Dermal transfer coefficients

USEPA (1998b). Laboratory and Field Methods to Establish a Dermal Transfer Coefficient for Residential Exposure Monitoring, Report No. 600/X-98/006, Office of Research and Development, Washington, DC, USA. 

Establishment of dermal transfer coefficients across a range of surface residue levels and work activities... 

Transfer coefficient for dermal surface region k (m /h) = (mass on dermal surface region fc)/[ (exposure duration) x (dislodgeable concentration in total environmental surface)] ... 

Risk certification accompanied by data requirements, such as individual product operator exposure studies, dermal absorption data and/or the range of results from biological monitoring of those exposed. This tier recognizes that where the best available knowledge used in the second tier still indicates risks, then exposure measurements for the actual product will be necessary. Exposure surveys need to be of adequate size, sufficiently reported and representative in order to be convincing. Exposure surveys may gather data that can be used for statistical relations, such as transfer coefficients, which may be used for other products in the lower tiers. 

The parameter a = kJ ID is the ratio of the rate of chemical transfer from the soil to the SC (a process that can be characterized by a mass transfer coefficient which has units of cm s ) to the rate of mass transport through the SC (i.e., DIL). Thus, when a > 1, the SC resistance is most important on the other hand, for a < 1, mass transfer from the soil to the SC limits the dermal absorption rate. When the mass transfer resistance from the vehicle is insignificant, the concentration of chemical on the skin surface (i.e., C ) is equal to (see Equation 11.2). By comparison, when the mass transfer resistance witliin the vehicle is not small relative to that in the SC (i.e., a is not very large), then at steady state is reduced relative to as specified by Equation 11.3. That is. 

The factor k (the transfer factor) was defined as a crop- and task-specific factor and is defined as the slope of the line that fits dermal exposure levels (g/hr) and corresponding levels of DFR (g/m2) on the crop (i.e., the regression coefficient of DFR). The DFR, according to the procedures described by Iwata et al. (1977), was considered to be a good estimate of source strength for re-entry exposure. 

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