It has been demonstrated that the human body is able to cleave the ingested azo dyes back into the parent substances at the azo bridge by reductive cleavage. This can be done by intestinal bacteria, by azoreductases of the liver or extrahepatic tissue. Therefore, there is a suspicion that all azo dyes containing a releasable carcinogenic aryl amine component have a carcinogenic potential.
Azo dyes composed of at least one of these carcinogenic amines are banned in consumer goods in Germany (Bedarfsgegenständeverordnung). Their use in consumer goods is also prohibited under EU Directive 2002/61/EC. This ban applies to textiles and leather that can come into direct and prolonged contact with human skin or the oral cavity.
Currently, 24 such amines are banned, of which benzidine (benzidine dyes) is the best known. Benzidine azo dyes are suspected of significantly increasing the risk of bladder cancer through occupational exposure. According to the EU Directive, azo dyes that can release such amines with more than 30 ppm in the finished product by reductive cleavage of azo groups may not be used and corresponding textile and leather products may not be placed on the market. The analytical limit of determination varies around 5 ppm depending on the amine. In the EU, these dyes have not been used in the textile and leather industry for years. Since imports of consumer goods (such as textiles from Asia) may also not be dyed with such health-endangering dyes, random tests are carried out by the responsible authorities.