Methylene chloride has been classified in group 2B as a possible human carcinogen by the IARC (1999). This is based on evidence of carcinogenicity in experimental animal studies. There is insufficient human data to allow a clear interpretation.
Epidemiological studies suggest that there may be long-term health effects from chronic exposure to methylene chloride in the form of pancreatic, hepatic or biliary cancer. Inhalation studies have demonstrated that methylene chloride causes hepatic neoplasms in animals (ATDSR, 1993a,b). Methylene chloride has been reported to cause benign mammary tumors and malignant liver and lung neoplasms in several animal species (ATSDR, 1993a,b; Huff et al., 1996).
Cantor et al. (1995) used mortality records from a five year period (1984-1989), coded for occupation and industry, and assessed them with regards to workplace exposures and possible breast cancer risk. Suggestive associations for probability and level of exposure were found for several organic solvents including methylene chloride.
A long term epidemiological study of workers chronically exposed to methylene chloride during the manufacture of photographic products, reported no statistically significant excesses in deaths from lung or liver cancer, or from ischaemic heart disease. These investigators did, however, report an increase in deaths from pancreatic cancer. Another study of employees at a fibre production plant reported an excess of deaths from liver and biliary tract cancer among workers exposed to methylene chloride. In both studies, however, the evidence was inconclusive (ATSDR, 1993a,b). In an epidemiological study of workers occupationally exposed to methylene chloride (some of them for over 20 years), there was no increase in the incidence of death from ischaemic heart disease or malignancies, compared with expected deaths in the general population (Friedlander et al., 1978).
In a study examining the occupational history of white men who had died of brain cancer an association of astrocytic brain cancer was observed with likely exposure to methylene chloride (Heineman et al., 1994).
In a review of the epidemiology literature on the potential cancer risks of methylene chloride, Dell et al. (1999) concluded that although the studies cannot rule out the possibility of any cancer risk associated with methylene chloride, there is no substantive cancer risk and that it appears likely that risks associated with methylene chloride exposure are small and limited to rare cancers. The carcinogenicity of methylene chloride is related to the metabolic activation of methylene chloride via the glutathione transferase pathway (Jonsson et al., 2001).
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