The International Agency for Research on Cancer (IARC) Working Group met on September 30-October 7, 2014, to review the carcinogenicity of fluoro-edenite, silicon carbide (SiC) fibers and whiskers, and carbon nanotubes (CNT). A summary of the evaluations has now been published in The Lancet Oncology. The summary states:
Fluoro-edenite fibrous amphibole was classified as carcinogenic to humans (Group 1) on the basis of sufficient evidence in humans that exposure to fluoro-edenite causes mesothelioma. Sufficient evidence of carcinogenicity was also reported in experimental animals, with increased incidences of mesotheliomas observed in one study in male and female rats given fibrous fluoro-edenite by intraperitoneal or intrapleural injection. The results of the few available mechanistic studies were consistent with proposed mechanisms of fibre carcinogenicity.
According to the summary, SiC particles are manufactured mainly by the Acheson process, with SiC fibers being unwanted byproducts. The summary states:
Occupational exposures associated with the Acheson process were classified as carcinogenic to humans (Group 1) on the basis of sufficient evidence in humans that they cause lung cancer. Since the correlation between exposures to SiC fibres and cristobalite made it difficult to disentangle their independent effects, the Working Group concluded that fibrous SiC is possibly carcinogenic to humans (Group 2B) based on limited evidence in humans that it causes lung cancer. No data on cancer in humans exposed to SiC whiskers were available. In experimental animals, there was sufficient evidence for the carcinogenicity of SiC whiskers, with mesotheliomas observed in three studies in female rats treated by intrapleural implantation, intrapleural injection, or intraperitoneal injection, and in one inhalation study in rats that did not include concurrent controls. Although not unanimous, the Working Group classified SiC whiskers as probably carcinogenic to humans (Group 2A) rather than possibly carcinogenic to humans (Group 2B), on the basis that the physical properties of the whiskers resemble those of asbestos and erionite fibres, which are known carcinogens. In addition, the results of available mechanistic studies were consistent with proposed mechanisms of fibre carcinogenicity. The majority of the Working Group considered that differences in the nature of SiC fibres and SiC whiskers warranted separate evaluations.
The Working Group reviewed single-walled CNTs (SWCNT) and multi-walled CNTs (MWCNT). The summary states that no human cancer data were available, “indicating inadequate evidence for the carcinogenicity of CNTs in humans.” The Working Group reviewed studies in rodents and in cultured human lung or mesothelial cells. The summary states:
As a whole, the Working Group acknowledged that the above mechanisms are all relevant to humans. However, a majority did not consider the mechanistic evidence for carcinogenicity — especially concerning chronic endpoints — to be strong for any specific CNT. Furthermore, the lack of coherent evidence across the various distinct CNTs precluded generalisation to other types of CNTs. Thus, MWCNT-7 was classified as possibly carcinogenic to humans (Group 2B); and SWCNTs and MWCNTs excluding MWCNT-7 were categorised as not classifiable as to their carcinogenicity to humans (Group 3).