to MVF until the hazards posed by inhaled asbestos—including asbestosis, lung cancer, and mesothelioma—were identified in the early 1960s (Wagner et al. 1960; Bader et al., 1961). Asbestosis was the first disease that clearly defined the potential for inhaled fibers to cause human health effects. The first case of asbestosis was described in 1907 by Murray, who reported that a carding machine operator died of pulmonary injury associated with diffuse pulmonary fibrosis. In 1924, Merewether and Price (1930), reporting on a study of British workers noted the unequivocal relationship between asbestos and pulmonary fibrosis. In the 1950's and 1960's, the association between asbestos exposure and the development of lung cancer (Doll 1955) or mesothelioma (Wagner et al. 1960) was first recognized. Only recently has the latent nature of asbestos-related disease been fully appreciated, particularly in the case of mesothelioma, which can occur more than 20 years after exposure. Gilson (1966) concluded that the average latent interval for the development of carcinoma of the lung was 20 years, whereas the interval between first exposure to asbestos and onset of symptoms of mesothelioma can be 25-50 years, with an average latency of 33 years. During World Wars I and II, there was a need to insulate ships rapidly; as a result, the number of asbestos-exposed individuals increased. Their exposure was correlated with an increased incidence of asbestos-related lung and pleural disease, often manifesting itself 1 or 2 decades after the end of exposure (Kennedy and Kelly 1993).

MVF were designed as replacements for asbestos in various applications, including insulation. In light of the asbestos legacy and the absence of a toxicological database on vitreous fibers combined with the latency issue, it is not surprising that concerns have been raised about human health effects related to MVF exposure. Because of the many types of fibers in use in the 1970's, including MVF, a series of studies were conducted to evaluate their pathologic potential. Numerous types of fibers were instilled into the animal's pleural cavities (Stanton and Wrench 1972) or injected into animal's abdominal cavities (Pott and Friedrichs 1972) primarily to evaluate their carcinogenicity. Both groups of investigators found that the dimensions of the fibers were critical to their pathogenicity. Stanton et al. (1981) proposed that although fibers greater than 8 µm in length and less than or equal to 0.25 µm in diameter might be more carcinogenic, the ratio of fiber length to width—the aspect ratio—was more important for carcinogenicity than either dimension alone. Those studies were the original basis of two of the three legs



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