Asbestos

Asbestos

any of several minerals that readily separate into long, flexible fibres. Chrysotile, the fibrous form of the mineral serpentine, is the best-known type and accounts for about 95 percent of all asbestos in commercial use. It is a hydrous magnesium silicate with the chemical composition of Mg3Si2O5(OH)4. The other types all belong to the amphibole group of minerals and include the fibrous forms of anthophyllite, amosite (grunerite), crocidolite (riebeckite), tremolite, and actinolite. Though valued since ancient times for its resistance to fire, asbestos fibre did not achieve commercial importance until the 19th century. Modern asbestos production began in 1868 with the workings of a mine in Italy, and in 1878 large-scale production from deposits in Quebec began. Production slackened in the late 20th century owing to the health hazards posed by the mineral.

Chrysotile occurs chiefly in association with massive serpentine. After mining or quarrying, the asbestos fibre is freed by crushing therock and is then separated from the surrounding material, usually by a blowing process. Only the longest of the fibres, at least 1 cm (0.4 inch), are suitable for spinning into yarn. Shorter fibres are used in such products as paper, millboard, and asbestos-cement building materials. Asbestos' brittle, smooth-surfaced fibres are difficult to spin, tending to slip past each other unless blended with a rough-surfaced fibre, such as cotton, which typically makes up 10–25 percent of the blend. Chrysotile fibre usually has a whitish colour, but fibres of the amphibole minerals may be pale green, yellow, or blue. Asbestos cannot be dyed easily, and the dyed material is uneven and has poor colourfastness.

In addition to its resistance to the effects of heat and fire, asbestos is long-lasting and bonds well with many materials, to which it adds strength and durability. The fibre was formerly widely used in brake linings, gaskets, and insulation; and in roofing shingles, floor and ceiling tiles, cement pipes, and other building materials. Asbestos fabrics were used for safety apparel and for such items as theatre curtains and fire stop hangings in public buildings. By the 1970s Quebec in Canada and the Urals region of the Soviet Union were the major sources of asbestos fibre, and the United States led the world in the manufacture of asbestos products.

Reports of the harmful effects of asbestos fibres on human health caused increasing concern beginning in the 1970s, however. It was found that prolonged inhalation of the tiny fibres can result in a lung condition known as asbestosis (q.v.) or in mesothelioma, which is a rapidly fatal form of lung cancer. Once these health risks were firmly documented in the 1970s, regulatory agencies in the United States and other developed nations began placing tight restrictions on workers' exposure to asbestos in industrial plants. In 1989 the U.S. government instituted a gradual ban on the manufacture, use, and export of most products made with asbestos. Since the 1980s various substitutes for asbestos have been developed for use in many products.