data are considered “limited.” Most of the human studies cited by IARC involve the increased risk of leukemia and other lymphatic and hematopoietic cancers (IARC, 1987). The National Toxicology Program (NTP) has also classified benzene as “known to be a human carcinogen” in its most recent report on carcinogens on the basis of animal and human studies (NTP, 2001).
On the basis of animal studies, trichloroethylene has been associated with liver cancer in one strain of one species (mouse) (ATSDR, 1997b). Liver and renal cell cancers and mononuclear cell leukemia have typically been seen after exposure to tetrachloroethylene. According to the Agency for Toxic Substance and Disease Registry (ATSDR, 1997c), the relevance to humans of rodent toxicology studies on trichloroethylene and tetrachloroethylene is unclear, given that some mechanisms of action differ. However, a great deal of research has been conducted over the last decade, and some mechanisms of action appear to be similar in rodents and humans such as genotoxic and cytotoxic actions of mercapturic acid derivatives of both trichloroethylene and tetrachloroethylene in the kidney (see Chapter 4 for more information). IARC has also reviewed trichloroethylene and tetrachloroethylene and determined that both are “probably carcinogenic to humans.” The evidence from animal studies is stronger and considered to be “sufficient,” whereas the evidence from human studies is considered “limited” (IARC, 1995). In addition, the NTP has identified both trichloroethylene and tetrachloroethylene as “reasonably anticipated to be human carcinogens” (NTP, 2001).
Exposure to methylene chloride in some rodent species has consistently produced excess numbers of cancers of the liver and lung and benign mammary tumors (ATSDR, 2000). IARC has determined that exposure to methylene chloride is “possibly carcinogenic to humans,” and the NTP concluded that it is “reasonably anticipated to be a human carcinogen” (IARC, 1999; NTP, 2001). IARC has determined that toluene and xylene are “not classifiable as to their carcinogenicity to humans” in that there was inadequate evidence from studies of humans and animals (IARC, 1999).
Chloroform has produced liver and kidney tumors in a strain-, sex-, species-, and dose-dependent manner and, on the basis of sufficient evidence from animal studies, is “reasonably anticipated to be a human carcinogen” according to the NTP (ATSDR, 1997d; NTP, 2001). Chloroform was once used as an anesthetic, but its association with cancer in nonmedical exposures in humans has not been investigated extensively. The committee did not review studies on the efficacy of solvents as therapeutic agents (see Chapter 2). Chapter 4 provides details on the adverse effects of chloroform as observed in experimental studies.
In addition to evaluating the carcinogenicity of specific chemical agents, IARC has analyzed whether particular occupations pose a greater risk for exposure to carcinogenic agents. In fact, IARC has determined that working in the rubber industry and in boot and shoe manufacturing and repair pose such a risk (IARC, 1987), and it determined that there is “sufficient evidence for the carcinogenicity of occupational exposure as a painter” (IARC, 1989). Although IARC identifies exposures of concerns and specific cancer outcomes that demonstrate an increased risk, its overall charge is to determine whether a specific agent or occupation is carcinogenic, not whether an agent causes a specific cancer outcome. It is important to distinguish the objectives of IARC’s program and the charge of the present committee. The purpose of the IARC program is to determine whether agents or occupational exposures are carcinogenic, whereas this committee is charged with determining whether there is an association between exposure to a specific agent and