Amphibole fibers

The two main amphibole asbestos fibers are amosite and crocidoHte, and both are hydrated siHcates of iron, magnesium, and sodium. The appearance of these fibers and of the corresponding nonfibrous amphiboles is shown in Figure 1. Although the macroscopic visual aspect of clusters of various types of asbestos fibers is similar, significant differences between chrysotile and amphiboles appear at the microscopic level. Under the electron microscope, chrysotile fibers are seen as clusters of fibrils, often entangled, suggesting loosely bonded, flexible fibrils (Fig. 2a). Amphibole fibers, on the other hand, usually appear as individual needles with a crystalline aspect (Fig. 2b). 

Fig. 5. Microscopic stmcture of amphibole fibers (10). Reprinted with permission.

The morphological variance appears more important with chrysotile than with amphiboles. The intrinsic stmcture of chrysotile, its higher flexibiUty, and interfibnl adhesion (10) allow a variety of intermediate shapes when fiber aggregates are subjected to mechanical shear. Amphibole fibers are generally more britde and accommodate less morphological deformation during mechanical treatment. 

Also, the adsorption of anionic or neutral surfactants on chrysotile fibers in aqueous dispersions enhances fiber separation, with a concomitant increase of surface area (26). Such effects have not been reported for amphibole fibers. 

The evolution in the world production of asbestos fibers since 1950 is illustrated in Table 5 (5) after a peak near 1980, production leveled off after 1985 at 4.2 4.3 X 10 t. Changes in the production of the two main producers, Canada and the former USSR, over the same period are also illustrated. These figures show a substantial decrease in the Canadian production with a concomitant increase in the former USSR production. During recent years, several other countries, namely Brazil, Zimbabwe, and China, have substantially increased their production of chrysotile. Most of China s production, as well as the limited production of many other countries, is used in local industrial appHcations. South Africa is the only country where the three main types of asbestos are produced (chrysotile, crocidoHte, and amosite), and the only significant producer of amphibole fibers. 

A further consensus developed within the scientific community regarding the relative carcinogenicity of the different types of asbestos fibers. There is strong evidence that the genotoxic and carcinogenic potentials of asbestos fibers are not identical in particular mesothelial cancer is mostiy, if not exclusively, associated with amphibole fibers (43). 

Figure 23 Two varieties of asbestos fibers (A) amphibole and (B) serpentine. The thin amphibole fibers present a larger toxicological threat due to their greater ability to penetrate lung tissue and reach the alveoli.

Analyses of bronchoalveolar lavage fluid samples or sputum samples can directly reflect alveolar concentrations of retained fibers and, although they do not reflect the proportion of deposited fibers that may move to the interstitium (Case 1994 Pinkerton et al. 1984), can provide information regarding past exposure to asbestos, especially to amphibole fibers. Obtaining sputum samples is much less invasive than obtaining bronchoalveolar lavage samples. In Libby, Montana vermiculite miners and millers exposed to fibrous tremolite, counts of asbestos bodies in sputum samples closely reflected intensity and duration of past exposure (Sebastien et al. 1988b), but asbestos body counts in sputum samples from volunteers from other cohorts of workers exposed to asbestos (predominately chrysotile or lower levels of amphibole fibers than in Libby) did not reliably reflect past levels of exposure (McDonald et al. 1988, 1992). 

As discussed in Section 3.4.4 inhaled asbestos fibers that are deposited in the lung are principally removed by mucociliary transport into the alimentary canal and eventually are excreted in the feces. Chrysotile fibers appear to be cleared more readily than amphibole fibers, and long fibers are cleared more slowly than short fibers (Coin et al. 1992 Morgan 1991). 

Asbestos fibers are basically chemically inert, or nearly so. They do not evaporate, dissolve, bum, or undergo significant reactions with most chemicals. In acid and neutral aqueous media, magnesium is lost from the outer bmcite layer of chrysotile. Amphibole fibers are more resistant to acid attack and all varieties of asbestos are resistant to attack by alkalis (Chissick 1985 WHO 1998). Table 4-2 summarizes the physical and chemical properties of the six asbestos minerals. 

R6delsperger K, Woitowitz H-J, Briickel B, et al. 1999. Dose-response relationship between amphibole fiber lung burden and mesothelioma. Cancer Detect Prev 23(3) 183-193. 

Smith AH. 1998. Amphibole fibers, chrysotile fibers, and pleural mesothelioma [Letter]. Am J Ind Med 33 96. 

The photograph in Figure 2 shows a sample of raw vermiculite ore from Libby, Montana, with asbestiform amphibole fibers mixed in with the vermiculite. Figure 3 shows processed vermiculite concentrate (before expansion) and exfoliated vermiculite (after expansion). 

Important determinations of asbestos toxicity include exposure concentration, duration, fiber dimensions, and fiber durability. There is animal and human evidence that long fibers are retained in the lungs for longer periods than short fibers and that amphibole fibers, such as tremolite asbestos, are retained longer than chrysotile fibers. Short and long fibers may contribute to the pathogenesis of inflammation, fibrosis, and cancer in humans, but their relative importance is uncertain. 

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