Dermal resorption of azo dyes - Part 1: Properties, uptake via the skin and metabolism

Azo dyes are one of the most important commercial dyes worldwide. They can be divided in two subgroups, soluble azo dyes (hydrophilic and lipophilic) and insoluble azo pigments. Insoluble azo pigments are taken up primarily via the lung during manufacture, e.g., in form of particles, whereas soluble azo dyes are taken up by inhalation as well as dermal and oral absorption. The major route of uptake at the workplace but also via the environment must be considered dermal and oral absorption. Soluble azo dyes are cleaved to aromatic amines by bacterial and microsomal reduction in various human organs including the liver, gastrointestinal tract and the skin. However, the quantitative nature of cleavage and absorption remains unknown and can be estimated only based on the structure and solubility of the azo dye but also on the individual health status of the person in question. Our review summarizes the current state of knowledge on properties of azo dyes with particular focus on uptake and metabolism of azo dyes via the skin.