SIGNIFICANCE OF THE DERMAL ROUTE OF EXPOSURE TO RISK ASSESSMENT

The skin is a route of exposure that needs to be considered when condu cting a risk assessment. It is necessary to identify the potential for dermal penetration by a chemical as well as to determine the overall importance of the dermal route of exposure as compared with inhalation or oral routes of exposure. The physical state of the chemical, vapor or liquid, the concentration, neat or dilute, and the vehicle, lipid or aqueous, is also important. Dermal risk is related to the product o f the amounts of penetration and toxicity. Toxicity involves local eff ects on the skin itself and the potential for systemic effects. Dermal penetration is described in large part by the permeability constant. When permeability constants are not known, partition coefficients can be used to estimate a chemical's potential to permeate the skin. With these concepts in mind, a tiered approach is proposed for dermal risk assessment. A key first step is the determination of a skin-to-air or skin-to-medium partition coefficient to estimate a potential for derma l absorption. Building a physiologically-based pharmacokinetic (PBPK) model is another step in the tiered approach and is useful prior to cl assical in vivo toxicity tests. A PBPK model can be used to determine a permeability constant for a chemical as well as to show the distribu tion of the chemical systemically. A detailed understanding of species differences in the structure and function of the skin and how they re late to differences in penetration rates is necessary in order to extr apolate animal data from PBPK models to the human. A study is in progr ess to examine anatomical differences for four species.