Objective: To critically review and summarise all of the available epidemiological evidence, both published and unpublished, to date on the carcinogenicity of methyl methacrylate (MMA) to humans. Methods: The review focused on studies of cast acrylic sheet manufacturing workers because this industry has historically had a potential for exposure to high levels of MMA. The majority of papers for review were identified through Medline (National Library of Medicine) but there is some discussion of two cohort studies and a nested case–control study, which to date have not been published. Results: An increased risk of colorectal cancer was reported in one group of workers highly exposed to MMA and ethyl acrylate (EA) in the manufacture of acrylic sheet. Analysis of colon cancer by cumulative exposure to MMA indicated that the excess was largely confined to the group with the highest exposure. However, a large excess of colon cancer deaths occurred among workers who never worked in a job entailing more than minimal exposure. Studies of other large occupational cohorts of workers potentially exposed to MMA, including some with potentially comparable exposures, have failed to strengthen the evidence that there is a causal association between colorectal cancer and MMA exposure although one reported an excess that did not appear to be exposure-related. Excesses of cancers of the respiratory system and stomach were seen in some cohorts, but not among the acrylic sheet workers who had the increased risk of colorectal cancer. Conclusions: Excesses of respiratory, stomach and colorectal cancers were observed in some cohorts of workers exposed to MMA. There was little to suggest that MMA exposure was responsible for the excesses of respiratory and stomach cancer and it is more likely that they resulted from unexplained contributions of lifestyle exposures such as cigarette smoking and diet. An excess of colorectal cancer in one group of workers exposed to high levels of MMA and EA during the 1930s and 1940s remains unexplained. However, the lack of consistency in the results of various studies, the absence of dose response and the lack of support from animal toxicology do not provide persuasive evidence that exposure to MMA is a human carcinogen.
