Dermal Uptake of Organic Chemicals from a Soil Matrix

Uptake of chemicals from soil on human skin is considered. Based on a review of literature on the structure of human skin, the processes by which chemicals pass through this boundary, and experiments that reveal the rate and magnitude of this transport process; a two-layer model is presented for estimating how chemical uptake through the stratum corneum depends on chemical properties, skin properties, soil properties and exposure conditions. The model is applied to two limiting scenarios--(1) continuous deposition and removal of soil on the skin surface and (2) a one-time deposition of soil onto the skin surface. The fraction of soil-bound chemical that passes through the stratum corneum is dependent on the skin-soil layer thickness; the dimensionless Henry's law constant, Kh and the octanol-water partition coefficient, Kow of the soil-bound chemical. The nature of this dependence is discussed.     

Estimating Dermal Uptake of Nonionic Organic Chemicals from Water and Soil: I. Unified Fugacity-Based Models for Risk Assessments

Contamination of water and soil that might eventually contact human skin makes it imperative to include the dermal uptake route in efforts to assess potential environmental health risks. Direct measurements of dermal uptake from either water or soil are only available for a small number of the thousands of chemicals likely to be found in the environment. We propose here a mass-transfer model for estimating skin permeability and dermal uptake for organic chemicals that contaminate soil and water. Statistical relationships between measured permeabilities and chemical properties reveal that permeability varies primarily with the octanol-water partition coefficient (Kow) and secondarily with the molecular weight. From these results, we derive a fugacity-based model for skin permeability that addresses the inherent permeability of the skin, the interaction of the skin with the environmental medium on skin (water or soil), and retains a relatively simple algebraic form. Model predictions are compared to measured human skin permeabilities for some 50 compounds in water and four compounds in soil. The model is adjusted to account for dermal uptake during both short-term (10-20 min) and long-term (several hour) exposures. This model is recommended for compounds with molecular weight less than or equal to 280 g.     

Time- and Loading-Dependence in the McKone Model for Dermal Uptake of Organic Chemicals from a Soil Matrix

McKone has recently proposed an innovative two-layer model for dermal uptake of organic chemicals from a soil matrix that explicitly includes variables for properties of the chemical, the soil, the skin, and the exposure. In this note, we investigate the joint time- and loading-dependencies implicit in the model by using MATHEMATICA to find and plot a closed-form function for the uptake fraction for six aromatic hydrocarbons.     

Benzene Inhalation by Parts Washers: New Estimates Based on Measures of Occupational Exposure to Solvent Coaromatics

Questions persist regarding assessment of workers’ exposures to products containing low levels of benzene, such as mineral spirit solvent (MSS). This study summarizes previously unpublished data for parts‐washing activities, and evaluates potential daily and lifetime cumulative benzene exposures incurred by workers who used historical and current formulations of a recycled mineral spirits solvent in manual parts washers. Measured benzene concentrations in historical samples from parts‐washing operations were frequently below analytical detection limits. To better assess benzene exposure among these workers, air‐to‐solvent concentration ratios measured for toluene, ethylbenzene, and xylenes (TEX) were used to predict those for benzene based on a statistical model, conditional on physical‐chemical theory supported by new thermodynamic calculations of TEX and benzene activity coefficients in a modeled MSS‐type solvent. Using probabilistic methods, the distributions of benzene concentrations were then combined with distributions of other exposure parameters to estimate eight‐hour time‐weighted average (TWA) exposure concentration distributions and corresponding daily respiratory dose distributions for workers using these solvents in parts washers. The estimated 50th (95th) percentile of the daily respiratory dose and corresponding eight‐hour TWA air concentration for workers performing parts washing are 0.079 (0.77) mg and 0.0030 (0.028) parts per million by volume (ppm) for historical solvent, and 0.020 (0.20) mg and 0.00078 (0.0075) ppm for current solvent, respectively. Both 95th percentile eight‐hour TWA respiratory exposure estimates for solvent formulations are less than 10% of the current Occupational Safety and Health Administration permissible exposure limit of 1.0 ppm for benzene.     

Dermal Uptake of 18 Dilute Aqueous Chemicals: In Vivo Disappearance‐Method Measures Greatly Exceed In Vitro‐Based Predictions

Average rates of total dermal uptake (K_up) 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 (K_p) through skin into receptor solution. Widely used (“PCR‐vitro”) methods estimate K_up by applying diffusion theory to increase K_p predictions made by a physico‐chemical regression (PCR) model that was fit to a large set of K_p measures. Here, K_up 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 K_up. 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 K_up 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 (r² = 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 K_up 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.