Chloroethene. Documentation of proposed values of occupational exposure limits (OELs)
Authors: Pakulska Daria , Czerczak Sławomir 
Published: September 2018

Chloroethene (vinyl chloride) does not occur naturally in nature. It is obtained exclusively by chemical synthesis. It is a gas under normal pressure and temperature conditions. At 40-70 °C, it polymerizes to form polyvinyl chloride (PVC). It is a large-volume compound. Its annual global production exceeds 40 million t / year. About 98% of the total production is used to produce polyvinyl chloride (PVC) and copolymers. Chloroethene is also used in the synthesis of 1,1,1-trichloroethane (methyl chloroform)
Exposure to chloroethene occurs during its synthesis and polymerization as well as during plastification and processing of polymers and copolymers that take place in many industries, including plastics, footwear, rubber and pharmaceutical industries.
The main route of occupational exposure to chloroethene is inhalation. After cessation of exposure, the levels of chloroethene in blood fall sharply. Absorption of the compound through the respiratory tract is very rapid. Deposition of chloroethene in the body is limited due to its rapid metabolism and excretion. The largest amount of absorbed chloroethene accumulates in the liver, where it undergoes biotransformation. The intermediate products of chloroethene metabolism, chloroethylene oxide and 2-chloroacetaldehyde, are the most reactive metabolites of this compound. The detoxification process takes place in the liver and relies on their conjugation with glutathione. As a result of further metabolism, final metabolites are formed which are excreted mainly with urine. In low concentrations, this is the main route of excretion. With the increase in the exposure concentration, the amount of chloroethene excreted by the lungs in the unchanged form increases.
Chloroethene has a very low acute toxicity, which has been found in both volunteer and animal studies. In volunteers as a result of acute inhalation exposure to high concentrations, only neurological and psychiatric disorders were observed. In animal studies, depressive effects on the central nervous system were observed, and histopathological examination revealed damage of liver, lung, kidney, heart and blood clotting disorders.
In workers chronically exposed to high concentrations of chloroethene, a syndrome of vinyl chloride disease was found, which includes symptoms of Raynaud's syndrome (pain, numbness and tingling in the upper and lower limbs, cold feeling in the limbs), pseudoscleroderma, acroosteolysis, allergic dermatitis, peripheral polyneuropathy, neurological disorders, as well as hepatotoxic effects.
In animal studies chronically exposed by inhalation to chloroethene, the hepatotoxic effect of the compound is well documented. This effect has been found at a relatively low concentration of 26 mg / m3 (10 ppm). In addition, there is evidence that chloroethene affects the vascular and respiratory system. The effects of the compound on bones, kidneys, spleen, blood and animal skin are less well documented.
Chloroethene has mutagenic / genotoxic properties, as observed in in vitro tests both with and without metabolic activation, and in in vivo tests. In in vitro tests on bacterial strains, the activity of chloroethene was much stronger with the participation of an exogenous metabolic system. Epidemiological studies in workers exposed to chloroethene showed an increased incidence of chromosomal aberrations, sister chromatid exchanges, micronuclei in lymphocytes and DNA damage in peripheral blood lymphocytes. The highest frequency of genotoxic effects was observed among operators of polymerization reactors subject to periodic exposure to very high concentrations of chloroethene.
Chloroetene has been classified as a carcinogen by the International Agency for Research on Cancer, IARC (Group 1) and the European Union (Category 1A). It was concluded that there was sufficient evidence of a carcinogenic effect of chloroethene in humans and sufficient evidence of carcinogenicity in experimental animals.
Carcinogenic effect of chloroethene has a genotoxic basis and results from the formation of reactive metabolites, mainly chloroethylene oxide and 2-chloroacetaldehyde, which reacting with DNA act mutagenically on somatic cells, mainly endothelial cells and thus play a significant role in the etiology of angiosarcoma.
Epidemiological studies have demonstrated a significant causal link between exposure to chloroethene and the incidence of hepatic cancers: angiosarcoma of the liver (ASL) and hepatocellular carcinoma (HCC). Epidemiological studies have shown a correlation between the number of deaths from liver tumors and the duration and magnitude of exposure and the length of latency, which in the case of ASL ranges from 10 to> 30 years. Carcinogenic effects of chloroethene on the lungs, brain, lymphatic and circulatory systems, skin and digestive system (cancers other than liver cancer) are less documented and ambiguous.
There are reports of the effect of chloroethene on the reproductive functions of women and men and the defects of their offspring. Existing data do not provide unambiguous evidence of teratogenicity and reproductive effects in the case of occupational exposure. In animal studies, chloroethene affected fertility and prenatal development of rats at high concentrations, with a NOAEL of 2860 mg / m3 (1100 ppm).
Available data indicate that the target organ of chloroethene toxicity in chronic exposure in humans is the liver, and the critical effect of exposure is the development of liver tumors.
In epidemiological studies, the effect of occupational cumulative exposure dose (CED) on the development of angiosarcoma of the liver (ASL) is best documented. The SCOEL Scientific Committee using PBPK models estimated the risk of ASLs at 3 • 10-4 as a result of 40 years of occupational exposure to chloroethene in a concentration of 2.6 mg / m3 (1 ppm).
Taking into account the above calculations, as well as the accepted level of occupational risk for carcinogens in the range from 10-4 to 10-3, the TWA of chloroethene at the level of 2.6 mg / m3 (1 ppm) has been proposed. This means an increase in the incidence of 3 liver cancers (ASL) per 10,000 people. There is no substantive basis to determine a short-term exposure limit (STEL) and acceptable concentration in biological material (DSB). It is proposed to label the compound as "Carc. 1A "- carcinogen category 1A.
The proposed value is in line with the value adopted by ACGIH and in Canada and the binding value proposed by SCOEL for this compound, as well as the binding value included in Directive of the European Parliament and of the Council (EU) 2017/2398 of 12 December 2017 amending Directive 2004/37 / EC on the protection of workers from the risks related to exposure to carcinogens or mutagens at work.