Estimates of current numbers of exposed workers to azo dyes, which can break down in aromatic amines, in the EU are not available. Occupational exposure to azo dyes may occur, where azo dyes are produced and used for colouring or indicating purposes. Dermal exposure is the most important route of exposure as liquid formulations are now the norm. Inhalation exposure may still be relevant in individual cases.
Azo dyes are suspected of causing cancer as they can break down into aromatic amines. Some aromatic amines are known carcinogens (category 1A according CLP) e.g., 4-aminobiphenyl, benzidine, 4-chloro-o-toluidin, 2-naphtylamine, or presumed to be carcinogenic for humans (category 1B according CLP) e.g., o-toluidin, o-anisidin, 4-aminoazobenzene. Other aromatic amines released from azo dyes are suspected to be carcinogenic for humans.
Some aromatic amines are associated with occupational diseases that cause mucosal changes, cancer or other neoplasms of the urinary tract.
Where risks occur
In general, azo dyes are widely used in industry for colouring plastics and rubber products, wood, paper, paints and polishes, but also for colouring consumer products such as textile fibres, cosmetics, pharmaceuticals and even food. A minor application may include medical or biological research e.g., colourants for microscopy.
Former uses in colouring leather or textiles have been banned in Europe and uses have decreased since.
Azo dyes releasing o-toluidin, such as C.I. Solvent Red 24, 164 and 215, still find application in marking mineral oil or in non-destructive methods as crack detection dye penetrants, e.g. in the metal industry. Professions with high risk of dermal exposure include industrial dyers, beauticians and material inspectors.
More about the substance
There are more than 3000 different azo compounds. The economically important azo compounds include azo colourants, which are subdivided into azo dyes, which are practically insoluble in the application medium, and soluble azo dyes.
In the latter case, a distinction is made between water-soluble and organic solvent-soluble azo dyes, which play an important role for the dermal exposure. Soluble azo dyes on the skin can be converted by bacterial or enzymatic activity into soluble aromatic amines, which are absorbed through the skin.
Azo dyes can be assigned to different colour types, such as acid dyes, direct dyes or reactive dyes, disperse dyes etc. In the colour index system, like other dyes, azo dyes are classified according to their chemical behaviour and the way they are used for colouring, resulting in the colour-index name (C.I. generic name).
It is noteworthy to mention that aromatic amines can also be found in tabacco smoke, diesel exhaust and as impurities in coal tar or in additives in the manufacture of rubber articles, which affect other occupations.
Hazards that may occur
Soluble azo dyes are mainly taken up via dermal absorption. Inhalation can also play a role. However, depending on whether azo dyes are water-soluble or fat-soluble azo dyes, they are absorbed in different ways and at different rates.
Acute toxic effects in humans have so far only been described after exposure to soluble azo dyes, but not to insoluble azo pigments. However, after absorption via the lungs and depending on the size of insoluble pigments, they can develop particle-typical effects, including similarities to granular biopersistent dusts, e.g. starting with symptoms like coughing.
It has been reported that workplace exposures are most relevant to known occupational diseases such as changes to the mucous membranes, cancer or other growths in the urinary tract. Further, chronic exposure is also presumed to be associated with allergic skin effects.
Latency period between exposure and aromatic amine related cancer of the urinary tract (bladder cancer) varies strongly between 12 and up to more than 40 years. Relevant carcinogenic amines are o-toluidin, xenylamin, 2-naphtylamine, benzidine and 4-chloro-o-toluidin.
What you can do
The most effective way to prevent exposure is substitution. There are safer, alternative dyes on the market, e.g. alternative natural dyes or mineral/pigment dyes, especially for the textile and food industry. Where substitution of azo dyes is not possible and use of azo dyes cannot be avoided, measures to reduce exposure must be taken.
The most effective way to avoid exposure to azo dyes is to develop and use closed systems. Where this is not possible, technical measures such as effective local exhaust ventilation, or good ventilation of the workplace as well as checking their effectiveness should be implemented to ensure that exposure is minimised as much as technically possible. One standard requirement for the textile industry is the best available technology (BAT), which needs to be regarded to minimize environmental and health effects.
Implement regular exposure assessments to check if your protective measures in place are effective or whether further actions need to be taken. Workers need to be aware of the effects of exposure and should be trained regularly on the control measures necessary for working safely with azo dyes to prevent exposure. They should be encouraged to report early symptoms such as respiratory problems or skin allergies. Involving an occupational physician is recommended. In addition, train workers on effective hygiene measures.
Ensure that workers have adequate personal protective equipment, such as protective clothing and gloves, if necessary, as the primary exposure pathway is the skin. Personal protective equipment should only be used as a last resort and only considered temporarily, after the possible technical solutions have been exhausted. It is important that personal protective equipment, if reusable, should be cleaned after use and stored at a clean place.
References: BAuA, ECHA, DGUV, EU-Commission
