Asbestiform Fibers Nonoccupational Health Risks (1984) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-15
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To reach a better understanding of the relationship between characteristics of asbestiform fibers and possible adverse health effects from nonoccupational exposures, the U.S. Environmental Protection Agency asked the National Academy of Sciences to undertake a Study with two goals: · to evaluate the human health risks associated with nonoccupational exposure to asbestifo~" fibers, with emphasis on inhalation of outdoor arid indoor air, and .
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Evidence for the existence of the risk includes the following: · Large excesses of lung cancer, mesothelioma, pulmonary fibrosis, and other pleural abnormal it lea have been found among workers occupationally exposed to asbestos. Presumably, nonoccupational exposures would result in qualitatively similar effects.
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The risk to nonsmokers appears greater for mesothelioma than for lung cancer. Because of the great reliance on assumptions and on clearly deficient exposure and effects data, the committee views these risk estimates as guides to the qualitative assessment of nonoccupational health risks from asbestos and asbestiform fibers -- not as definitive estimates of the amount of disease to be anticipated.
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The time dependence factor also implies that restricting exposures of children to asbestos would be even more effective than a corresponding restriction for adults in reducing the lifetime risk of mesothelioma. Physicochemical Properties and Health Effects Some of the physical properties of asbestiform fibers appear to be important in causing adverse health effects, but the specific properties that are necessary and sufficient are not known.
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From page 5... ...
These materials include man-made mineral fibers such as fibrous glans and mineral wool, which are sometimes used as substitutes for asbestos, as well as certain natural asbestiform varieties of minerals not marketed as asbestos. Sources of exposure to asbestiform fibers may be roughly divides into three broad categories:
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nonmalignant pleural disease, including diffuse pleural thickening and effusions and the formation of fibrous and calcified plaques. The occurrence of these four diseases in various occupational settings and the presence of asbestiform fibers in the general environment led to current concern about potential health effects from nonoccupational exposures.
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The committee made quantitative risk assessments for lung cancer and mesothelioma from inhaled asbestos, but it did not attempt quantitative risk assessments for other cancers, from inhalation of other asbestiform fibers, or from ingestion of asbestifo~m fibers in water or food. Materials of Concern For purposes of this report, the term "asbestiform fibers" is used broadly to include both naturally occurring and certain synthetic inorganic and carbon fibers that share some specific physical properties with asbestos.
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As a result, it may be impossible to discern the composition of materials studied and extremely difficult to draw conclusions about their physical properties and biological effects. Relationship of Fiber Characteristics to Health Effects Various physical properties of asbestiform fibers appear to play a role in causing adverse health effects; however, the specific properties that are necessary and sufficient to produce such effects have not been postively identified.
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Some experimental studies have indicated that surface charge appears to be involved in cytotoxicity. Although chemical composition is related to physical properties of asbestiform fibers, a direct role for chemical composition Per se in biological activity has not been demonstrated.
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However, material already in place would also contribute to total exposure. Health Effects Methodology To develop an understanding of the health risks associated with exposure to environmental agents such as asbestiform fibers, investigators usually evaluate data from clinical, epidemiological, and laboratory studies.
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Although exposure to asbestos has been strongly associated with most mesothelioma cases studied, some cases may occur without apparent asbestos exposure. The evidence does not exclude the possibility that ambient exposure to asbestiform fibers was associated with mesotheliomas for which exposure court not be documented.
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Moreover, because of inherent limitations in the epidemiological studies, including the limited sizes of the exposed populations and the lack of individual exposure data, it has not been possible to determine from these studies the extent to which there may be an association between GI cancers in humans and the presence of asbestiform fibers in drinking water. Wealth Effects of Nonasbestos Asbestiform Fibers .
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From page 13... ...
Entimates of exposure outside the workplace are particularly difficult to obtain, ant it is the risk from such exposure that is the focus of this report. Other factors that introduce uncertainty into risk assesaments for nonoccupational exposures include assumptions about the magnitude of effects at low doses; differences in the characteristics of fibers in the
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Smal ler f iber s ize in the ambient environment would probably tend to lead to lower risk. The comparative or qualitative risk assessments for the other asbestiform fibers were based on chrysotile and lung cancer an the baseline case.
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The risks for developing lung cancer or mesothelioma as a result of exposure to the other materials considered by the committee were usually much lower than those for chrysotile, principally because of a lower potential for airborne exposure or because the fibers are less respirable -- not because their intrinsic toxicity is necessarily less. For example, both ceramic and carbon fibers can be found in respirable size ranges and may have some biological properties similar to those of asbestos, but production and opportunities for exposure are low, although increasing.
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