Solvent molecules can gain entry into a worker’s body by inhalation, skin absorption, or ingestion. In biological monitoring of solvent exposure, it is necessary for the occupational health professional to be well aware of the metabolic pathway(s) of the foreign chemical in question and in what biological specimen the index chemical can be determined. The determinant can be the chemical itself or its metabolite(s) or a characteristic biochemical change induced by the chemical. Depending on the chemical and the analyzed biological parameter, the term internal exposure or internal dose may cover different concepts. Human breast milk analysis has not been used to monitor exposure to industrial chemicals even though milk could be considered as an excretory mechanism. Ionization in turn depends on the degree of polarizability of the chemical. Polarizability of a chemical is related to the geometry and molecular size.