Manufactured vitreous fibers (MVF), also known as synthetic vitreous fibers, are considered to be less hazardous than asbestos to human health. They are used in many thermal- and acoustical-insulation applications as an asbestos substitute or as a filtration medium. The Navy uses MVF in shipboard and onshore applications. To protect Navy personnel from harmful exposures to MVF, the U.S. Navy Environmental Health Center (NEHC) developed occupational exposure standards. The documentation assists industrial hygienists, occupational medicine physicians, and other Navy health professionals in assessing and controlling the health hazards linked with exposure to MVF. In 1997, the National Research Council (NRC) was asked to conduct an independent review of the Navy's toxicological assessment of MVF and to evaluate the scientific validity of its exposure standard of 2 fibers per cubic centimeter of air (f/cm3). The NRC assigned the task to the Committee on Toxicology, which established the Subcommittee on Manufactured Vitreous Fibers, a multidisciplinary group of experts, to determine whether all relevant toxicological and epidemiological data were correctly considered in developing the exposure standard; and to examine the uncertainty, variability, and quality of data and the appropriateness of assumptions used in the derivation of the exposure standard. The subcommittee was also asked to identify deficiencies in the MVF database and, where appropriate, to make recommendations for future research and data development. Review of the U.S. Navy's exposure Standard for Manufactured Vitreous Fibers represents the subcommittee's final report. The committee had expanded its review when in January 1999, the Navy revised its Occupational Safety and Health Program Manual (CNO 1999), changing the occupational exposure limit for MVF to the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV) of 1 f/cm3. The report features recommendations by the subcommittee as well as information gaps found throughout investigation. Overall, the subcommittee found that the Navy made a good start in assessing the health effects of MVF, but needed further research.