Thioacetamide – inhalable fraction. Documentation of proposed values of occupational exposure limits (OELs)

Abstract

Thioacetamide occurs in the form of colorless crystals with a characteristic smell of mercaptans. It was used in the past as a fumigant to prevent oranges from rotting, in rubber vulcanization and as a diesel stabilizer. It is currently used in a qualitative analysis as a source of hydrogen sulfide. According to information from the Central Register of Data on Exposure to Carcinogenic or Mutagenic Substances, Mixtures, Factors or Technological Processes in 2005-2016 from 486 to 1137 people were exposed to thioacetamide in Poland. Most of them were women. The LD50 value after intragastric administration of the compound to rats is 301 mg/kg. Thioacetamide is a strong hepatotoxic agent, its single dose caused hepatic necrosis. Administered repeatedly it induced liver damage, which was indicated by biochemical changes and cirrhosis. The effects of thioacetamide toxicity in chronic animal experiments indicated a relationship to exposure time. After chronic exposure of rats to thioacetamide in drinking water (at 0.03%, i.e., approximately 35 mg/kg/day) or in feed (0.5% in feed, i.e., approximately 28 mg/kg/day), hepatitis and local hepatic foci were noted after 4 months, these changes later intensified, and after 8–17 months chronic hepatitis, cirrhosis and cancer of the liver and bile ducts occurred. The results of mutagenicity and genotoxicity studies of thioacetamide are inconclusive. It can be assumed that the compound may damage genetic material in vivo after biotransformation to a highly hepatotoxic metabolite. The metabolism of thioacetamide by S-oxidation (mainly with the participation of CYP2E1) leads to the production of sulfoxide (TASO), and then to hepatotoxic, highly reactive sulfone (TASO2). The latter is of fundamental importance for the mechanism of toxic action of thioacetamide (by binding with hepatic macromolecules). Thioacetamide metabolites also induce oxidative stress. Because of neoplasms observed in chronic studies, International Agency for Research on Cancer (IARC) included thioacetamide in group 2B – agents probably carcinogenic to humans. According to the CLP classification, thioacetamide is a category-1B carcinogen with a "H350 – May cause cancer" note. The hepatotoxic effects of thioacetamide in rats after repeated administration were used as the basis for determining the maximum acceptable concentration (MAC; TLV-TWA – threshold limit value-time weighted average). A concentration of 1.5 mg/m3 was proposed as the MAC value. There are no bases to determine the short-term exposure limit (STEL) and the biological limit value (BLV). "Carc. 1B" marking is also proposed, as thioacetamide is a category-1B carcinogen. This article discusses the problems of occupational safety and health, which are covered by health sciences and environmental engineering.