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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers
showed that at the end of the exposure period the lengths of MMVF 10a, MMVF 33, and amosite fibers in the lung were reduced by 64.2%, 44.2%, and 8.7%, respectively, in comparison with the original aerosolized-fiber lengths. McConnell et al. (1999), showed that after a single 6-hr exposure in hamsters, almost twice as many WHO amosite fibers and approximately equal numbers of fibers longer than 20 µm were deposited in the lung. The results of CLSM constitute additional evidence that differential shortening occurs in fibers retained in the lung. However, the value of CLSM is limited as it can only be used to visualize to a depth of a few millimeters in the lung, whereas more-centralized locations cannot be visualized without cutting the lung, which would diminish the advantages of the technique. At present, CLSM should be viewed as a research tool rather than as a means of assessing biopersistence definitively.
The potential hazards posed by a given MVF is directly related to its ability to persist in the lung long enough to cause chronic disease. This persistence has been termed biopersistence. Research has shown that persistence in the lung is directly related to the chemical composition and dimensions of fibers. Biopersistence can be measured with long-term and short-term studies. It has also been proposed that simple in vitro fiber solubility studies can reasonably predict what will happen in vivo. Most of the fibers that are of concern to the Navy have been investigated with one or more of these techniques, and review of the resulting data can be valuable for estimating the hazards potentially associated with the fibers.