3,3'-Dimethylbenzidine and its salts – inhalable fraction. Documentation of proposed values of occupational exposure limits (OELs)

Abstract

3,3'-Dimethylbenzidine (3,3'-DMB, DMB, o-tolidine) is a solid used (as water-soluble dihydrochloride salt (dimethylbenzidine · 2HCl)) in the production of azopigments, polyurethane elastomers and plastics for coating. Small amounts are also used in diagnostic tests in laboratories. Occupational exposure to dimethylbenzydine occurs mainly during the production and use of pigments to dye textiles, plastics, paper and leather. In 2005–2014, dimethylbenzidine was used in Poland in 18–30 workplaces, where 135–280 people each year (mainly women) were exposed. No epidemiological data and information related to toxic effects of DMB in humans was found in the available literature. The LD50 value (median lethal dose) after single intragastric administration of 3,3'-dimethylbenzidine to rats was 404 mg/kg. After repeated exposure of laboratory animals, liver, kidney, thyroid injury and hematological changes were noted. In the Ames tests with metabolic activation, it was found that metabolites of 3,3'-dimethylbenzidine show stronger mutagenic action than the parent compound. 3,3'-DMB induced also chromosome aberrations and exchange of sister chromatids in in vitro tests. Although 3,3'-dimethylbenzidine is a derivative of carcinogenic benzidine, carcinogenic effects on humans have not been proven. However, research provides data about carcinogenic effect of 3,3'-DMB in animals. After subcutaneous administration of 3,3'-dimethylbenzidine and its dihydrochloride salt in drinking water, Zymbal's and mammary glands tumors, and cancers of uterus, skin, liver, hematopoietic system, small and large intestine were observed in rats. IARC classified 3,3'-dimethylbenzidine in the 2B group (a supposed carcinogenic agent for humans), whereas ACGIH – in the A3 group (proved carcinogenic effect on animals and unknown carcinogenic effect for humans). The European Union (according to the CLP classification) has listed 3,3'-DMB in the 1B category with the inscription "H350 – can cause cancer". The permissible concentrations for 3,3'-dimethylbenzidine have been established in some European countries only (Austria, Slovenia and Switzerland) as 0.03 mg/m3. The basis of the proposed maximum concentration value (MAC-TWA) for 3,3'-dimethylbenzidine and its salts was a risk assessment of cancer in male rats chronically exposed to 3,3'-dimethylbenzidine dihydrochloride in drinking water. Taking into account the cancer risk at the level of 10-4, a concentration of 0.03 mg/m3 for the MAC-TWA value was proposed. There are no basis to determine the short-term value (STEL) and biological limit values (BLV). It was also proposed to label the compound with "Carc 1B", which indicates that it is a carcinogen category 1B, and "skin" – the absorption of substances through the skin may be as important as an inhalation route.