Skin.

Human skin provides a barrier between the host and the physical, chemical, and biological environment. It is also a potential portal of entry for hazardous or infectious agents and a potential target of environmental toxins. Cutaneous vulnerability may take on many forms in the embryo, infant, child, and adolescent. Teratogenic agents may occasionally target skin, as appreciated in the proposed association of the antithyroid medication methimazole, with the congenital malformation known as aplasia cutis congenita. Percutaneous absorption of topically applied substances and the potential for resultant drug toxicities are important considerations in the child. Many topical agents have been associated with systemic toxicity, including alcohol, hexachlorophene, iodine-containing compounds, eutectic mixture of local anesthetics, and lindane. Percutaneous toxicity is of greatest concern in the premature infant, in whom immaturity of the epidermal permeability barrier results in disproportionately increased absorption. Immature drug metabolism capabilities may further contribute to the increased risk in this population. Ultraviolet (UV) radiation exposure, which increases an individual's risk of cutaneous carcinogenesis, may be a particularly significant risk factor when it occurs during childhood. The "critical period hypothesis" suggests that UV exposure early in life increases the risk of eventual development of malignant melanoma. Other risk factors for malignant melanoma may include severe sunburns during childhood, intense intermittent UV exposure, and increased susceptibility of pediatric melanocytes to UV-induced DNA damage. Last, percutaneous exposure to environmental toxins and chemicals, such as insecticides and polychlorinated biphenyls, may differ between children and adults for several reasons, including behavioral patterns, anatomic and physiologic variations, and developmental differences of vital organs.