Abstract

The term asbestos is a generic designation referring usually to six types of naturally occurring mineral fibers that are or have been commercially exploited. These fibers belong to two mineral groups: serpentines and amphiboles. The serpentine group contains a single asbestiform variety: chrysotile; five asbestiform varieties of amphiboles are known: anthophyllite asbestos, grunerite asbestos (amosite), riebeckite asbestos (crocidolite), tremolite asbestos, and actinolite asbestos. These fibrous minerals share several properties that qualify them as asbestiform fibers: they are found in bundles of fibers that can be easily separated from the host matrix or cleaved into thinner fibers; the fibers exhibit high tensile strengths, they show high length: diameter (aspect) ratios, from a minimum of 20 up to >1000; they are sufficiently flexible to be spun; and macroscopically, they resemble organic fibers such as cellulose. Since asbestos fibers are all silicates, they exhibit several other common properties, such as incombustibility, thermal stability, resistance to biodegradation, chemical inertia toward most chemicals, and low electrical conductivity.

The term asbestos has traditionally been attributed only to those varieties that are commercially exploited. The industrial applications of asbestos fibers have now shifted almost exclusively to chrysotile. Two types of amphiboles, commonly designated as amosite and crocidolite are no longer mined. The other three amphibole varieties, anthophyllite asbestos, actinolite asbestos, and tremolite asbestos, have no significant industrial applications presently.

The microscopic and macroscopic properties of asbestos fibers stem from their intrinsic, and sometimes unique, crystalline features. As with all silicate minerals, the basic building blocks of asbestos fibers are the silicate tetrahedra that may occur as double chains [Si₄O₁₁]^6–, as in the amphiboles, or in sheets [Si₄O₁₀]^4–, as in chrysotile. In the case of chrysotile, an octahedral brucite layer having the formula [Mg₆O₄ (OH)₈]^4– is intercalated between each silicate tetrahedra sheet. Asbestos fibers used in most industrial applications consist of aggregates of smaller units (fibrils), which is most evident with chrysotile that exhibits an inherent, well-defined unit fiber.

The identification of asbestos fibers can be performed through morphological examination, together with specific analytical methods to obtain the mineral composition and/or structure. Morphological characterization in itself usually does not constitute a reliable identification criterion. Hence, microscopic examination methods and other analytical approaches are usually combined.

Most of the asbestos mining operations are of the open pit type, using bench drilling techniques. The fiber extraction (milling) process must be chosen so as to optimize recovery of the fibers in the ore, while minimizing reduction of fiber length. Dry milling operations are the most widely used. In the production, or industrial applications, of asbestos fibers, several parameters are considered critically important. The measurement of fiber length is important since the length determines the product category in which the fibers will be used and, to a large extent, their commercial value. The most widely accepted method for chrysotile fiber length characterization in the industry is the Quebec Standard test. A second industrially important fiber-length evaluation technique is the Bauer–McNett classification.

Asbestos fibers historically have been used in a broad variety of industrial applications. Because of recent restrictions, many of these applications have now been abandoned and others are pursued under strictly regulated conditions. The main characteristic properties of asbestos fibers that can be exploited in industrial applications are their thermal, electrical, and sound insulation; nonflammability; matrix reinforcement (cement, plastic, and resins); adsorption capacity (filtration, liquid sterilization); wear and friction properties (friction materials); and chemical inertia (except in acids). These properties led to several main classes of industrial products or applications: fire protection and heat or sound insulation, fabrication of papers and felts for flooring and roofing products, pipeline wrapping, electrical insulation, thermal and electrical insulation, friction products in brake or clutch pads, asbestos–cement products, reinforcement of plastics, fabrication of packings and gaskets, friction materials for brake linings and pads, reinforcing agents, vinyl or asphalt tiles, and asphalt road surfacing. Of these, asbestos–cement products, roof coatings, brake pads and shoes, and clutches are the major markets for asbestos.

The relationship between workplace exposure to airborne asbestos fibers and respiratory diseases is one of the most widely studied subjects of modern epidemiology. The research efforts resulted in significant consensus in some areas, although strong controversies remain in other areas. Typically, it is widely recognized that the inhalation of long (considered usually as >5 µm), thin, and durable fibers can induce or promote lung cancer. It is also widely accepted that asbestos fibers can be associated with three types of diseases: asbestosis, lung cancer, mesothelioma. 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 mostly associated with amphibole fibers. The identification of health risks associated with asbestos fibers has prompted strict regulations to limit the maximum exposure of airborne fibers in workplace environments.