Skin Penetration and Safety of Nanoparticles

Human beings are exposed or otherwise a subjected to a various chemical compounds. Various nanomaterials are contained in the chemical compounds which are used in many fields. Nanomaterials are also used in cosmetics: titanium dioxide and zinc oxide are examples. Consumers who apply cosmetics to their skin as well as workers at industrial plants may thus be exposed to these nanoparticles. Therefore, it is of great importance to evaluate the safety of these nanoparticles. In this review, we describe the possibility of nanoparticle penetration to skin following exposure, which makes it urgent to evaluate the safety factors. In general, it is necessary to take account of the desquamation rate of the stratum corneum and the permeation pathway and size of nanoparticles when considering such penetration. One layer of the human stratum corneum is peeled off per day. Therefore, a chemical compound of which the skin penetration is lower than the desquamation rate does not permeate through the skin, when the compound infiltrates the stratum corneum. Hence, compounds with a molecular weight of more than 500 Daltons do not permeate through the stratum corneum. However, we must also pay attention to the appendage routes, although the aforementioned layer is the primary permeation route of nanoparticles. The contribution of appendage routes must be taken into consideration.