Abstract:
Analysis of selected data from USEPA's Children's Total Exposure to Persistent Pesticides and Other Persistent Pollutants (CTEPP) study reveals that observed outputs of urinary metabolites cannot be fully accounted for by conventionally estimated inputs of those pesticides for which mass balance can be feasibly attempted. Evidence that much of this shortfall can be explained by a phenomenon designated persistent low-level ambient contact exposure (PLACE) has been presented previously. An assumption essential to this hypothesis is that dermal absorption of semi-volatile organic chemicals (SVOCs) is more efficient at low (~1 ng/cm2) loads than at skin loads typically applied in laboratory absorption experiments and that therefore the uptake of SVOCs into skin under common conditions is significantly greater than previously assumed. While this assumption is theoretically well-founded, difficulties in reliably applying very small quantities of SVOCs to human skin while maintaining uniform spatial distribution impede empirical quantification of the efficiency of dermal absorption at low loads. To address this issue, a glass and Teflon chamber was constructed that permits deposition of aerosols generated by a Collison nebulizer onto skin coupons. Results from experiments in which a fluorescent tracer was employed provide visual evidence that distribution of tracer on human cadaver skin following low-load aerosol deposition differs substantially from that observed following application in solvent by pipette. Subsequent experiments involving nebulization of an ethanol-based solution of 14C-labeled pentachlorophenol (PCP) have demonstrated that PCP loads on the order of 1 ng/cm2 can be reproducibly applied to, and quantitatively recovered from, human cadaver skin. In the current phase of experimentation, residue remaining in/on skin following soap and water washing serves as a surrogate for dermal absorption. Greater absorption efficiency at lower initial loads is hypothesized. Further research into the PLACE hypothesis will lead to a better understanding of long-term residential exposure to potentially harmful chemicals.