Azo dyes are a type of direct dye - meaning that they color the fibers "directly", which eliminates the need for a mordant. Azo dyes are one of the main types of dye used by the textile industry. They are known to be potential carcinogens. They were first regulated in the EU by Germany and the list is commonly called the "German banned dye list". If a product has not been made containing these compounds, it is sometimes called "azo free". That's misleading because the number of dyes on the list is very small compared to the total number of azo dyes which are still used. And with a few exceptions, the banned dyes on the list have not been used in the past 20 years, so the bans didn't change manufacturing much.
Studies have found that the dyes are resistant to municipal waste water treatment facilities and pass into the environment relatively unchanged.1 Textile mill effluent contains 5 – 20% of original dyestuff, plus salt and dye fixing agents.
Some azo dyes break down during use and release chemicals known as aromatic amines, some of which can cause cancer – typically bladder and liver cancers,2 but studies also link them to breast cancer.3 They also cause damage to human DNA.4 They are easily absorbed by skin exposure.5 The European Union has banned the use of these azo duyes that release cancer-causing amines in any textiles that come in contact with human skin.6
1Ferraz, Elisa R.A. et al.; "The azo dye Disperse Orange 1 induces DNA damage and cytotoxic effects but does not cause ecotoxic effects in Daphnia similis and vibrio fischeri", Journal of Hazardous Materials, Aug. 30, 2011, Vol 192, Issue 2, pp. 628-633
2Dom N, Knapen D, Benoot D, Nobels I & Blust R (2010). Aquatic multi-species acute toxicity of (chlorinated) anilines: Experimental versus predicted data. Chemosphere 81(2): 177-1
4Chequer, F.M.D. et al., "Azo dyes and their metabolites: Does the discharge of the Azo dye into water bodies represent human and ecological risks", http://cdn.intechopen.com/pdfs-wm/22392.pdf
6Since 1991, all PCP-containing products sold and used in the EU have been imported (EU production was banned under Directive 76/769/EEC). Now entry number 22 of Annex 17 of the EU chemical law REACH prohibits the marketing and use in the EU of PCP and its salts and esters in products in a concentration equal to or greater than 0.1 per cent. Commission Regulation (EC) No 552/2009 of 22 June 2009, op cit. (REACH)