Understanding the Relationship Between Percentage and Actual Amount Absorbed in Topical Formulations Under Finite and Infinite Dose Scenarios

Risk assessment is the product of hazard and exposure. The exposure itself is the product of two components: the percent absorbed and the applied dose. For a topical application, the absorbed dose includes the fraction of a chemical able to enter the skin. It can be expressed in different units, i.e. , amount absorbed per exposure area, and percentage absorbed. The way it is expressed is related to the protocol used and the context of use. The applied dose for a topical product depends on two factors, i.e. , the amount of formulation applied and the concentration of the chemical in the formulation. Calculation of the applied dose is generally well understood. A common misconception in dermal risk assessment is that percent absorbed remains constant across different exposure conditions. The purpose of this review article is to demonstrate that this assumption is not universally valid. The implications of expressing absorption using different units are discussed, together with their impact on chemical risk assessment outcomes.