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Introduction

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Asbestos is a fibrous hydrated magnesium silicate with commercial use due to its indestructible nature, fire resistance, and easy transformability into industrial products from yarn to insulation block to brakes. Asbestos fibers are generally defined as long, thin fibers with a length to width ratio (aspect ratio) of 3:1. There are six commercial forms of asbestos: chrysotile, crocidolite, amosite, anthophyllite, actinolite, and tremolite. Most of the asbestos used in the United States has been chrysotile, a serpentine form of asbestos. Other asbestos types are the amphiboles—notably amosite, mined in South Africa, and crocidolite, mined in the Cape Province of South Africa and in Western Australia. These asbestos fiber types have strikingly different physical characteristics: chrysotile tends to be wavy and long, and occurs in bundles; crocidolite is needle-shaped with many long fibers; and amosite is similar to crocidolite but generally thicker. Asbestos fibers accumulate in the interstitium of the lung and are coated by iron and hemosiderin in a beaded, clubbed fashion referred to as ferruginous or asbestos bodies.

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Initially, asbestos was widely used in fireproof textiles and later as insulation for boilers and pipes. Thereafter, asbestos was used in yarn, felt, paper, millboard, shingles, paints, cloth, tape, filters, wire insulation, cement pipes for potable water, gaskets, and in friction materials, including brake linings, and roofing and floor products. Asbestos was extensively used for ship construction during World War II. Occupational exposure to asbestos in the United States now primarily occurs during maintenance activities or remediation of buildings containing asbestos. The Occupational Safety and Health Administration (OSHA) has estimated that 1.3 million workers in general industry continue to be exposed to asbestos, and the National Institute for Occupational Safety and Health (NIOSH) estimates that 44,000 mine workers might be exposed to asbestos fibers or amphibole cleavage fragments, especially tremolite. Although the European Union has banned imports and use of asbestos, approximately 2 million tons are used worldwide. Asbestos is mined in Eastern Europe and Asia where asbestos products are manufactured for use in insulation, roofing, and construction. Globalization has resulted in export of asbestos to developing countries for manufacture and export to other developing countries with concomitant increase in asbestosis and mesothelioma.1

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Types of Exposure

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Primary asbestos exposures occurred in miners and millers. Secondary exposures occurred in manufacturing plants using asbestos in the production of textiles, friction materials, tiles, and insulation materials. Epidemiological studies focused on cohorts in these plants, since asbestos fiber type was often specified and dust measurements were obtained. These studies demonstrated that intensity and duration of exposure play an important role in the prevalence of asbestos-related disease. Common trades with asbestos exposure include sheet metal work, plumbing, pipefitting, insulation, railroad and utility work, and school or building custodians. Prevalence of radiographic opacities in asbestos insulation workers exposed prior to the 1970s reached 50% in long-term workers and was approximately 15% in sheet metal workers. There is a ...

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