Two samples of refractory ceramic fibers (RCF1 and RCF1a) were tested in rats exposed by inhalation for 3 wk and followed thereafter for 12 mo. RCF1 and RCFIa have similar fiber chemistry but differ in the fiber size distributions and the contents of nonfibrous particles (2% for RCFIa, 25% for RCFli. For both test samples the target aerosol concentration was 130 fibers/ml > 20 µm but the RCF1 aerosol contained more short fibers (less than 20 µm long) and more nonfibrous particles. Radioactive tracer measurements carried out during a period of 90 days after exposure demonstrated an almost complete abolition of alveolar clearance in RCFI-exposed animals. With RCFIa the half-time of(46)Sc2O3 particles was 80 days, compared to 60 days for controls (not statistically different). Biochemical and cytological analyses were carried out in bronchoalveolar lavage at days 3, 17, 31, 94, and 365 postexposure. They revealed a more important and more persistent inflammation in RCFI-exposed animals compared to RCFIa. These observations show that the biological activity of synthetic vitreous fibers (SVFs) assessed by inhalation experiments depends not only on the chemical composition of the fiber type but also on the physical characteristics of the test sample. They are in line with results of previous studies and support the hypothesis of a synergistic effect between fibrous and nonfibrous particles. They raise questions about the interpretation of the so called RCC experiments in which the several SVFs samples tested were not prepared using the same method and differed in their physical characteristics. In particular, these differences existed especially between the RCF and MMV F samples.
