DISSOLUTION OF FIBERS INHALED BY RATS

This article provides an analysis and an interpretation of the results of an inhalation study of the durability of several man-made vitreous fiber (MMVF) insulation wool samples in rats. The study was sponsored by the Thermal Insulation Manufacturers Association and was conducted by the Research and Consulting Company in Geneva with fiber size measurements made by Schuller Mountain Technical Center in Denver, CO. In this study, rats were exposed for 5 days to 4 types of airborne, respirable-sized MMVF and to crocidolite asbestos fibers. The MMVF included two compositions of glass wool, and one each of rock and slag wool. After exposure, animals were sacrificed at intervals up to 18 mo, and the number, length, and diameter of a representative sample of fibers in their lungs were measured. These data show that the long fibers (>20 mu m) are eliminated from the rats' lungs at a rate that is predicted from the dissolution rate measured in vitro for these fiber compositions. In particular, the long MMVF were nearly completely eliminated in several months, whereas the long crocidolite asbestos fibers remained in significant numbers at the end of the study. The number, length, and diameter distributions of fibers remaining in the rats' lungs agreed well with a computer simulation of fiber clearance. This simulation assumed that the long fibers dissolved at the rate measured for each fiber in vitro, and that the short fibers of every type were removed at the same rate as short crocidolite asbestos. The results are strong evidence that long MMVF are cleared by complete dissolution al the rate measured in vitro and that short fibers do not dissolve but are cleared by macrophage-mediated physical removal.