Aerosols of synthetic amorphous silica do not induce fibrosis in lungs after inhalation: Pathology working group review of histopathological specimens from a subchronic 13-week inhalation toxicity study in rats:

In a subchronic (13-week) inhalation toxicity study with a terminal sacrifice (after 13 weeks inhalation) and several recovery period sacrifices (13, 26, 39, and 52 weeks), the effects of AEROSIL® 200 (pyrogenic synthetic amorphous silica (SAS)), AEROSIL® R 974 (surface-treated pyrogenic SAS), and SIPERNAT® 22 S (precipitated SAS) were tested in rats at multiple dose levels. The aforementioned materials are all SAS products. A comparative group of animals was exposed to quartz dust. This study attempts to reexamine the lung tissues originally evaluated in a study published by Reuzel et al. using the current standards. To reach a high level of credibility, the results of the reevaluation were subsequently examined by a pathology working group (PWG). In particular, the reevaluating pathologist and the PWG concluded that, even though quartz (crystalline silica) persisted, induced alterations in the lungs following 13 weeks of exposure to amorphous silicas were reversible following 52 weeks of recovery. A lon...