National Industrial Chemicals Notification and Assessment Scheme
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IARCIARC (International Agency for Research on Cancer) (IARC, 1995) considered three cohort studies to be relevant for the evaluation of trichloroethylene. Meta-analysis of the three studies (Spirtas et al., 1991; Axelson et al., 1994 and Anttila, 1995) by IARC indicated an excess relative risk for cancer of the liver and biliary tract (23 observed cases whereas 12.7 expected) and non-Hodgkin’s lymphoma (27 observed and 18.9 expected). Results for liver cancer were given separately in the study by Anttila et al (1995) and for the maintenance workers in the study by Spirtas et al (1995). A total of 7 cases were observed whereas 4 were expected. On the basis of these findings and the induction of tumours in animals at sites other than the liver, IARC concluded that trichloroethylene is probably carcinogenic to humans (Group 2A), that is limited evidence in humans and sufficient evidence in experimental animals. GermanyThe Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (1996) considered that the study by Henschler et al (1995) indicates an increased incidence of renal tumours in workers exposed to high concentrations of trichloroethylene. The Commission states that the findings of this study were confirmed by a recent case control study carried out by Vamvakas et al (cited in Deutsche Forschungsmeinschaft, 1996) suggesting an association between renal cell tumours and exposure to trichloroethylene. The tumours were histopathologically similar to those found in rats. In addition, the metabolic pathway postulated to be responsible for nephrocarcinogenicity has been found to be similar in rats and humans (Bernauer et al., 1996). On the basis of these three findings ie increased incidence of renal cell tumours in exposed workers, nephrocarcinogenicity in rats and the molecular mechanism of renal toxicity, trichloroethylene is classified by the Commission in category IIIA1, ie compound capable of inducing malignant tumours as shown by experience with humans. UK HSEThe HSE (United Kingdom, 1996) consider that the majority of epidemiological studies, including the studies by Axelson et al (1994) and Spirtas et al (1991) that had substantial power to detect an effect, did not show any evidence of an association between trichloroethylene exposure and increased incidence of cancer. However, they noted that there is limited evidence of an increased risk of liver cancer in one cohort study (Anttila et al., 1995) and of renal cancer in another study (Henschler et al., 1995). These two studies indicate that trichloroethylene has some carcinogenic potential. HSE have concluded that the liver tumours in mice are due to peroxisomal proliferation and are of no relevance to humans but that there is no evidence to indicate that the mechanism of induction of kidney tumours in rats and lung tumours in mice is not applicable to humans. On the basis of the uncertainties of the epidemiological data and the tumours in animals, the HSE have proposed that trichloroethylene be classified as a category 3 carcinogen, ie a substance which causes concern for humans owing to possible carcinogenic effects, but in respect of which the available information is not adequate to make a satisfactory assessment. CanadaThe Canadian Priority Substances List Assessment (Government of Canada, 1993), conducted by Environment Canada and Health Canada, in accordance with the Canadian Environmental Protection Act (CEPA), states that an association between exposure to trichloroethylene and the development of any specific type of tumour has not been consistently observed in the epidemiological studies, including those by Spirtas et al (1991); Axelson et al (1984); Tola et al (1980) and Shindell and Ulrich (1980). The Canadian report concluded that the increased incidence of hepatic tumours in mice appears to be induced by a mechanism not relevant to humans and that the relevance of renal tumours in male rats to humans is unclear. The most pertinent results in assessing the carcinogenicity of trichloroethylene are the pulmonary tumours in mice reported by Maltoni et al (1986, 1988) and Fukuda et al (1983) and the increases in testicular tumours in rats (Maltoni et al., 1986; Maltoni et al., 1988; US National Toxicology Program NTP, 1988). Trichloroethylene also appears to be weakly genotoxic in in vitro and in vivo assays. The Canadian report categorised trichloroethylene in Group II, ie probably carcinogenic to humans.
ECETOC noted that animal studies have shown liver and lung tumours in mice and kidney tumours in rats. The mechanisms in these cases are linked to species specific metabolism of trichloroethylene or to biochemical responses which are specific to rodents. These tumours are therefore considered to be of no relevance to humans. ECETOC concluded that trichloroethylene does not present a carcinogenic hazard to man.
ACGIHACGIH (1992) found no evidence in six epidemiological studies to suggest that an association between trichloroethylene exposure and increased cancer in humans. The six studies considered were Spirtas et al (1991); Axelson et al (1978); Tola et al (1980) and Shindell and Ulrich (1980) Paddle (1983) and Novotna et al (1979). ACGIH concluded that the hepatocellular tumours in mice (National Cancer Institute (NCI), 1976) occur via a nongenetic mechanism following liver injury. No carcinogenic effects were seen in animals in other studies either orally (Maltoni & Maioli, 1977) or by dermal application (Van Duuren et al., 1979). ACGIH categorised trichloroethylene in Group A5, ie not suspected as a human carcinogen.
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