin permeability or decrease in resistance (less than 40 % changes) except 25 % ethanol. The receptor solution containing 25 % ethanol induced an approximately twofold average increase in skin permeability and reduced skin electrical resistance by approximately threefold. The receptor solution of 2.5 % HPCD provided the highest levels of solubility for the model lipophilic permeants, while 0.2 % Brij-98 and 5 % ethanol showed the lowest solubility enhancement from those in PBS.

Human skin permeation rates ex vivo following exposures to mixtures of glycol ethers

Skin exposure to cleaning products in the general and occupational population are a public health concern. Among the most frequently identified amphiphilic organic solvents in cleaning products are propylene glycol monomethyl ether (PGME) and propylene glycol n-butyl ether (PGBE). Internal dose from skin exposure may be efficiently evaluated using in vitro flow-through diffusion cells with excised human skin. Our aim in this study was two-fold; 1) characterize the permeation rates (J), time lag (T_lag), and permeation coefficients (Kp) of PGME and PGBE in human ex-vivo skin permeation assays, and 2) determine a possible mixture effect on skin permeation characteristics when applied together. Our results showed a short T_lag for PGME and was reduced further depending on the amount of PGBE in the mixture (T_lag was reduced from 2 h to 1-1.7 h) for fresh skin. PGBE T_lag slightly increased when mixed with 50 % or more PGME. Permeation rate decreased to half for both PGME and PGBE in mixture at any concentration. This substantial permeation was greater with previously frozen skin. This mixture effect could favor permeation of other compounds through human skin.

Percutaneous absorption of thirty-eight organic solvents in vitro using pig skin

Percutaneous absorption is highly variable between chemicals but also within chemicals depending on exposure conditions and experimental set up. We tested a larger number of organic solvents with the same experimental set up, using skin from new-born piglets and static diffusion cells. Thirty-six common organic solvents were studied neat (and 31 of them also in water dilution): acetone, acetonitrile, n-butanol 2-butanone 2-butoxyethanol, 1-butoxy-2-propanol, n-butyl acetate, butyl acrylate, cyclohexane, cyclohexanone, 1,2-dichloroethane, dichloromethane, ethanol, 2-ethoxyethanol, ethyl acetate, ethyl acrylate, ethylbenzene, furfuryl alcohol, n-hexane, 2-hexanone, 2-isopropoxyethanol, methanol, 1-methoxy-2-propanol, methyl acrylate, 3-methyl-1-butanol, methyl tertiary butyl ether, 4-metyl-2-pentanol, methyl methacrylate, 2-propanol, 2-propen-1-ol, 2-propoxyethanol, 1-propoxy-2-propanol, styrene, trichloromethane, toluene and m-xylene. In addition, a mixture of 2-methylbutyl acetate and n-pentyl acetate was tested. For most of the solvents, little or no percutaneous absorption data have been published. Lag times, steady-state fluxes and apparent permeability coefficients were obtained from the time courses of solvent appearance in the receptor medium, as measured by gas chromatography. The use of the same methodology and kind of skin resulted in small variability within experiments, underlining the need for consistent methodology for useful results for developing predictive models. Furthermore, a comparison of the neat and diluted data shows that water dilution affects all these variables and that the direction and magnitude of the effects vary between chemicals. This comparison strongly supports that prediction of percutaneous absorption of neat and water diluted chemicals requires different models.

Modeling Aggregate Exposures to Glycol Ethers from Use of Commercial Floor Products

Computer modeling of aggregate exposure provides the capability to estimate the range of doses that can occur from product use and to understand the relative importance of different routes of exposure. This paper presents an assessment of aggregate occupational exposure to two glycol ethers used as solvents in floor maintenance products for industrial and institutional facilities, using a simulation tool named PROMISE. Three commercial floor-care products were assumed to be applied in sequence—a floor stripper, then a floor cleaner, and lastly a protective coating. The glycol ethers modeled were ethylene glycol butyl ether (EGBE) in the floor stripper and in the floor cleaner, and dipropylene glycol methyl ether (DPGME) in the coating. Modeling uncertainty was assessed through a comparison of the PROMISE inhalation exposure estimates with those from an independent model (MCCEM), and parameter uncertainty was investigated using PROMISE software’s Monte Carlo simulation capabilities. Modeling results indicated that inhalation is the dominant exposure route. The predicted average air concentration and inhalation dose from PROMISE agreed with the second model (MCCEM) within 10%. Monte Carlo simulation indicated that the upper end of the aggregate-dose distribution for the scenario was more than 50% higher than the value of the point estimate. The modeled 8-h TWA concentrations for EGBE and DPGME were lower than the corresponding permissible exposure limits American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLV) by at least a factor of 20, indicating that under the assumed conditions work-place exposures to glycol ethers are below levels of concern.

Evaluation of in vitro vs. in vivo methods for assessment of dermal absorption of organic flame retardants: a review

There is a growing interest to study human dermal exposure to a large number of chemicals, whether in the indoor or outdoor environment. Such studies are essential to predict the systemic exposure to xenobiotic chemicals for risk assessment purposes and to comply with various regulatory guidelines. However, very little is currently known about human dermal exposure to persistent organic pollutants. While recent pharmacokinetic studies have highlighted the importance of dermal contact as a pathway of human exposure to brominated flame retardants, risk assessment studies had to apply assumed values for percutaneous penetration of various flame retardants (FRs) due to complete absence of specific experimental data on their human dermal bioavailability. Therefore, this article discusses the current state-of-knowledge on the significance of dermal contact as a pathway of human exposure to FRs. The available literature on in vivo and in vitro methods for assessment of dermal absorption of FRs in human and laboratory animals is critically reviewed. Finally, a novel approach for studying human dermal absorption of FRs using in vitro three-dimensional (3D) human skin equivalent models is presented and the challenges facing future dermal absorption studies on FRs are highlighted.

Recent advances in predicting skin permeability of hydrophilic solutes

Understanding the permeation of hydrophilic molecules is of relevance to many applications including transdermal drug delivery, skin care as well as risk assessment of occupational, environmental, or consumer exposure. This paper reviews recent advances in modeling skin permeability of hydrophilic solutes, including quantitative structure-permeability relationships (QSPR) and mechanistic models. A dataset of measured human skin permeability of hydrophilic and low hydrophobic solutes has been compiled. Generally statistically derived QSPR models under-estimate skin permeability of hydrophilic solutes. On the other hand, including additional aqueous pathway is necessary for mechanistic models to improve the prediction of skin permeability of hydrophilic solutes, especially for highly hydrophilic solutes. A consensus yet has to be reached as to how the aqueous pathway should be modeled. Nevertheless it is shown that the contribution of aqueous pathway can constitute to more than 95% of the overall skin permeability. Finally, future prospects and needs in improving the prediction of skin permeability of hydrophilic solutes are discussed.

Design and performance of a spreadsheet-based model for estimating bioavailability of chemicals from dermal exposure

A comprehensive transient model of chemical penetration through the stratum corneum, viable epidermis and dermis formulated in terms of an Excel™ spreadsheet and associated add-in is presented. The model is a one-dimensional homogenization of underlying microscopic transport models for stratum corneum and dermis; viable epidermis is treated as unperfused dermis. The model's salient features are a detailed structural description of the skin layers, a combination of first-principles based transport equations and empirical partition and diffusion coefficients, and the capability of simulating a variety of exposure scenarios. Model predictions are compared with representative in vitro skin permeation data obtained from the literature using as summary parameters total absorption (Q(abs)), maximum flux (J(max)) and skin permeability coefficient (k(p)). The results of this evaluation demonstrate the current state-of-the-art in prediction of transient skin absorption and highlight areas in which further elaborations are needed to obtain satisfactory predictions.

Explaining skin permeation of 2-butoxyethanol from neat and aqueous solutions

Absorption of 2-butoxyethanol (BE) from neat and aqueous solutions of BE was measured through rat skin in vitro and in vivo and through silicone membranes. Like previous studies in human and guinea pig skin, BE flux increased proportional to BE concentration only when the weight fraction of BE (w(BE))<about 0.2. The flux of BE was relatively constant for 0.2<w(BE)<0.8, and it decreased dramatically for w(BE)>0.8. Experimental values of thermodynamic activity for BE and water in aqueous solutions of BE are presented. Except when the water content in the vehicle is small, skin is fully hydrated and the flux of a BE through it is proportional to the thermodynamic activity of BE. When w(BE)>0.8, there is a sharp drop in the activity-normalized BE flux through skin, which coincides with a decrease in water activity from 0.9 at w(BE)=0.8 to zero for neat BE. These observations are consistent with reduced BE flux arising from skin dehydration. From an analysis of previously published data, the activity-normalized flux of BE through hydrated human skin was determined to be 2-4 mg cm(-2)h(-1), which is in reasonable agreement with predictions of its maximum flux.

Evaluation of in-vivo animal and in-vitro models for prediction of dermal absorption in man

Risk assessment of dermal exposure to chemicals requires percutaneous absorption data to link the external exposure to the systemic uptake. The most reliable data on percutaneous absorption are obtained from in-vivo human volunteer studies. In addition to ethical constrains, the conduct of these studies is not feasible for the large number of industrial chemicals in use today. Therefore, there is an increasing need for alternative methods to determine percutaneous absorption such as in-vitro assays and methods performed in vivo in experimental animals. In this article, recent comparative in-vitro and in-vivo studies on percutaneous absorption have been addressed with emphasis on the factors that may affect the predictive value of the in-vitro models. Furthermore, the use of animal models, in particular the rat skin, in prediction of percutaneous absorption in the human skin has been reviewed. In-vitro assays showed to be largely influenced by the experimental circumstances, such as type and thickness of the skin, receptor fluid, and the way in which percutaneous absorption is calculated. Rat skin showed consistently to be more permeable than human skin. However, the difference between human and rat skin does not show a consistent pattern between chemicals hampering prediction of human percutaneous absorption. To increase predictive value of in-vitro and animal models, the influence of experimental factors on the percutaneous absorption should be systematically investigated by comparison with human in-vivo data, resulting in more prescriptive guidelines.

Human volunteer study on the inhalational and dermal absorption of N-methyl-2-pyrrolidone (NMP) from the vapour phase

N-Methyl-2-pyrrolidone (NMP) is a versatile organic solvent frequently used for surface cleaning such as paint stripping or graffiti removal. Liquid NMP is rapidly absorbed through the skin but dermal vapour phase absorption might also play an important role for the uptake of the solvent. This particular aspect was investigated in an experimental study with 16 volunteers exposed to 80 mg/m(3) NMP for 8 h under either whole-body, i.e. inhalational plus dermal, or dermal-only conditions. Additionally, the influence of moderate physical workload on the uptake of NMP was studied. The urinary concentrations of NMP and its metabolites 5-hydroxy-N-methyl-2-pyrrolidone (5-HNMP) and 2-hydroxy-N-methylsuccinimide (2-HMSI) were followed for 48 h and analysed by gas chromatography-mass spectrometry (GC-MS). Percutaneous uptake delayed the elimination peak times and the apparent biological half-lives of NMP and 5-HNMP. Under resting conditions, dermal-only exposure resulted in the elimination of 71 +/- 8 mg NMP equivalents as compared to 169 +/- 15 mg for whole-body exposure. Moderate workload yielded 79 +/- 8 mg NMP (dermal-only) and 238 +/- 18 mg (whole-body). Thus, dermal absorption from the vapour phase may contribute significantly to the total uptake of NMP, e.g. from workplace atmospheres. As the concentration of airborne NMP does not reflect the body dose, biomonitoring should be carried out for surveillance purposes.

Hematological Follow‐up of an Intervention Program Adding Rubber Glove‐Wearing to Local Ventilation for 2‐Ethoxyethanol Acetate‐Exposed Workers

The objective of this study was to evaluate an intervention program, implemented in year 1999, of wearing rubber gloves in addition to engineering control, and to follow-up the hematological effects of 2-ethoxyethanol acetate (2-EEA) exposure among workers in a silk-screening factory. All workers from the printing department with direct exposure to 2-EEA were recruited as the exposed group. Workers from the other departments were recruited as the comparison group. Hematological parameters were measured during health surveys conducted 3 times every two years. Information on personal characteristics and working habits was obtained through a structured questionnaire. More female workers were involved in manual printing resulting in higher exposure to 2-EEA. Hemoglobin and haematocrit levels in female exposed workers were significantly lower than those of female comparison workers in the 1st (1998) health survey, but not in the 2nd (2000) and 3rd (2002) health surveys. No difference was found between male exposed and comparison workers for all three surveys. Longitudinal analysis after adjusting for confounders using the general estimating equation model showed the hemoglobin, haematocrit, and RBC count were significantly higher for 2-EEA exposed workers than comparison workers across the 3 surveys (n=42). The results show that wearing rubber gloves in addition to local ventilation was effective at preventing direct dermal exposure to 2-EEA and ameliorated the hematological effects of 2-EEA exposure.

Discrepancies between different rat models for the assessment of percutaneous penetration of hazardous substances

By regulatory authorities the rat is considered to be a suitable animal model to predict the percutaneous absorption of hazardous substances in humans. In our study, the percutaneous penetration of 2-butoxyethanol (BE) and toluene was compared in different rat models. Intradermal microdialysis and static diffusion cells were used in in vivo and in vitro experiments with haired Wistar and hairless Lewis rats. Microdialysis experiments showed a steady-state penetration for BE and a penetration maximum for toluene in both rat strains at approximately 60 min after beginning of exposure. However, in diffusion cell experiments the penetration of the test compounds in both rat strains increased until the end of exposure (4 h). Additionally, in microdialysis experiments BE penetrated in hairless rats in a higher amount than in haired rats (factor: 1.4; P < 0.01), for toluene it was just the opposite (factor: 1.9; P < 0.001). In diffusion cell experiments, the penetrated amounts of both compounds were higher in hairless rats compared to haired rats. The fluxes for BE were in diffusion cell experiments at a factor of 14.5 (haired rat) and 18.1 (hairless rat) higher than in microdialysis experiments, the difference factor for toluene was 2.6 (haired rat) and 12.9 (hairless rat). The lag times indicate a significantly faster penetration in microdialysis experiments compared with diffusion cell experiments (P < 0.001). There are great differences in percutaneous penetration behaviour between the techniques and the rat strains. The diffusion cell method has difficulties to describe the percutaneous penetration kinetics, whereas microdialysis describes it more reliable. Due to these differences the reliability of a conversion factor for the transfer of percutaneous absorption data from rat to human skin, as proposed in the literature, is questionable.

The influence of water mixtures on the dermal absorption of glycol ethers

Glycol ethers are solvents widely used alone and as mixtures in industrial and household products. Some glycol ethers have been shown to have a range of toxic effects in humans following absorption and metabolism to their aldehyde and acid metabolites. This study assessed the influence of water mixtures on the dermal absorption of butoxyethanol and ethoxyethanol in vitro through human skin. Butoxyethanol penetrated human skin up to sixfold more rapidly from aqueous solution (50%, 450 mg/ml) than from the neat solvent. Similarly penetration of ethoxyethanol was increased threefold in the presence of water (50%, 697 mg/ml). There was a corresponding increase in apparent permeability coefficient as the glycol ether concentration in water decreased. The maximum penetration rate of water also increased in the presence of both glycol ethers. Absorption through a synthetic membrane obeyed Fick's Law and absorption through rat skin showed a similar profile to human skin but with a lesser effect. The mechanisms for this phenomenon involves disruption of the stratum corneum lipid bilayer by desiccation by neat glycol ether micelles, hydration with water mixtures and the physicochemical properties of the glycol ether-water mixtures. Full elucidation of the profile of absorption of glycol ethers from mixtures is required for risk assessment of dermal exposure. This work supports the view that risk assessments for dermal contact scenarios should ideally be based on absorption data obtained for the relevant formulation or mixture and exposure scenario and that absorption derived from permeability coefficients may be inappropriate for water-miscible solvents.

Penetration of β-naphthylamine and o-toluidine through human skin in vitro

Several aromatic amines (AAs) are known to be carcinogens for humans. AAs are considered to be substantially absorbed through the skin. However, the database for dermal absorption of AAs in general is limited and no specific studies on dermal absorption of beta-naphthylamine (BNA) and o-toluidine (OT) have been published. In the present study using diffusion cells, we investigated dermal penetration of BNA and OT through human skin. We have demonstrated that both AAs penetrate through human skin fast (lag time: approximately 1.2 vs. 0.8 h) and in high percentages (54 vs. 50%, respectively, of the applied dose within 24 h). A skin notation is therefore justified for these substances.

Evaluation of exposure to 1-alkoxy-2-propanols and 1-(2-methoxy-1-methylethoxy)-2-propanol by the analysis of the parent compounds in urine

Floor lacquerers' inhalation and total exposure to 1-alkoxy-2-propanols and 1-(2-methoxy-1-methylethoxy)-2-propanol (DPGME) were measured. The total exposure was biomonitored by urinalysis of free unchanged 1-alkoxy-2-propanols and DPGME. The floor lacquerers' 8-h inhalation exposures to 1-methoxy-2-propanol (PGME), 1-butoxy-2-propanol (PGBE) and DPGME were 1.9+/-1.3 (mean+/-S.D., n=15), 1.0+/-1.4ppm (n=11) and 0.2+/-0.3ppm (n=11), respectively. The gravity-corrected urinary excretions of PGME, PGBE and DPGME were 5.3+/-5.4mumol/l, 0.9+/-0.9mumol/l and 1.5+/-2.8mumol/l, respectively. A linear relationship was found between the gravity-corrected urinary excretion of PGME (R(2)=0.82), PGBE (R(2)=0.93) and DPGME (R(2)=0.93) and their preceding 8-h inhalation exposure. The correlations between the uncorrected urinary excretions and inhalation exposures to PGME, PGBE and DPGME was also calculated and found good (R(2)=0.82-0.95). The effect of work strain on the total exposure seemed to be more relevant in the exposure to hydrophilic PGME than in the exposure to more lipophilic PGBE.

In vitro studies-how good are they at replacing in vivo studies for measurement of skin absorption?

Measures of percutaneous penetration are required for risk assessment of exposure of man to chemicals. In vitro approaches and QSAR predictions can be used and reduce the use of in vivo animal experiments. The OECD Guidelines on in vitro dermal absorption studies were recently accepted but progress was hampered by a lack of direct in vitro/in vivo comparisons in humans or in rodents. Either flow through diffusion or static cell systems with full thickness, dermatomed skin or membranes can be used. In a study of the robustness of in vitro techniques, inter-skin variability was greater than inter-laboratory or between cell variability. Recent studies with a number of chemicals have shown a reasonably good prediction but the difference between in vitro and in vivo results was greater for lipophilic molecules as lipophilic molecules which were retained in the stratum corneum. The experimental flux obtained in vitro using conditions that reflect the potential occupational exposure may be the most appropriate figure for risk assessment purposes. A database of in vitro and in vivo dermal penetration has been established. Dermal absorption data using infinite doses has been combined in a number of databases used for predictive QSAR modelling approaches to dermal absorption. However, absorption values derived from QSAR may over estimate the actual absorption from a finite dose.

Interactions of skin thickness and physicochemical properties of test compounds in percutaneous penetration studies

OBJECTIVES

To determine the effect of skin thickness on the percutaneous penetration and distribution of test compounds with varying physicochemical properties using in vitro systems. Studies were carried out in accordance with OECD guidelines on skin absorption tests.

METHODS

Percutaneous penetration of caffeine (log P -0.01), testosterone (log P 3.32), propoxur (log P 1.52) (finite dose in ethanol to water vehicle ratio) and butoxyethanol (log P 0.83) (undiluted finite dose or as an infinite dose 50% [v/v] aqueous solution) through skin of varying thicknesses under occluded conditions was measured using flow through cells for 8-24 h. Saline (adjusted to pH 7.4) was used as receptor fluid, with BSA added for studies with testosterone and propoxur. Following exposure, the remaining surface dose was removed by swabbing and the skin digested prior to scintillation counting.

RESULTS

The maximum flux of caffeine was increased with decreasing skin thickness, although these differences were found to be non-significant. The presence of caffeine in the skin membrane was not altered by skin thickness. Maximum flux and cumulative dose absorbed of testosterone and butoxyethanol (in both finite and infinite doses) were markedly reduced with full thickness (about 1 mm thick) skin compared with split thickness skin (about 0.5 mm). Maximum flux of propoxur (dissolved in 60% ethanol) was clearly higher through skin of 0.71 mm than through skin of 1.36 mm, but no difference was found between 0.56 and 0.71 mm. The proportion of propoxur present in the membrane after 24 h increased significantly over the complete range of thicknesses tested (0.56-1.36 mm).

CONCLUSIONS

A complex relationship exists between skin thickness, lipophilicity and percutaneous penetration and distribution. This has implications for risk assessment studies and for the validation of models with data from different sources.

Is the permeability coefficient Kp a reliable tool in percutaneous absorption studies?

In percutaneous absorption studies the potency of penetration of chemical substances is often described by the permeability coefficient Kp. The experimentally determined Kp is characterized according to Fick's first law of diffusion by the ratio of flux and the concentration of the test compound (Kp=Flux/C). This equation implies that in percutaneous absorption studies Kp is theoretically a more reliable parameter than flux taking the concentration into consideration, and should remain constant for each compound independent from the grade of dilution. In our study we evaluated the course of the percutaneous absorption parameters flux and Kp of neat and of 50% aqueous solution of 2-butoxyethanol (BE). An infinite dose of neat and 50% aqueous solution of non-radiolabeled BE were applied on excised human skin from two donors in static diffusion cells in parallel (for each test setting n=21). The flux of 50% aqueous BE (0.704+/-0.152 mg/cm2/h) was about 15-fold higher than that of neat BE (0.045+/-0.014 mg/cm2/h). The comparison of the Kp values of both test settings showed with a factor of about 31 (Kp=1.563 x 10(-3) cm/h) much higher values for 50% aqueous BE and Kp=0.050 x 10(-3) cm/h for neat BE. Although the flux does not consider the chemical concentration, it showed a smaller difference in both test settings as Kp; however, the flux remains a non-specific parameter for the description of percutaneous absorption. The results of our experiments showed that the permeability coefficient Kp was not able to adjust the flux of BE to the concentration. This is in agreement with the evaluation of Kp from BE data described in the literature.

Percutaneous penetration and metabolism of 2-butoxyethanol

2-Butoxyethanol (2-BE) is widely used as an industrial solvent, which may result in human dermal exposure within the workplace. This study compares in vivo and in vitro skin absorption of 2-BE using similar application regimes and determines the potential of skin to metabolise this chemical prior to entering the systemic blood circulation. Following topical application of undiluted [1-14C] 2-BE to occluded rat skin in vivo, 28% of the dose was absorbed after 24 h. The major routes of excretion included the urine (19%), expiration as carbon dioxide (6%) and faeces (0.4%) whilst little of the dose remained in the carcass (1.3%). Free 2-BE (0.5%), butoxyacetic acid (8%), glucuronide conjugate (3%), sulphate conjugates (0.7%) and ethylene glycol (0.6%) were detected in urine. Permeation rates of 2-BE through unoccluded rat dermatomed skin (16%) were greater than rat whole skin (8%) whilst absorption through human dermatomed skin (4%) was lower than the rat. Absorption of undiluted 2-BE through occluded rat dermatomed skin in vitro (18%) most accurately predicted absorption through rat skin in vivo. However, 2-BE absorption (23%) was enhanced by application in methanol. Distribution analysis and microautoradiography demonstrated the lack of 2-BE accumulation within the skin in vitro or in vivo. This was reflected in the absence of first pass metabolism of 2-BE during percutaneous penetration through viable human or rat skin in vitro or rat skin in vivo, despite rat skin cytosol having the potential to metabolise 2-BE. In conclusion, the in vitro system provided a reasonable estimate of dermal absorption in vivo for the rat. Therefore, by extrapolation of the comparative in vitro data for human and rat skin in vitro, dermal absorption of 2-BE in man was about one-fifth of that in the rat. However, the rapid penetration through skin in vitro prevented local metabolism and systemic exposure after skin contact with 2-BE in vivo was likely to be to the parent compound. Thus, in vitro skin systems can be used to model dermal absorption of volatile glycol ethers, to predict how much compound enters the circulation and allows the toxicologist to evaluate the body burden of a chemical and potential systemic toxicity.