Effect of soil loading on dermal absorption efficiency from contaminated soils

The effect of soil loading on the dermal uptake of soil-borne contaminants was examined using an in vitro evaporation/penetration apparatus and abdominal skin from human cadavers. Dermal uptake of two C-14-labeled pesticides, lindane and 2,4-dichlorophenoxyacetic acid (2,4-D), was assessed at nominal soil loadings of 1, 5, and 10 mg/cm(2). Sub-150-mu m fractions of two soils with differing organic carbon contents were employed. Mean 24-h dermal absorption values ranged from 0.45 to 2.35% for lindane and from 0.18 to 1.64% for 2,4-D, depending upon soil load and type. Mean mass fluxes ranged from 8.8 to 32 pg/cm(2)/h for lindane and from 1.9 to 6.4 pg/cm(2)/h for 2,4-D. Results were aggregated as ratios of flux or percent absorption at 1 and 10 mg/cm(2) to corresponding values obtained at 5 mg/cm(2). Fluxes at 5 and 10 mg/cm(2) did not differ significantly, but flux at 1 mg/cm(2) was about one-half the value observed al the higher loadings. The most plausible explanation for this decrease in mass flux is incomplete (submonolayer) coverage of the skin. Evidence in the form of electron micrographs is presented in support of this conclusion. Relative percent absorption increased significantly with decreases in soil load from 10 to 5 and from 5 to 1 mg/cm(2). This effect was inversely proportional to loading reduction in the former case, bur was less than proportional due to the impact of contact area reduction (and in the case of lindane, volatilization losses) in the latter. Percent dermal absorption data obtained in the laboratory require adjustment for differences in loading and coverage before application to assessment of exposure to contaminants in soils. Description of dermal absorption from soil in a manner comparable to that used to describe absorption from a liquid or vapor (i.e., using a driving force and a mass transfer coefficient) would reduce confusion on this point and is recommended.