Asbestos Selected Cancers (2006) / Chapter Skim
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4 Exposure and Disposition
4 Exposure and Disposition
Pages 63-80
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From page 63... ...
In 1973, the US Environmental Protection Agency (EPA) prohibited spraying of asbestos insulation, and further restrictions were later applied.
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Occupational Exposure Asbestos concentrations observed in occupational settings have been orders of magnitude higher than the highest concentrations observed in residential settings, but some in-home activities, such as shaking out work clothes, can produce levels that may rival those found in the workplace. The highest well-documented exposures have been among workers manufacturing asbestos products or employed in mining and milling operations.
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For example, in the mid 1960s, mean exposures of 20-100 f/ml were reported at the Wittenoom crocidolite mines and mills in Western Australia; they may have been higher in earlier decades (Armstrong et al.
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Limits considered acceptable for occupational exposure have dropped over time. The American Conference of Governmental Industrial Hygienists first proposed a Threshold Limit Value (TLV)
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The sources include disturbed natural deposits, improper disposal or transportation of asbestoscontaining wastes, and uses that result in friable asbestos, such as motorvehicle brake pads. Although exposure from undisturbed natural sources is possible, it has not been documented.
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Asbestos may enter drinking water from erosion of natural deposits, mining operations, or asbestos-containing cement pipes (ATSDR 2001)
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In considering the potential risk posed by inhaled pollutants, including fibers, the critical determinant of injury is the amount of material that reaches the target site -- a measure generally referred to as the biologically effective dose. As depicted in Figure 4.2, dose, without qualification, gener FIGURE 4.1 Effect of aerodynamic diameter on deposition of particles in the respiratory tract.
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Particles may also penetrate the respiratory epithelium and remain in the airways or migrate to bronchopulmonary lymph nodes. Experimental studies show that particles in the ultrafine fraction (less than 0.1 µm in aerodynamic diameter)
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In general, amphibole asbestos fibers are more persistent than chrysotile asbestos fibers. In contrast with studies of fiber deposition in the lower respiratory tract, little is known about fiber deposition and clearance from the upper respiratory tract, particularly the larynx.
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. Fibers that persist in the alveolar region may be directly toxic to alveolar epithelial cells or incite a chronic inflammatory response that perpetuates tissue injury followed by episodes of epithelial cell proliferation and fibrosis (Oberdörster 1996)
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Anthracotic particles, as well as asbestos fibers, were detected near lymphatic openings on the surface of the parietal pleura. Scanning electron microscopy of these "black spots" revealed focal accumulation of inflammatory cells.
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. Asbestos fibers that are cleared from the lower respiratory tract by the mucociliary escalator can be swallowed from the pharynx and gain access to the upper and lower gastrointestinal tract.
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Of organs with fewer samples available, stomach, duodenum, and colon were among those in which ABs were found; the others with positive findings were brain, prostate, thyroid, mediastinal lymph nodes, bone, omentum, and spinal cord. The prevalence of ABs in other organs reflected the number of asbestos fibers found in a case's lung sample.
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a prerequisite of accumulation of fibers and development of cancer at these sites? Finally, the finding of asbestos fibers or ABs in tissue samples obtained from gastrointestinal tract tumors raises the question of the causal significance of this observation: do the fibers accumulate secondarily at sites of mucosal damage or ulceration associated with a growing tumor?
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British Journal of Industrial Medicine 45(1)
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1990. Asbestos content of lung tissue, lymph nodes, and pleural plaques from former shipyard workers.
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1994. Human Respiratory Tract Model for Radiolgical Protection.
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From page 80... ...
2001. Asbestos tissue burden study on human malignant mesothelioma.
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