: Muhle, H.; Kittel, B.; Ernst, H.; Mohr, U.; Mermelstein, R.

Abstract
Croups of 100 SPF Fischer-344 rats were exposed 6 h a day, 5d a week for 24 months to crystalline silica (1 mg . m-3, DQ 12 quartz) or titanium dioxide (5 mg . 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 modular hyperplasia lesions were characterized by moderate central fibrosis (abstract truncated)