carcinogenicity of multi-walled carbon nanotubes: challenging issue on hazard assessment

Objectives: this report reviews the carcinogenicity of multi-walled carbon nanotubes (mwcnts) in experimental animals, concentrating on mwnt-7, a straight fibrous mwcnt. methods: mwcnts were administered to mice and rats by intraperitoneal injection, intrascrotal injection, subcutaneous injection, intratracheal instillation and inhalation. results: intraperitoneal injection of mwnt-7 induced peritoneal mesothelioma in mice and rats. intrascrotal injection induced peritoneal mesothelioma in rats. intratracheal instillation of mwcnt-n (another straight fibrous mwcnt) induced both lung carcinoma and pleural mesothelioma in rats. in the whole body inhalation studies, in mice mwnt-7 promoted methylcholanthrene-initiated lung carcinogenesis. in rats, inhalation of mwnt-7 induced lung carcinoma and lung burdens of mwnt-7 increased with increasing concentration of airborne mwnt-7 and increasing duration of exposure. conclusions: straight, fibrous mwcnts exerted carcinogenicity in experimental animals. phagocytosis of mwcnt fibers by macrophages was very likely to be a principle factor in mwcnt lung carcinogenesis. using no-observed-adverse-effect levelbased approach, we calculated that the occupational exposure limit (oel) of mwnt-7 for cancer protection is 0.15 μg/m 3 for a human worker. further studies on the effects of the shape and size of mwcnt fibers and mode of action on the carcinogenicity are required.