[Cutaneous absorption of chemicals]

Chemicals have become indispensible for the maintenance of health in animals and man. The route of administration of each medicament is decided by factors such as site of desired action, chemistry of the active ingredient, age and species of the patient, and frequency of administration (or desired duration of activity). In situations where the oral and hypodermic routes, which are used most frequently, are inadequate or unsatisfactory, dermal application can provide a valuable alternative method to achieve systemic activity. Examples of formulations currently available for dermal application contain diverse chemicals and are intended for a variety of purposes, such as crufomate against cattle grubs, fenthion against cattle lice, levamisole against gastrointestinal nematodes, nitroglycerine for angina pectoris, and scopolamine for motion sickness. The skin acts as a barrier to penetration by chemicals and micro-organisms by virtue of its morphology and chemical composition. Chemicals which do penetrate, do not necessarily pass through the appendages (hair follicles and gland ducts), but mostly penetrate through the interjacent epidermis, either through the cells, or via the intracellular spaces. These spaces have recently been shown by electron microscopy to be filled by an amorphous substance which exudes on the skin surface in convex ridges. This substance has a lipid nature, but is not hydrophobic as is often accepted. For a chemical to be able to penetrate the skin, it must be partially water and lipid soluble, polar, and weakly ionizing. A variety of factors can possibly affect the permeability of skin for a chemical. These include species differences in morphology (skin thickness, tightness of intercellular junctions, density of hair follicles and other appendages), biochemistry, and physiology; seasonal and climatic variations; and differences between breeds and genders. Species differences in skin permeability are largely unpredictable and inconsistent. An observed difference between two species for one chemical cannot necessarily be extrapolated to another.(ABSTRACT TRUNCATED AT 250 WORDS)