The pulmonary response and clearance of Ludox colloidal silica after a 4-week inhalation exposure in rats.

Rats were exposed to Ludox colloidal silica (CS) at concentrations of 0, 10, 50, and 150 mg/m3 for 6 hr/day, 5 days/week for 4 weeks. Rats were killed after 4 weeks of exposure and 10 days or 3 months postexposure (PE). The exposure concentration of 10 mg/m3 Ludox CS is considered to be the no-effect concentration. There were no exposure-related clinical signs in any group. After 4 weeks exposure, lung weights were increased significantly in rats exposed to 50 and 150 mg/m3 Ludox CS, but lung weights were similar to those of controls at 3 months PE. After 4 weeks exposure to 50 mg/m3 Ludox CS, a slight alveolar macrophage response, polymorphonuclear leukocytic infiltration, and Type II pneumocyte hyperplasia in alveolar duct regions were present. After 3 months PE, these pulmonary lesions had almost disappeared with removal of most dust-laden alveolar macrophages (AMs). The pulmonary response to 150 mg/m3 Ludox CS was similar in character but increased in magnitude from that seen at 50 mg/m3. At 3 months PE, most particle-laden AMs had disappeared and the remaining AMs were aggregated and sharply demarcated. A few aggregates of particle-laden AMs appeared to transform into silicotic nodules comprising macrophages, epithelioid cells, and lymphocytic infiltration in some animals. Some silicotic nodules showed reticular fiber networks with minute collagen fiber deposition. Tracheobronchial lymph nodes were enlarged with aggregates of particle-laden AMs and hyperplastic histiocytic cells. Lung-deposited Ludox cleared rapidly from the lungs with half-times of approximately 40 and 50 days for the 50 and 150 mg/m3 groups, respectively.