cations. For these workers, exposures can be highly variable in terms of concentration and relatively acute, in contrast with the generally continuous and longer-term exposures that take place in the manufacturing industry. For RCF that have experienced repeated thermal stress, the resulting airborne fibers can have altered physical and chemical properties, including length distributions and crystalline forms that affect their biopersistence (TIMA 1993, p. 36). Non-RCF MVF, however, melt when exposed to heat so transformation to crystalline forms is of less concern (TIMA 1993, p. 28). The Navy should evaluate how appropriate the exposure standards are for protecting the health of the its personnel who are exposed to MVF that might have been altered by thermal stress.