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 center dot m-3, DQ 12 quartz) or titanium dioxide (5 mg center dot m-3) or air only. The animals were kept without further exposure for an additional 1.5 months. In the group exposed to crystalline silica a significantly increased incidence of 20 primary lung tumors was observed among 19 animals. The distribution of tumor types consisted of 3 adenomas, 11 adenocarcinomas, 4 benign cystic keratinizing squamous-cell tumors, 1 adenosquamous carcinoma, and 1 squamous-cell carcinoma. There were also 13 nodular hyperplasia lesions, which were interpreted to be borderline cases of adenomas. Approximately half of the adenoid tumors and all of the nodular hyperplasia lesions were characterized by moderate central fibrosis. The principal nonneoplastic 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.