NEOPLASTIC LUNG LESIONS IN RAT AFTER CHRONIC EXPOSURE TO CRYSTALLINE SILICA

Groups of 100 SPF Fischer-344 rats were exposed 6 h a day, 5 d a week for 24 months to crystalline silica (1 mg . m(-3), DQ 12 quartz) or ti tanium dioxide (5 mg . m(-3)) or air only. The animals were kept witho ut further exposure for an additional 1.5 months. In the group exposed to crystalline silica a significantly increased incidence of 20 prima ry lung tumors was observed among 19 animals. The distribution of tumo r types consisted of 3 adenomas, 1 1 adenocarcinomas, 4 benign cystic keratinizing squamous-cell tumors, 1 adenosquamous carcinoma, and 1 sq uamous-cell carcinoma. There were also 13 modular hyperplasia lesions, which were interpreted to be borderline cases of adenomas. Approximat ely half of the adenoid tumors and all of the nodular hyperplasia lesi ons were characterized by moderate central fibrosis. The principal non neoplastic findings in the silica-exposed group were lipoproteinosis, inflammation, epithelial hyperplasia, and fibrosis. The results can be considered significant due to the increased lung tumor incidence at a relatively low exposure level.