Equation 1.  According to the well-known Fick’s First Law of
Diffusion, the flux, J, of a chemical through the skin equals to
Equation 1 in which kp is the permeability coefficient, ΔC the
concentration difference between the concentrations at the top
and bottom of the stratum corneum, D the diffusion coefficient,
K the partitioning coefficient and L the length of the pathway
of diffusion.