Five-day inhalation toxicity study of three types of synthetic amorphous silicas in Wistar rats and post-exposure evaluations for up to 3 months

Evidence suggests that short-term animal exposures to synthetic amorphous silicas (SAS) and crystalline silica can provide comparable prediction of toxicity to those of 90-day studies, therefore providing the opportunity to screen these types of substances using short-term rather than 90-day studies. To investigate this hypothesis, the inhalation toxicity of three SAS, precipitated silica Zeosil 45, silica gel Syloid 74, and pyrogenic silica Cab-O-Sil M5 was studied in Wistar rats. Rats were exposed nose-only to concentrations of 1, 5 or 25 mg/m3 of one of the SAS 6 h a day for five consecutive days. Positive controls were exposed to 25 mg/m3 crystalline silica (quartz dust), negative controls to clean air. Animals were necropsied the day after the last exposure or 1 or 3 months later. All exposures were tolerated without serious clinical effects, changes in body weight or food intake. Differences in the effects associated with exposure to the three types of SAS were limited and almost exclusively confined to the 1-day post-exposure time point. Silicon levels in tracheobronchial lymph nodes were below the detection limit in all groups at all time points. Silicon was found in the lungs of all high concentration SAS groups 1-day post-exposure, and was cleared 3 months later. Exposure to all three SAS at 25 mg/m3 induced elevations in biomarkers of cytotoxicity in bronchoalveolar lavage fluid (BALf), increases in lung and tracheobronchial lymph node weight and histopathological lung changes 1-day post-exposure. Exposure to all three SAS at 5 mg/m3 induced histopathological changes and changes in BALf only. With all three SAS these effects were transient and, with the exception of slight histopathological lung changes at the higher exposure levels, were reversible during the 3-month recovery period. No adverse changes were observed in animals exposed to any of the SAS at 1 mg/m3. In contrast, with quartz-exposed animals the presence of silicon in the lungs was persistent and toxicological effects differed from those seen with SAS both with regard to the type and severity as well as in the time-response profile. In quartz-exposed animals silicon in the tracheobronchial lymph nodes was below the detection limit but silicon was found in the lungs at comparable levels 0-, 1- and 3-months post-exposure. One-day post-exposure to quartz, elevations in biomarkers of cytotoxicity in BALf, increases in lung and tracheobronchial lymph node weight and histopathological lung changes were minimal. These effects were present at 1-month post-exposure and progressively more severe at 3-months post-exposure. Overall, the results of the current study are similar to those of other published studies that had a 90-day exposure period and both types of studies indicate that the lack of lung clearance is a key factor in the development of silicosis.