ABSTRACT
This chapter focuses on assessing the bioactivity of both cosmetic products and cosmetic ingredients on the skin. The legal definition of a cosmetic is a product which enhances appearance, aids in personal hygiene, and does not affect the structure or function of the skin. Cosmetic products, therefore, contain no active drug ingredient. Almost all moisturizers are cosmetics. There are a few exceptions, such as OTC drugs under the skin protectant and analgesic monographs. Structurally, the skin absorbs through a dual membrane system perforated with shunts. The first rate-limiting membrane is the stratum corneum of the epidermis. The second rate-limiting membrane is the dermal-epidermal junction or basement membrane. Human skin is the site of extrahepatic metabolism for certain types of chemicals. Hydration of the skin increases percutaneous absorption potential. Externally absorbed cutaneous water increases the partitioning of certain chemicals in the skin, and there are differences in water uptake by the skin versus the hair.
