Absorption (skin)

Skin absorption is a route by which substances can enter the body through the skin. Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for toxic substances and route of administration for medication. Absorption of substances through the skin depends on a number of factors, the most important of which are concentration, duration of contact, solubility of medication, and physical condition of the skin and part of the body exposed.

Skin (percutaneous, dermal) absorption is a term that describes the transport of chemicals from the outer surface of the skin both into the skin and into circulation. Skin absorption relates to the degree of exposure to and possible effect of a substance which may enter the body through the skin. Human skin comes into contact with many agents intentionally and unintentionally. Skin absorption can occur from occupational, environmental, or consumer skin exposure to chemicals, cosmetics, or pharmaceutical products. Some chemicals can be absorbed in enough quantity to cause detrimental systemic effects. Skin disease (dermatitis) is considered one of the most common occupational diseases. In order to assess if a chemical can be a risk of either causing dermatitis or other more systemic effects and how that risk may be reduced one must know the extent to which it is absorbed, thus dermal exposure is a key aspect of human health risk assessment.

Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for bioactive substances including medications. Absorption of substances through the skin depends on a number of factors:

In general the rate of absorption of chemicals through skin follows the following scheme from fastest to slowest: Scrotal > Forehead > Armpit≥ Scalp > Back = Abdomen > Palm = under surface of the foot.

To be absorbed through the skin, a chemical must pass through the epidermis, glands, or hair follicles. Sweat glands and hair follicles make up about 0.1 to 1.0 percent of the total skin surface. Though small amounts of chemicals may enter the body rapidly through the glands or hair follicles, they are primarily absorbed through the epidermis. Chemicals must pass through the seven cell layers of epidermis before entering the dermis where they can enter the blood stream or lymph and circulate to other areas of the body. Toxins and toxicants can move through the layers by passive diffusion. The stratum corneum is the outermost layer of the epidermis and the rate-limiting barrier in absorption of an agent. Thus, how quickly something passes through this thicker outer layer determines the overall absorption. The stratum corneum is primarily composed of lipophilic cholesterol, cholesterol esters and ceramides. Thus lipid-soluble chemicals make it through the layer and into the circulation faster, however nearly all molecules penetrate it to some minimal degree.

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