Asbestos

Asbestos characterizes different natural silicates, which form fibrous and flexible crystals. The term is derived from the ancient Greek adjective for durable, which refers to its specific thermal, mechanical, and chemical stability. These properties made asbestos especially attractive for the production of heat-resistant materials and inert components for construction, industry, and households. Since the last decades of the 20th century, the use of asbestos became increasingly criticized and banned because of health risks associated with exposure to asbestos, including asbestosis, mesothelioma, and other cancers. Corresponding pathogenic effects are likely after long-term exposure to its respirable tiny fibers during mining, processing, weathering, and demolition of the mineral or its products. The particle spectrum of the released fibers may even range down to the nanoscale.

The typical length of asbestos fibers varies between 200 nm and 200 μm, while their mean diameter ranges from 20 to 90 nm. The fibrous nature of asbestos reflects its internal structure, which is given by parallel alignment of either chains or bands of hydrated silicate molecules. The composition of the more abundant “chrysotile” type is Mg3Si2O5(OH)4. Asbestos from the amphibole group is variable, with different sets of cations (Mg, Fe, Na, Ca) bound to the complex [Si4O11(OH)]7-anion. Asbestos remains stable below temperatures from 1,350 to 1,750 K, depending upon its magnesium content. Inversely, its chemical inertness weakens with rising proportions of magnesium. Asbestos is an excellent thermal and electrical isolator, with high tensile strength and elasticity. It has a specific affinity to certain substances like benzopyrene, which might be adsorbed at the fibers' surfaces.

A typical appearance of asbestos on a site contaminated with asbestos-containing materials in Oregon.

Asbestos has been known since ancient times. The era of its industrial use began in the 19th century. Main producers of asbestos are currently Russia, Kazakhstan, Brazil, Canada, and South Africa where it is mined from certain crystalline rocks. Its fibrous nature enables the production of fire-resistant clothes (e.g., for fire departments) and other woven fabrics for heat protection. Asbestos might be used for construction purposes, while being added to cement mixtures for fabrication of durable roof and facing tiles as well as for sewage pipes. It has been also utilized for manifold other products (floor covering, seals, brake pads, insulators) as well as for fire-resistant spray layers on steel constructions of large tower blocks. Overall, asbestos has been used in thousands of applications. However, recent research has demonstrated that exposure to asbestos increases risk for several serious diseases, and as a consequence, some uses of asbestos are now regulated or prohibited, and in some cases removal or containment of asbestos materials has been mandated in locations (such as schools and other public buildings) where they would expose humans to health risks.

The pathogen potential of asbestos is related to the specific size and shape of asbestos fibers. Fibers below 3 μm diameter may enter the respiratory organs as airborne dust after being mobilized by production, handling, and demolition of asbestos-bearing materials. Respired asbestos mostly accumulates on the lung tissues and may stimulate enhanced cell proliferation as well as chromosomal aberrations. Long-term exposure to inhaled asbestos fibers might induce bronchial tumors and malignant pleural diseases besides asbestosis, which results in an irreversible decrease of pulmonary function. Corresponding health effects come about after decades in a stochastic manner. The risk of malignant disease depends upon the concentration of fibers and duration of exposure. Moreover, individual risks rise extraordinarily in combination with loads from tobacco smoke or radon. In contrast, asbestos contaminated drinking water poses relatively low risks [Page 32]to human health compared to burdens from airborne particles. Permanently tied asbestos is even harmless.

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