Because it resists acids, is noncombustible, and can be woven into fabrics, asbestos was commonly used as fireproofing and insulation in many build ings built before the mid-1970s as well as in brake linings, heat-proof gloves, and other commercial products. Con cern about adverse effects on health from exposure to asbestos originally centered on miners, insulation workers, and others who were exposed to large amounts of asbestos in their jobs. But recent studies indicate that even low levels of airborne asbestos may cause cancer, and concern has mounted over the effects on the general public of ex posure to asbestos in the environment. Because most of the methods that were developed for the analysis of bulk as bestos samples are not appropriate for the analysis of air samples, new meth ods capable of detecting small amounts of asbestos in ambient air have been developed. Asbestos is a generic name used to
FOCUS describe a variety of hydrated silicate materials that can, under certain con ditions, crystallize in bundles of fibrils that look like organic fibers. Although there are several kinds of asbestos, most of the asbestos used in the United States is chrysotile (or white) asbestos. The average asbestos fiber is ~100 μνα long and 100 nm wide when mined. It is then manufactured into fabrics and boards and used in a variety of com mercial applications. Asbestos can get into the air from many sources, includ ing natural rock, mining and manufac turing operations, brake linings of cars, and aging or damaged insulation. Its widespread use over the past 100 years has caused asbestos to be generally present in the atmosphere. Microscopic methods, including phase-contrast microscopy (PCM), po
larized light microscopy (PLM), trans mission electron microscopy (TEM), scanning electron microscopy (SEM), and analytical electron microscopy (AEM), are the most common means of determining asbestos in air samples. Other methods that can be used for the analysis of bulk asbestos samples, such as infrared spectrometry (IR) and X-ray diffraction (XRD), cannot be used for air samples because they do not have sufficient sensitivity to see the low levels of asbestos present in air samples. Optical microscopy The first two microscopic methods, PCM and PLM, are optical or light mi croscopic methods. In the PCM meth od, differences in refractive index be tween an object and its surrounding medium are made visible in the form of a black-and-white image as a result of differences in phase of some of the light rays passing through the system. For many years PCM was the standard method for determining airborne as bestos. But PCM has two limitations. First, it cannot distinguish asbestos from other types of fibers (such as glass, cellulose, or carbon)—all fibers are counted as asbestos. In addition, PCM does not have sufficient resolu tion to see asbestos fibers that are less than ~0.25 μπι in diameter, and many asbestos fibrils found in air samples are only 0.03-0.04 μηι in diameter. Thus for chrysotile asbestos, only those as bestos fibers found in bundles can be seen by PCM. Despite these limita tions, PCM can be a useful technique for an asbestos mill or plant, where it is reasonable to assume that most of the fibers in the sample are asbestos and bundles of asbestos fibers will be com mon. For example, PCM is the method currently recommended by OSHA and NIOSH for monitoring occupational exposure to asbestos and for evaluating asbestos abatement efforts. PLM also uses refractive index dif-
can be used to determine asbestos in ambient air ferences to visualize the asbestos fi bers, but it differs from PCM in that these refractive index differences are used to distinguish between different types of asbestos rather than between the fibers and the background. Optical properties, including refractive index, birefringence, morphology, extinction angle, and sign of elongation, are mea sured to distinguish the fiber types. Like PCM, PLM can only detect fairly large asbestos fibers and thus is not particularly useful for the analysis of ambient air samples. It is used, howev er, for determination of asbestos in bulk samples. Electron microscopy The greater magnification and resolu tion of electron microscopes compared with optical microscopes allows smaller asbestos fibers to be detected. Three types of electron microscopy are used to determine asbestos fibers in ambient air samples: TEM, SEM, and a combi nation of the two, known as AEM. In TEM, a beam of electrons is passed through the sample, where some of the electrons are diffracted, to a detector. The pattern of electron scattering generates an image in much the same way that the image in a light microscope is formed as a result of the reduction of light intensity by the ob ject. Because TEM normally includes electron diffraction capability, which
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FOCUS can provide the crystallographic structure of a fiber, it can distinguish asbestos from other types of fibers. It can also distinguish chrysotile asbestos from other forms of asbestos. In SEM, the electron beam is collimated into a small (100 À) spot, and the spot is swept across the sample surface, which has been coated with a thin layer of gold or other conductive substance. As the beam impinges on the surface and penetrates into it a short distance, electrons are emitted either as secondary emissions or as directly backscattered electrons. The image is then formed from the collected electrons. SEM is usually used in combination with an energy-dispersive X-ray spectroscopic detection system (EDXS), which can qualitatively and semiquantitatively identify the elemental content of a fiber. An AEM has both the high-resolution and high-contrast imaging capabilities of a TEM and the chemical analysis capabilities of an S E M / EDXS. The TEM imaging mode of the AEM is used to find the asbestos fibers, and the TEM electron diffraction capabilities and the SEM X-ray spectrometry capabilities are used to identify the fibers. Chrysotile asbestos can be identified using electron diffraction, whereas X-ray analysis generally is necessary to identify other types of asbestos. Asbestos analysis with TEM or AEM is performed at 20.000X magnification; a PCM analysis is performed at a magnification of 500 X. Although it takes longer to analyze an entire sample by TEM or AEM, these methods may often see ~10 times the numbers of chrysotile asbestos fibers in a given sample. Put another way, PCM will only see ~10-20% of the asbestos fibers seen with a TEM or AEM. SEM lies somewhere between PCM and AEM in capability but is not recommended for asbestos determination in air samples because it has not been characterized as completely as PCM or AEM and researchers do not know its limitations. Sampling and analysis Although all the methods use essentially the same sample collection techniques, sample preparation after collection varies according to the method used. For sample collection, a known volume of air is drawn through a cellulose acetate or perforated polycarbonate membrane filter, which is then analyzed by either optical or electron microscopy. Because it isn't feasible to evaluate the complete sampling area of the filter, a random selection of fields of view is evaluated and the rest of the
sample is assumed to have an even distribution. The measured concentration of the original air sample is calculated from the total number of fibers estimated to be on the filter and the amount of air drawn through the filter. For light microscopy, a section of the filter is cut and put in an acetone vapor bath, which causes the filter to become transparent. It can then be observed under the microscope. The procedure is a bit more complicated for electron microscopy. For SEM, the surface of the filter normally is coated with an electrical conducting material such as gold, silver, carbon, or silicon monoxide and then viewed under the microscope. Because the electron beam actually passes through the
The trend in airborne asbestos monitoring methodology is primarily toward TEM or AEM... 4 * sample, TEM analysis requires that the matrix be nearly transparent to electrons. This involves making a carbon film replica of the filter sample, which is done by coating the filter with carbon and then gently washing the filter away, leaving a replica of the filter on a special grid. Although this procedure is not particularly labor intensive, it does require quite a bit of time— anywhere from 4 to 48 h. Future trends Although asbestos analysis is an area in which existing methodology is primarily being used and refined, some work is being done on new methodology. Fred Adams and his colleagues at the University of Antwerp are investigating the use of LAMMA (laser microprobe mass analysis) to detect chemical impurities on the asbestos fiber surface. It has been postulated that the toxicity of asbestos fibers may result not only from the fibers acting as an irritant in the lungs but also from the asbestos carrying carcinogenic compounds to the lungs. Carcinogenic organic pollutants or their precursors may adsorb onto the surface of the asbestos fibers; they could then be transported to the lung and deposited in the tissue. Adams's group has used LAMMA, in which laser desorption is used to obtain mass spectra of the sample, to detect trace quantities of benzi-
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dine, JV,iV-dimethylaniline, and benzo[a]pyrene on doped asbestos fibers as well as to determine phthalate contamination resulting from polyethylene packing material. Detection limits are better than 500 Mg/g for the phthalate experiments with reproducibility of -20%. Adams and his co-workers admit that more work needs to be done before LAMMA can be considered a routine method for the determination of asbestos in air samples. And although the use of LAMMA for detection of impurities is intriguing, it will probably not gain wide acceptance for routine asbestos analysis for two reasons. First, there are very few LAMMA instruments, and second, like PCM, LAMMA has an optical resolution limit of ~0.25 μπι, so it cannot see all of the asbestos fibers present in a particular sample. Never theless, it may prove useful in correlat ing carcinogenic impurities with asbes tos toxicity. The trend in airborne asbestos moni toring methodology is primarily toward TEM or AEM, in part because they provide superior resolution and in part because these are the methods recom mended by the U.S. Environmental Protection Agency. New regulations re quire accreditation of asbestos-testing laboratories, and the National Bureau of Standards (NBS) will add two new accreditation programs within the next two years. In October 1988, NBS will start an accreditation program for the use of PLM for bulk sample analysis in response to passage of the Asbestos Hazard Emergency Response Act last October. In October 1989, NBS will be gin a lab accreditation program for the use of TEM in the determination of airborne asbestos. These programs will join one that is already in operation by the American Industrial Hygiene Asso ciation for the use of PCM in asbestos analysis. Mary Warner Suggested reading Beckett, S. T. In Asbestos: Properties, Ap plications, and Hazards; Michaels, L.; Chissick, S. S., Eds.; John Wiley & Sons: New York, 1980. De Waele, J. K.; Vansant, E. F.; Van Epsen, P.; Adams, F. C. Anal. Chem. 1983, 55, 671-7. De Waele, J. K.; Gybels, J. J.; Vansant, E. F.; Adams, F. C. Anal. Chem. 1983,55, 225560. Proceedings of Workshop on Asbestos: Def initions and Measurement Methods; Gravett, C. C; LaFleur, P. D.; Heinrich, K.F.J., Eds.; National Bureau of Stan dards: Gaithersburg, Md., 1978. Small, J. Α.; Steel, E. B.; Sheridan, P. J. Anal. Chem. 1985, 57, 204-8. Steel, E. B.; Small, J. A. Anal. Chem. 1985, 57, 209-13. Zurer, P. S. Chem. Eng. News, March 4, 1985, pp. 28-41.
