Individual And Regional Variation

In vivo and in vitro percutaneous absorption studies give data as mean absorption ± some standard deviation. Some of this variability is attributable to conduct of the study and is called experimental error. However, when viewing a set of absorption values it is quite clear that some people (as well as some rhesus monkeys) are low absorbers and some are high absorbers. This becomes evident with repeat studies. This is individual variation.

The first occupational disease in recorded history was scrotal cancer in chimney sweeps. The historical picture of a male worker holding a chimney brush and covered from head to toe with black soot is vivid. But why the scrotum? Percutaneous absorption in humans and animals varies depending on the area of the body on which the chemical resides. This is called regional variation. When a certain skin area is exposed, any effect of the chemical will be determined by how much is absorbed through the skin. Feldmann and Maibach [5-7] were the first to systemically explore the potential for regional variation in percutaneous absorption. The first absorption studies were performed on the ventral forearm because this site is convenient to use. However, skin exposure to chemicals exists over the entire body. The scrotum was the highest-absorbing skin site (scrotal cancer in chimney sweeps is the key). Skin absorption was lowest for the foot area, and highest around the head and face (Fig. 4). There are two major points. First, regional variation was confirmed with the different chemicals. Second, those skin areas that would be exposed to cosmetics—the head and face—were among the higher absorbing sites.

Figure 4 Percutaneous absorption of parathion from various parts of the body varies with region of the body.

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Figure 5 Lidocaine percutaneous absorption through human skin. Formulation determines the initial absorption.

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