HISTOPATHOLOGIC EXAMINATIONS OF RATS TREATED BY CHRONIC INHALATION EXPOSURES TO GLASS-FIBERS

Effects on rats of a chronic inhalation exposure to glass fibers were examined histopathologically with special reference to pulmonary fibro sis and tumor genesis. Twenty-seven male Wistar rats of 6 wk of age we re exposed to glass fibers, the exposure concentration was 2.2 +/- 0.6 mg/m(3), and the mass median aerodynamic diameter 2.6 mm (the geometr ic standard deviation 1.9), for 1 yr. Among them 6 and 9 rats were sac rificed just after the 1-yr exposure period and after a further 1-yr c learance period, respectively. Among 25 control rats who did not under go the exposure, 5 and 15 rats were sacrificed at the same times as ea ch exposure group. Twelve exposure and five control rats died during t he exposure and clearance period, respectively. No tumors were found i n both the exposure and control of rats immediately after the inhalati ons. There were no pneumoconiotic nodules or interstitial fibrosis in any of the rats, though a somewhat shorter survival was observed in th e exposure group than in the control group. As for the group after the clearance period, increasing macrophages aggregated around the termin al bronchioles and a small number of lung tumors; for example, two ade nomas in the exposure group, and one adenoma and one squamous-cell car cinoma in the control group were noted. Sporadic tumors in other organ s were also noted. However, there was no significant difference in the occurrence of these tumors between the exposure and control groups. I t was concluded that the chronically inhaled glass fibers showed no ap parent promoting effects of lung fibrosis or tumor genesis in this stu dy, although a cumulative survival rate was somewhat more diminished i n the exposure group than in the control.