SERA TR 98-21-08-01d1 A Reevaluation of Methods for Assessing Dermal Exposure

PREFACE Under purchase order 43-57G3-3-C5374 from USDA/APHIS, SERA (1993) prepared a review and evaluation of methods used to estimate exposure to pesticides. The report, Special Issues Related to the Assessment of Pesticide Absorption, includes methods for estimating dermal absorption rates, the uptake of pesticides from contaminated vegetation, and worker exposure involving the application of pesticides. The analysis of dermal absorption rates and the uptake from contaminated vegetation are published in the open literature (Durkin et al. 1995). The analysis of worker exposure is not published in the open literature but is used by the Forest Service to conduct human health risk assessments under the current contract. The following paper presents a reanalysis of the dermal absorption data that is suitable for publication. Additional details of the analyses that would be too voluminous for a typical publication are included as part of the report submitted under this task [SERA TR 98-21-08-01d, dated May 15, 1998]. In preparing this paper for publication, some additional analyses were conducted that are not included in the formal report. ABSTRACT This paper reexamines an earlier analysis by the Durkin et al. (1995) on the different types of dermal exposure scenarios that may be characterized by either first-order or zero-order (Fick's first law) absorption models. In addition, this paper presents a substantial expansion and analysis of a unique and highly relevant series of studies by Feldmann and Maibach (1969, 1970, and 1974) that may be used to estimate first-order dermal absorption rate coefficients. The reanalysis of the Feldmann and Maibach data is based on estimated absorption rate coefficients rather than simple observed absorption rates, as well as a much less arbitrary censoring of the data compared to the earlier analysis by Durkin et al. (1995). In addition, the equations that describe the relationship of the absorption rate coefficients to physicochemical properties (Equations 12 and 13) are qualitatively similar to the equations recommended by U.S. EPA (1992) in the estimation of dermal permeability. While the underlying processes involved in dermal absorption are likely to be zero-order in cases where the individual is effectively immersed in a large volume of contaminated fluid, the present analysis suggests that 'dry deposition' scenarios are likely to be better modelled using the assumption of first-order dermal absorption. For highly lipophilic compound, nonetheless, the differences between these two methods when applied to comparable exposure scenarios is, in general, not likely to be substantial. For …