Effects of inhaled chromium dioxide dust on rats exposed for two years.

Rats were exposed by inhalation to chromium dioxide (CrO2) dust at design concentrations of 0, 0.5 mg/m3 (stabilized and unstabilized, respectively), or 25 mg/m3 (stabilized) for 6 hr/day, 5 days/week for 2 years. No dust-exposure-related pathological changes other than lung lesions were observed in all exposed rats. There were no significant differences in pulmonary responses between unstabilized and stabilized CrO2 at the 0.5 mg/m3 exposure level. The lungs showed minute dust deposition in the alveoli adjacent to the alveolar ducts, but maintained an intact general architecture. The pulmonary responses satisfied the biological criteria for a nuisance dust. At 25 mg/m3, dust deposition was sharply confined to the alveoli in the alveolar duct region. Alveolar walls enclosing dust-laden macrophage (dust cell) aggregates were thickened with hyperplastic Type II pneumocytes and slightly collagenized fibrosis. Alveoli adjacent to the terminal bronchioles were lined with bronchiolar epithelium (alveolar bronchiolarization). In addition, lungs showed foamy macrophage response, cholesterol granulomas, alveolar proteinosis, and minute fibrotic pleurisy. These pulmonary lesions occurred predominantly in female rats. Of 108 female rats, 6 developed keratin cysts and 2 had cystic keratinizing squamous cell carcinoma (CKSCC). None of 106 male rats had either a keratin cyst or a CKSCC. The lung tumors developed from metaplastic squamous cells in the areas of alveolar bronchiolarization at the alveolar duct region. The lung tumors were well differentiated and devoid of characteristics of true malignancy. The CKSCC is an experimentally induced, unique tumor type and is different from the type of spontaneous lung tumor seen in man or animals. The relevance to man of this type of lung tumor appears to be negligible.