On March 11, 2013, at the annual meeting of the Society of Toxicology, researchers from the National Institute for Occupational Safety and Health (NIOSH) reported preliminary findings from a new laboratory study in which mice were exposed by inhalation to multi-walled carbon nanotubes (MWCNT). According to NIOSH, the study was designed to investigate whether these tiny particles have potential to initiate or promote cancer. One group of laboratory mice was injected with a chemical that is a known cancer initiator, and another group of mice was injected with a saline solution as a control group. The mice then were exposed by inhalation either to air or to a concentration of MWCNT. According to NIOSH, mice receiving both the initiator chemical plus exposure to MWCNT were significantly more likely to develop tumors and have more tumors than mice receiving the initiator chemical alone. Additionally, mice exposed to MWCNT and to MWCNT plus the initiator chemical had larger tumors than the respective control groups. NIOSH concluded that the results indicate that MWCNT can increase the risk of cancer in mice exposed to a known carcinogen. NIOSH notes that the study does not suggest that MWCNTs alone cause cancer in mice.

NIOSH states that the study is the first to show that MWCNT is a cancer promoter in a laboratory experiment, and reports the growth of lung tumors in laboratory mice following inhalation exposure to MWCNT rather than injection, instillation, or aspiration. According to NIOSH, inhalation exposure most closely resembles the exposure route of greatest concern in the workplace. NIOSH notes that the research is an important step in understanding the hazard associated with MWCNT, but that it needs more information about actual exposure levels and the types and nature of MWCNT being used in the workplace, and how that compares to the material used in this study, before it can determine whether MWCNT pose an occupational cancer risk. Workplace studies are underway at NIOSH to learn more about actual worker exposure and to develop guidance on how to contain and control MWCNT processes to eliminate exposures, based on advancing knowledge about exposures. NIOSH issued a draft Current Intelligence Bulletin on single-walled and MWCNT in December 2010, and expects to issue a final version, incorporating its review of the public comments, “soon.” NIOSH will submit the results of the laboratory study on MWCNT to a peer-reviewed journal for publication.