Margin of safety and exposure of nanomaterials used in cosmetics

It is an urgent problem to evaluate safety concern of nanomaterials such as titanium oxide- and zinc oxide-nanoparticles that are broadly used as an UV-scattering cosmetic. The primary diameter of the nanomaterials is 15-50 nm. Possible exposure sites to human are the GI tracts, respiratory tracts (nasal cavlty, trachea, tracheal branch and alveolus of the lung) and skin. Skin must be most important exposure site among them especlally for general consumers. As easily understood by the 500 Dalton rule (where these small molecules have 0.2-0.4 nm in diameter) , nanomaterials are hardly penetrated into the skin tissues through stratum corneum, primary permeation pathway of most compounds. We have to pay attention to the second permeation pathway (transfollicular pathway) of skin entry in case of nanomaterials. We also consider several wound skins which have low skin barrier function compared with normal skin for nanomaterial penetration into skin. In addition, we must consider the nanomaterial exposure in a quantitative way not qualitative way. For example, the resquamation rate and permeability coefficient of water are I x 10 -9 and 2 x 10 -7 cm/s, respectively. Physicochemical properties of nanomaterials are also important determinants for their skin entry. Evaluation study on the nanomaterial safety is just the mark of the start line. We have to pay attention to this issue.特集 紫外線防御試験法の国際的動向と紫外線防御剤の開発の課