amorphous, they can be partially converted to a crystalline form such as mullite or cristobalite, when heated to above 1800°F.

Each of the three categories of fibers contains fibers with different chemical compositions and different sizes. Animal and human studies that address potential health consequences of exposures to MVF should be based on knowledge of the chemical and size characteristics of the fibers under study.

The Navy's review of the production, use, and chemical and physical properties of MVF covers technologies only through 1993. Of particular concern to the subcommittee is the Navy's understanding of the effects of time and temperature on the composition of MVF. The Navy does indicate in the Section “Chemical and Physical Properties” in Man-Made Vitreous Fibers, that MVF have high melting points, which make them good candidates for some applications, such as high temperature insulation, but it does not cite any studies on the wearing of these fibers and what happens to them when they are exposed to high temperatures. Since the anticipated exposure of Navy personnel is primarily to worn fibers, the subcommittee believes it would be helpful if the Navy included any relevant references on this topic or indicated that relevant data were not available.

Because of the dynamic nature of the development of “new” fibers, which are being used in a myriad of applications, one can expect that MVF in the future will be different from those in use or in production today. Therefore, the Navy will have to be cognizant of those differences both with regard to current and future use, but also, and just as importantly, with regard to “tear out” and replacement of older fibers. More recent advances in the production of MVF are not included in the Navy's documentation (Maxim et al. 1999b). New uses for MVF and the properties of the fibers may have a substantial impact on the types of exposures that may be anticipated for Navy personnel, now and in the future.