Dissolution of Slagwool Fibers in the Filter Clearing and Mounting Step in Preparation for Counting

Abstract

Slagwool can be used in studies of fiber deposition as a surrogate for more toxic fibers. However, we found that slagwool fibers are quickly dissolved in the process of mounting filters onto slides for counting. The mixture of 35% dimethylformamide, 15% acetic acid, and 50% water (DMF) is used widely to clear filters made of mixed cellulose ester in the determination of particle number by the membrane filter method. Although the DMF/Euparal technique has been thought to be the best method compared to other clearing and mounting techniques for membrane filters, we found fiber dissolution to be a serious limitation. We examined the dissolution of three man-made vitreous fibers (slagwool, rockwool, and fiberglass) by the DMF mixture. Each fiber was soaked in the DMF mixture and monitored for 1 h using phase-contrast microscopy at 400x. Only slagwool fibers were dissolved rapidly and severely. Possible explanations for the dissolution of slagwool fibers include low pH (2.5) of the DMF mixture, a high percentage (37.5%) of CaO in the slagwool fibers, and the matrix structure of these fibers. Fiber number, dimension, and biopersistence are important parameters of even short-term deposition studies. The membrane filter method for assessing total fibers in digested lung must not modify these parameters. The purpose of this article is to point out that techniques of clearing filters should be carefully assessed for any fibers used in experimental studies.