[Estimation of skin absorption of chemicals with Fick's diffusion model].

The skin is not only the site of drug application, but also the site of exposure to various chemical substances. Prediction of skin permeability, body absorption, and local skin concentration of chemicals are very important to assure efficacy and safety. Most in silico models for skin permeation of chemical are based on the prediction of its permeability coefficient (P, unit cm/s), and a certain level of prediction accuracy has been obtained. On the other hand, the amount absorbed in the body of a chemical is calculated from the sum of the amount of skin permeation and the amount under the stratum corneum (viable epidermis and dermis). The amount of skin permeation can be calculated from the P value as mentioned above, however, the amount under the stratum corneum, which is related to the local skin safety of the chemical, cannot be predicted. This article describes the principles of permeation of chemicals across the skin, the relationship between skin permeability and concentration in the skin, and the method for calculating the chemical concentration under the stratum corneum using skin permeation parameters.