Objectives: To determine the dermal absorption rates of vaporous 1,1,1-trichloroethane (111TRI), trichloroethene (TRI), tetrachloroethene (TETRA), hexane (HEX), toluene (TOL) and m-xylene (XYL) in humans. The determined absorption data were used for the validation of two published models for prediction of non-steady-state skin absorption.
Methods: Five volunteers were dermally exposed on an area of about 1,000 cm2 (forearm and hand) for 20 or 30 min. An inhalation exposure with a known dose rate served as a reference. Using the solvent concentrations in exhaled air, measured after both inhalation and dermal exposure, we calculated the maximum absorption rate into the blood, and the average absorption rates into the skin throughout the exposure, using the linear system dynamics method.
Results: The absorption rates into the skin, normalised for exposure concentration, amounted to 0.021 cm/h (111TRI), 0.049 cm/h (TRI), 0.054 cm/h (TETRA), 0.013 cm/h (HEX), 0.14 cm/h (TOL), and 0.12 cm/h (XYL). The maximum absorption rates into the blood ranged from 0.005 nmol/h for 111TRI and HEX to 0.050 nmol/hr for TOL. The ratios between the predicted and experimental values of the absorption rates into the skin ranged, for the model of Cleek and Bunge [4], from 0.3 (HEX) to 1.1 (TRI and TETRA), and for the model of Wilschut and Ten Berge [22], from 1.1 (HEX) to 4.7 (XYL).
Conclusion: The linear system dynamics method allowed us to calculate not only the total amount absorbed by the skin but also the maximum absorption rate into the blood. The steady-state absorption rate, usually described by a permeability constant, will be below the absorption rate into the skin and above the maximum absorption rate into the blood. The skin absorption rates predicted by the models showed a good agreement with the experimental values. A comparison of the estimated whole-body skin uptake with the inhalatory uptake from the same atmosphere, revealed that the dermal uptake contributed from 0.1% (HEX) to 1% (TOL and XYL) to the total uptake.