Differential pulmonary responses in rats inhaling crystalline, colloidal or amorphous silica dusts

Pulmonary responses in rats were compared after short-term inhalation exposure to polymorphs of silica dust. Groups of CD rats were exposed 6 h a day for 3 d to crystalline silica or amorphous silica. Another group was exposed to Ludox colloidal silica for 6 h a day, 5 d a week for two or four weeks. Thereafter the groups were killed, and the lungs washed at several postexposure times. The crystalline silica produced persistent pulmonary inflammatory responses characterized by neutrophil recruitment and consistently elevated biomarkers of cytotoxicity in bronchoalveolar lavage fluids, and progressive histopathological lesions were observed within one month of the exposure. Amorphous silica produced a transient pulmonary inflammatory response, and Ludox elicited transient pulmonary inflammatory responses at 50 or 150 mg center dot m-3 but not at 10 mg center dot m-3. After three months most of the biochemical values of the Ludox-exposed animals had returned to the control level. These results demonstrate that crystalline silica dust is more potent in producing pulmonary toxicity when compared with amorphous or colloidal silica particles.