Carbon black particles
Inhalation exposures to high concentrations of carbon black (CB) particles have produced lung tumours in rats, but not mice or hamsters, presumably due to secondary genotoxic mechanisms involving persistent lung inflammation and injury. Accordingly, for this study it was postulated that the lung inflammation and injury induced by sub chronic, 90-day inhalation exposures of CB would be pronounced in rats than in mice and hamsters. Particle retention kinetics, inflammation, and histopathology were examined in female rats, mice and hamsters exposed for 13 weeks to high surface area CB (HSCb) at doses chosen to span a no-observable adverse effects level (NOAEL) to particle overload (0, 1, 7, 50 mg/m³). Rats were also exposed to low surface area CB (50 mg/m³, nominal; LSCb). Retention and effects measurements were performed immediately after exposure and 3 and 11 months post-exposure. Equivalent or similar mass burdens were achieved in rats exposed to high-dose HSCb and LSCb, whereas surface area burdens were equivalent for mid-dose HSCb and LSCb. Prolonged retention was found in rats exposed to mid- and high-dose HSCb and to LSCb, but LSCb was cleared faster than HSCb. Retention was also prolonged in mice exposed to mid- and high-dose HSCb and to LSCb, and in hamsters exposed to high-dose HSCb. The results demonstrated that pulmonary inflammation and histopathological effects were more severe and prolonged in rats when compared to mice or hamsters, and both lung effects were similar in rats exposed to mid-dose exposures to the two forms of carbon black, i.e., HSCb and LSCb. Similar to the results of both TiO2 sub chronic inhalation studies, the findings demonstrated that hamsters have the most efficient clearance mechanisms and least severe responses of the three species. The results from rats also show that particle surface area is an important determinant of target tissue dose. The authors concluded that a subchronic NOAEL of 1 mg/m³ respirable HSCb (Printex 90) was determined for female rats (Elder et al, 2005).