Editors' Column Asbestos: Definition and Measurement Asbestos, fiber, asbestiform, amphi bole, serpentine. These terms are seen and heard more and more frequently today as a result of the current atten tion asbestos pollution is creating. Ac cording to Tibor Zoltai of the Depart ment of Geology and Geophysics at the University of Minnesota, these mineralogical terms have been misin terpreted in the current literature. Environmental and public health sci entists, engineers, and lawyers have adopted popular redefinitions that are not the terminology used by mineralo gists. For example, asbestos was ini tially the name of an independent mineral species and now has gradually become a collective term applied to all asbestiform varieties of minerals. In recent literature asbestos is defined as hydrated silicates that, when crushed or processed, separate into fi bers. A fiber is defined by its shape, a length-to-width ratio of at least 3:1. Asbestiform is used to describe a min eral known to occur as asbestos and/or produce fibers when crushed. Zoltai called for the establishment of a clear, interdisciplinary terminolo
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gy during his presentation at a recent meeting on asbestos. A workshop on asbestos definitions and measurement methods, held July 18-20 in Gaithersburg, Md., was cosponsored by the National Bureau of Standards and the Occupational Safety and Health Ad ministration. The meeting consisted of a series of technical presentations by some 30 scientific, medical, and government authorities covering all aspects of the asbestos issue. Sessions were held on mineralogical nomencla ture, the relationship between chemi cal and physical properties of fibers and health effects, analytical tech niques for measuring asbestos, and regulatory aspects. The two main asbestos-forming minerals are amphibole and serpen tine. Their crystal structure is quite different. The amphiboles are chain silicates, where S1O4 tetrahedra are linked to form bands four tetrahedra wide and very long. The bands run parallel to the asbestos fiber axis and are linked laterally by cations (calci um, magnesium, sodium, iron). Tremolite, crocidolite, amosite, and an-
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thophyllite fall into this category. The serpentines are layered silicates where the S1O4 tetrahedra are linked to form large, thin sheets. In chrysotile asbes tos the layers form either scrolls or concentric cylinders. Chrysotile, am osite, crocidolite, anthophyllite, and tremolite particles with length greater than 5 μπι, diameter less than 5 μνα, and a length-to-width ratio greater than 3:1 are currently defined as as bestos by the U.S. Occupational Safe ty and Health Administration (Feder al Register, October 9,1975). The major controversies in the health issue today concern the extent of toxicity of the two major classes of asbestos and of the various fiber sizes. The hazards of prolonged asbestos in halation have been well established. Irving Selikoff and associates at the Mt. Sinai School of Medicine, the City University of New York, have shown that asbestos inhalation can lead to cancer after a latency period of 20-40 years. Although the risk is greater for asbestos workers, there is growing concern for the potential hazards of nonoccupational exposures. There is
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no clear answer yet concerning the toxicity of ingested asbestos, that is, asbestos found in drinking water and foods. Research is currently underway in this area. Work is also being carried out on possible hazardous effects of mineral fibers resembling asbestos. Because of the growing concern for the accurate detection of asbestos, re liable analytical methods are needed to monitor asbestos levels. Toxicity of mineral fiber fragments has been shown to be related to the geometry of the fibers, fiber diameter, and fiber length. Thus, the scientist must obtain length and width data as well as con centration and mineral species. In en vironmental samples the fibers are generally 0.1-20 μπι in length and 0.03-1 μπι wide. Various techniques used by the analytical chemist include light microscopy, x-ray diffraction, thermal analysis, and electron micros copy. X-ray diffraction is a good screening technique; however, inter ferences make it difficult to determine morphology and identify the species. The very small size of these fibers and the need to distinguish between asbestos and other fibers mistaken for asbestos have prompted the use of electron microscopy. Techniques being used include transmission elec tron microscopy (TEM) with selected area electron diffraction and scanning electron microscopy (SEM) with en ergy-dispersive x-ray methods. Clay ton Ruud of the Denver Research In stitute discussed these two methods at the workshop. Advantages of the SEM system include easier sample preparation and the ability to identify species from elemental data obtained with the energy-dispersive x-ray methods. However, Ruud felt that TEM gives a sharper image and that analysis time is slightly faster than with SEM. Don Beaman of Dow Chemical Co. reported on the use of TEM with energy-dispersive spec trometry. Because it is possible to ob tain morphology, crystal structure, and elemental composition on one in strument, the reliability of fiber analy sis is improved with this method. Each of these individually determined can yield ambiguous information. The burden now rests on the envi ronmental and health scientists. They must answer the fundamental ques tion of just which fibers are harmful and need to be detected and analyzed. The analytical chemists have the tech niques available for asbestos analyses, although work needs to be done in re fining them and reducing the time and cost involved. The questions and problems raised by asbestos pollution will only be solved after much addi tional study and a great deal of inter disciplinary cooperation. Deborah C. Stewart
