Asbestos fibers, types

In accordance with demonstrated differences between the various asbestos fiber types, the workplace regulation in many countries specifies different exposure limits for chrysotile and the amphiboles (45). Moreover, to alleviate estabHshed, or apprehended, risk from substitute fibers, the regulation often specifies maximum exposure limits for synthetic fibers (46) values of exposure limits adopted in leading industrial countries are coUected in Table 8. 

SVFs have been widely used as insulation material in places in which asbestos was used many years ago, and therefore the hazards have been compared. Because the three principal types of asbestos fibers types have caused lung cancer at high exposures, there is a widely held belief that all fibers are carcinogenic if inhaled in large enough doses." Hence, on a morpho-... 

Asbestos fibers, Type II, fibrated lap cement (asbestos) ... 

Available evidence indicates that all asbestos fiber types are fibrogenic, although there may be some differences in potency among fiber types (Bignon and Jaurand 1983 Churg 1993 Davis 1972 EPA 1986a Kamp and Weitzman 1997 McDonald et al. 1999). Most studies in humans have involved exposure to predominantly chrysotile, the most widely used type of asbestos (Albin et al. 1996 Berry et al. 1979 BOHS 1983 Case and Duffesne 1997 Cullen and Baloyi 1991 Dement et al. 1983 McDonald et... 

Case BW, Dufresne A, McDonald AD, et al. 2000. Asbestos fiber type and length in lungs of chrysotile textile and production workers Fibers longerthan 18 m. Inhal Toxicol 12 411-418. 

McDonald JC, Armstrong B, Case B, et al. 1989. Mesothelioma and asbestos fiber type. Evidence from lung tissue analysis. Cancer 63 1544-1547. 

Nicholson WJ. 1991. Comparative dose-response relationships of asbestos fiber types Magnitude and uncertainties. Ann NY Acad Sci 74-84. 

Rogers AJ, Ixigh J, Berry G, et al. 1991. Relationship between lung asbestos fiber type and concentration and relative risk of mesothelioma. Cancer 67 1912-1920. 

Roggli VL, Pratt PC, Brody AR. 1993. Asbestos fiber type in malignant mesothelioma An analytical scanning electron microscopic study of 94 cases. Am J Ind Med 23(4) 605-614. 

Mineralogy Asbestos is a name for a set of naturally fibrous silicate minerals. Asbestos minerals have been used because of their high tensile strength, high heat resistance, resistance to chemical attack, and ability to be woven into cloth (Dupres et al. 1984). These properties vary by asbestos fiber type. There are six accepted types of asbestos fibers namely, chrysotile, amosite, crocidolite, antho-phyllite, tremolite, and actinolite. These fibers can be separated into two broad mineralogical groups chrysotile or amphiboles, the latter including amosite, crocidolite, tremolite, actinolite, and anthophyllite. Table 2.1.6 lists the diseases caused by asbestos. While Table 2.1.6 is correct in broad outline, there are considerable discrepancies in the abil-... 

The possible increased risk for bronchogenic carcinoma in patients with pleural plaques is still a matter of debate (Smith 1984). Epidemiological studies are difficult to conduct, as they should adequately control for asbestos fiber type, duration and intensity of asbestos exposure, as well as cigarette smoking habits, race, gender, and exposure to other potential carcinogens (Smith 1984). Never-... 

Acryhcs and modacryhcs are also useflil industrial fibers. Fibers low in comonomer content, such as Dolan 10 and Du Font s PAN Type A, have exceptional resistance to chemicals and very good dimensional stabihty under hot—wet conditions. These fibers are useflil in industrial filters, battery separators, asbestos fiber replacement, hospital cubical curtains, office room dividers, uniform fabrics, and carbon fiber precursors. The exceUent resistance of acryhc fibers to sunlight also makes them highly suitable for outdoor use. Typical apphcations include modacryhcs, awnings, sandbags, tents, tarpauhns, covers for boats and swimming pools, cabanas, and duck for outdoor furniture (59). 

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). 

Tensile Strength. The inherent tensile strength of a single asbestos fiber, based on the strength of Si—O—Si bonds ia the siUcate chain, should be near 10 GPa (1.45 x 10 psi) (18). However, iadustrial fibers exhibit substantially lower values, because of the presence of various types of stmctural or chemical defects. 

Adsorption and Surface Chemical Grafting. As with siHca and many other siHcate minerals, the surface of asbestos fibers exhibit a significant chemical reactivity. In particular, the highly polar surface of chrysotile fibers promotes adsorption (physi- or chemisorption) of various types of organic or inorganic substances (22). Moreover, specific chemical reactions can be performed with the surface functional groups (OH groups from bmcite or exposed siHca). 

Instrumental Methods for Bulk Samples. With bulk fiber samples, or samples of materials containing significant amounts of asbestos fibers, a number of other instmmental analytical methods can be used for the identification of asbestos fibers. In principle, any instmmental method that enables the elemental characterization of minerals can be used to identify a particular type of asbestos fiber. Among such methods, x-ray fluorescence (xrf) and x-ray photo-electron spectroscopy (xps) offer convenient identification methods, usually from the ratio of the various metal cations to the siUcon content. The x-ray diffraction technique (xrd) also offers a powerfiil means of identifying the various types of asbestos fibers, as well as the nature of other minerals associated with the fibers (9). 

Thermoanalytical methods (tga, dta) often enable definite identification of the type of asbestos fibers (Fig. 7). For example, the strong exotherm observed with chrysotile at 830°C can be used as a routine indicator for determining the chrysotile content of talc (4,10). Thermal methods are also usefiil for determining certain mineral contaminants of asbestos fibers, for example bmcite and calcite in chrysotile. 

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. 

Asbestos fibers have been used in a broad variety of industrial appHcations. In the peak period of asbestos consumption in industrialized countries, some 3000 appHcations, or types of products, have been Hsted. Because of recent restrictions, many of these appHcations have now been abandoned and others are pursued under strictly regulated conditions. 

In the various appHcations, each type or group of products usually requires a selected asbestos fiber grade (or range of grades). Table 6 Hsts various... 

The substitution of asbestos fibers by other types of fibers or minerals must, ia principle, comply with three types of criteria (36) the technical feasibihty of the substitution the gain ia the safety of the asbestos-free product relative to the asbestos-containing product and the availabiHty of the substitute audits comparative cost. 

Table 7 Hsts some of the materials and fibers that have been suggested or used ia the development of asbestos-free products. These are Hsted by categories, and no attempt is made to provide a detailed coverage of the respective properties of these materials. Table 7 also provides an estimate of the cost ranges of asbestos fibers and several types of substitution materials.

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). 

Country Asbestos exposure limits Fiber type Exposure limits for SMF and nuisance dust... 

The use of flax fibers in car disc brakes as a replacement of asbestos fibers is another example of an application of this type of material [7]. 

Older appliances (such as ovens) still sometimes contain asbestos. A waste from which asbestos fibers may be released is classified as a hazardous waste and must be disposed of as specified in the appropriate environmental laws. The heat-transfer oils of older types of mobile convector heaters still sometimes contain PCBs. These fluids must be disposed of as hazardous waste. 

---