The skin is constantly exposed to foreign compounds such as gases, solvents, and substances in solution, and so absorption through the skin is potentially an important route.The outer layer of the nonvascularized epidermis, the stratum corneum, consists mainly of cells packed with keratin, which limits the absorption of compounds, and a few hair follicles and sebaceous glandsHowever, although the skin has a large surface area, some 18,000 cm2 in humans, fortunately, it represents an almost continuous barrier to foreign compounds as it is not highly permeable.