Percutaneous absorption and metabolism of 2-butoxyethanol in human volunteers: A microdialysis study

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

New experimental data on the human dermal absorption of Simazine and Carbendazim help to refine the assessment of human exposure

Due to their widespread usage, people are exposed to pesticides on a daily basis. Although these compounds may have adverse effects on their health, there is a gap in the data and the methodology needed to reliably quantify the risks of non-occupational human dermal exposure to pesticides. We used Franz cells and human skin in order to measure the dermal absorption kinetics (steady-state flux, lag time and permeability coefficient) of Carbendazim and Simazine. These parameters were then used to refine the dermal exposure model and a probabilistic simulation was used to quantify risks resulting from exposure to pesticide-polluted waters. The experimentally derived permeability coefficient was 0.0034 cm h(-1) for Carbendazim and 0.0047 cm h(-1) for Simazine. Two scenarios (varying exposure duration and concentration, i.e. environmentally relevant and maximum solubility) were used to quantify the human health risks (hazard quotients) for Carbendazim and Simazine. While no risks were determined in the case of either scenario, the permeability coefficient, which is concentration independent and donor, formulation, compound and membrane specific, may be used in other scenarios and exposure models to quantify more precisely the dermally absorbed dose during exposure to polluted water. To the best of our knowledge, the dermal absorption kinetics parameters defined here are being published for the first time. The usage of experimental permeability parameters in combination with probabilistic risk assessment thus provides a new tool for quantifying the risks of human dermal exposure to pesticides.

Dermal uptake of 18 dilute aqueous chemicals: in vivo disappearance-method measures greatly exceed in vitro-based predictions

Average rates of total dermal uptake (Kup ) from short-term (e.g., bathing) contact with dilute aqueous organic chemicals (DAOCs) are typically estimated from steady-state in vitro diffusion-cell measures of chemical permeability (Kp ) through skin into receptor solution. Widely used ("PCR-vitro") methods estimate Kup by applying diffusion theory to increase Kp predictions made by a physico-chemical regression (PCR) model that was fit to a large set of Kp measures. Here, Kup predictions for 18 DAOCs made by three PCR-vitro models (EPA, NIOSH, and MH) were compared to previous in vivo measures obtained by methods unlikely to underestimate Kup . A new PCR model fit to all 18 measures is accurate to within approximately threefold (r = 0.91, p < 10(-5) ), but the PCR-vitro predictions (r > 0.63) all tend to underestimate the Kup measures by mean factors (UF, and p value for testing UF = 1) of 10 (EPA, p < 10(-6) ), 11 (NIOSH, p < 10(-8) ), and 6.2 (MH, p = 0.018). For all three PCR-vitro models, log(UF) correlates negatively with molecular weight (r(2) = 0.31 to 0.84, p = 0.017 to < 10(-6) ) but not with log(vapor pressure) as an additional predictor (p > 0.05), so vapor pressure appears not to explain the significant in vivo/PCR-vitro discrepancy. Until this discrepancy is explained, careful in vivo measures of Kup should be obtained for more chemicals, the expanded in vivo database should be compared to in vitro-based predictions, and in vivo data should be considered in assessing aqueous dermal exposure and its uncertainty.

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.

In vivo monitoring of epidermal absorption of hazardous substances by confocal Raman micro-spectroscopy

BACKGROUND

Presently, percutaneous absorption of potentially hazardous chemicals in humans can only be assessed in animal experiments, in vitro, or predicted mathematically. Our aim was to demonstrate the proof-of-principle of a novel quantitative in vivo assay for percutaneous absorption: confocal Raman micro-spectroscopy (CRS). The advantages and limitations of CRS for health risk assessments are discussed.

PATIENTS AND METHODS

2-butoxyethanol, toluene, and pyrene were applied in pure form, diluted in water, or in ethanol on the skin of three healthy volunteers. CRS measurements were done following application for 15 min and 3 hours. The concentrations of the three substances as a function of distance to the skin surface were calculated and further analyzed with regard to mass transport into the stratum corneum (μg/cm(2)) and the flux through the stratum corneum (μg/cm(2)h). The results were compared with the available data from literature.

RESULTS

Considering the preliminary nature of these data, good accordance with data from the literature was observed. In addition, we observed that 2-butoxyethanol penetrates markedly faster when dissolved in water as compared to ethanol. This observation is also in agreement with previous results.

CONCLUSIONS

CRS has the potential to provide fast, accurate and reliable results for advanced studies of in vivo percutaneous absorption kinetics of hazardous substances in human skin. This will require further research with other substances and under differing conditions.

Common household chemicals and the allergy risks in pre-school age children

Background

The risk of indoor exposure to volatile organic compounds (VOCs) on allergic airway diseases in children remains unknown.

Objective

We examined the residential concentrations of VOCs, emitted from building materials, paints, furniture, and other lifestyle practices and the risks of multiple allergic diseases as well as the IgE-sensitization in pre-school age children in Sweden.

Methods

In a case-control investigation (198 case children with asthma and allergy and 202 healthy controls), air samples were collected in the room where the child slept. The air samples were analyzed for the levels of eight classes of VOCs.

Results

A natural-log unit of summed propylene glycol and glycol ethers (PGEs) in bedroom air (equal to interquartile range, or 3.43 – 15.65 µg/m³) was associated with 1.5-fold greater likelihood of being a case (95% CI, 1.1 – 2.1), 1.5-fold greater likelihood of asthma (95% CI, 1.0 – 2.3), 2.8-fold greater likelihood of rhinitis (95% CI, 1.6 – 4.7), and 1.6-fold greater likelihood of eczema (95% CI, 1.1 – 2.3), accounting for gender, secondhand smoke, allergies in both parents, wet cleaning with chemical agents, construction period of the building, limonene, cat and dog allergens, butyl benzyl phthalate (BBzP), and di(2-ethylhexyl)phthalate (DEHP). When the analysis was restricted to the cases, the same unit concentration was associated with 1.8-fold greater likelihood of IgE-sensitization (95% CI, 1.1 – 2.8) compared to the non-IgE sensitized cases. No similar associations were found for the other classes of VOCs.

Conclusion

We propose a novel hypothesis that PGEs in indoor air exacerbate and/or induce the multiple allergic symptoms, asthma, rhinitis and eczema, as well as IgE sensitization respectively.

Association between indoor exposure to semi-volatile organic compounds and building-related symptoms among the occupants of residential dwellings

The aim of this study was to evaluate the levels of semi-volatile compounds (SVOCs) in residential detached houses in Sapporo, Japan, and whether exposure to these SVOCs was associated with the development of building-related symptoms named 'sick house syndrome' (SHS). The definition of SHS is fundamentally the same as that of the sick building syndrome (SBS). The presence of symptoms of SHS was evaluated using a validated self-administered questionnaire. Surveys and samplings of air and house dust in 41 dwellings were performed from October 2006 to January 2007, and 134 occupants responded to questionnaires. Samples were analyzed to quantify the concentrations of eight plasticizers, eleven phosphate triester flame retardants, two alkyl phenols used as anti-oxidants, and one organochlorine synergist called s-421, by gas chromatography-mass spectrometry and gas chromatography-flame photometry. The compounds frequently detected were di-n-butylphthalate, di(2-ethylhexyl)phthalate (DEHP), and dibutylhydroxytoluene in air, and DEHP and tris(2-butoxyethyl)phosphate (TBEP) in dust. Tributylphosphate was strongly and directly associated with mucosal symptoms of SHS; s-421 was also directly associated with mucosal symptoms of SHS. On the contrary, some chemicals such as diethylphthalate and TBEP were inversely associated with SHS. In future studies, we plan to assess these associations in a larger population.

PRACTICAL IMPLICATIONS

This study suggests that it may be possible to reduce building-related symptoms by altering exposure to certain SVOCs, such as tributylphosphate commonly found in ceiling and wall coverings and s-421 used as a synergist for pyrethroids. The association between SHS and s-421 suggested that use of pyrethroid insecticides could elicit symptoms of SHS. However, further studies are necessary to test the associations observed in the present study and to examine whether the SVOCs associated with symptoms are causative agents or simply surrogates for some other factor that is causing the symptoms.

Potential for metabolism locally in the skin of dermally absorbed compounds

Dermally absorbed chemicals can be locally metabolized in the skin during absorption but it is difficult to distinguish this metabolism from liver metabolism by biological monitoring in vivo. Studies with sub-cellular fractions have showed the presence of metabolizing enzymes in the skin but with loss of cellular localization. Studies in HaCat cells in culture maintain cellular localization and in skin, in short-term culture, the chemical can be applied to the skin surface and metabolism during absorption can be monitored. Flow though diffusion systems with tissue culture medium as receptor fluid have maintained the viability of skin and supported metabolism, but dilution of the metabolites in the receptor fluid has limited detection. This article uses data derived by a range of techniques from the Newcastle laboratory to discuss the importance of local metabolism in the skin of butoxyethanol to butoxyacetic acid and parabens to p-hydroxybenzoic acid during dermal absorption, following application to the skin surface.

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.