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.
A chemical may be directly applied to the skin followed by blood and urine measurements at set time points after application to assess the amount of chemical that entered the body. The concentration in the blood or urine at particular time points can be graphed to show and area under the curve and the extent and duration of absorption and distribution to provide a measure of systemic absorption.