Biological effect of carbon graphite whisker in rat lung by long-term Inhalation

Carbon graphite whisker (CGW) was used in a 1-year inhalation study in male Wistar rats and its biological effect was observed until the 1-year clearance period. The inhalation study was conducted at 2.6 +/- 0.5 mg/m(3) (equivalent to 44.5 +/- 15.0 fibers/mL) for 6 hours a day, 5 days a week for 1 year. There were no differences in survival rate between the exposure and control groups during this examination; however, the body weights were significantly different at the end of the 1-year clearance. The lung weight at 3 days and 1 year after the end of exposure was not significantly different in both groups. The deposited amount of CGW was 6.83 +/- 0.75 mg at 3 days post-exposure; the deposition rate was 17.6%. Only around 30% of the total deposited CGW was cleared during the 1-year clearance period. The geometric means of CGW in the lung, i.e. CMD (count median diameter) and CML (count median length), hardly changed, and the clearance was delayed. In the histopathological examination, there was mild fibrosis in all exposed rats irrespective of the clearance period. One adenoma was observed in a single animal at 3 days post-exposure, while no adenomas were observed in the exposure group after the 1-year clearance. Epithelial hyperplasia was found in some animals.