<P> The amount of chemical that is absorbed through the skin can be measured directly or indirectly . Studies have shown there are species differences in the absorption of different chemicals . Measurements in rats, rabbits or pigs may or may not reflect human absorption . Finding the rate at which agents penetrate the skin is important for assessing the risk from exposures . </P> <P> 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 . This can be done in animals or humans with a dry chemical powder or a chemical in solution . Rats are commonly used for these experiments . An area of skin is shaved before the chemical is applied . Often the area of chemical application is covered to prevents ingestion or rubbing off of the test material . Samples of blood and urine are taken at specific time intervals following application (0.5, 1, 2, 4, 10, and 24 hours) and in some protocols at the chosen end time the animal maybe necropsied and tissue samples may also be evaluated for the presence of the test chemical . In some test protocols many animals may be tested and necropsies may occur at set intervals after exposure . Biomonitoring, such as taking urine samples at intervals, from workers exposed to chemicals may provide some information but it is difficult to distinguish dermal from inhalation exposure using this method . </P> <P> The permeability properties of the stratum corneum are, for the most part, unchanged after its removal from the body . Skin that has been removed carefully from animals may also be used to see the extent of local penetration by putting it in a chamber and applying the chemical on one side and then measuring the amount of chemical that gets into a fluid on the other side . One example of this ex vivo technique is the isolated perfused porcine flap . This method was first described in 1986 as a humane alternative to in vivo animal testing . </P> <P> Techniques such as static diffusion cells (Franz cells) and flow - through diffusion cells (Bronaugh cells) have also been used . The Franz Cell apparatus consists of two chambers separated by a membrane of animal or human skin . Human skin is preferred but due to ethical and other considerations is not always available . Human skin often may come from autopsies or plastic surgeries . The test product is applied to the membrane via the top chamber . The bottom chamber contains fluid from which samples are taken at regular intervals for analysis to determine the amount of active that has permeated the membrane at set time points . </P>

What part of the skin absorbs the most