Dermal absorption may be an important route of exposure in several exposure scenarios for workers and the general public. Because criteria (e.g., RfDs or MRLs) for chemical exposures are usually expressed in units of mg/kg/day, risk assessments often attempt to convert dermal exposure data to units of mg/kg/day absorbed dose. For some types of dermal exposure involving direct and continuous contact with liquids, Fick's first law may be used. In other cases, such as those involving spills onto the skin surface or dermal exposure to contaminated vegetation, the applicability of Fick's first law is limited.

This analysis focuses on a method for estimating absorbed dose from dermal contact with contaminated vegetation or other surfaces. The method involves two steps: estimating the transfer rate from contaminated vegetation to the skin surface, and estimating the extent of absorption from the skin surface into the body.

A generic equation can be derived for estimating the transfer rate (TR) from dislodgeable foliar residues (DFR):

logTR = 1. 09 logDFR + 0. 05.

Given the surface area of the exposed skin and the duration of contact, this equation can be used to estimate the amount of a chemical deposited on the surface of the skin. This equation is based on data from eight different studies using 16 different pesticides. Excluding one outlier (Vinclozolin), the squared correlation coefficient for this equation is 0.78, and the model is significant at p < 0. 00001.

Data from a series of studies by Feldmann and Maibach (1969, 1970, 1974) are used to estimate dermal absorption. These studies were selected as the most relevant for risk assessment because most of the experimental subjects are human and because the nature of the dermal exposures was closely related to many exposure scenarios used in risk assessments. For all 47 compounds included in this series of studies, there were no significant correlations between commonly available physical or chemical properties and dermal absorption. For those compounds with a K_o/w > 1.85, however, the average daily absorption rate (AR) over a five-day postexposure period can be estimated from the molecular weight:

logAR = -0.04MW + 1.5.

The squared correlation coefficient for this equation is 0.68, and the model is significant at p < 0.00001.

The usefulness of this approach is evaluated using a study by Harris and Solomon (1992) in which the absorption of 2,4-D from contaminated turf was measured in a group of volunteers. The estimated absorbed dose using the equations above is very close to the measured values.

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Methods of Assessing Dermal Absorption With Emphasis On Uptake From Contaminated Vegetation