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 Often the discussion of fibers assumes that fibers are straight objects that can be well defined by several parameters as indicated above. However, many real-world particles are not so simple to describe. In fact, the detailed features of many fibers can aid in their identification [12]. Fibers are often curved, have splayed ends, or differ in other ways from a cylindrical shape. Asbestos mineral is composed of fibrils (about 0.03-:m diameter) that are packed together. This fibrillar structure is characteristic of asbestiform minerals. When the mineral is broken apart mechanically, the material separates primarily between fibrils and the resulting fibers are usually bundles of fibrils. The ends of the fibers can be broken apart, with smaller bundles or individual fibrils spread apart, yet still be part of the fiber. Fibers can be contaminated by attachment of other dust particles, creating a complex structure with aerodynamic behavior not matching that of cylindrical fibers. The complexity of fiber shapes affects all of the measurement and separation techniques described below and frequently makes it difficult to compare one method to another. In addition to asbestos fibers, there are many types of fibrous materials being produced for commercial purposes. These include fibrous glass, mineral wool, refractory ceramic fibers, wood and other plant fibers, and synthetic organic fibers. Most of these fibers tend to have larger diameters than asbestos fibers. On the other hand, carbon nanotubes (<0.005-:m diameter) have recently been produced in small-scale commercial quantities and because of their high tensile strength, high conductivity, and other special properties, show great promise as a commercial material [13]. Measurement techniques must be tailored to the size distribution and physicochemical properties of the fibers. This review primarily relates to measurement of fibers in air. There are a several techniques that address concentration of asbestos and other fibers in bulk material and measurement of mass concentration of fibers [14]. One of these bulk methods, polarizing light microscopy, will be discussed below. 3.

PHASE CONTRASTING LIGHT MICROSCOPE COUNTING (PCM) As asbestos-induced disease became widely studied in the 1960s, cellulose-based membrane filter sampling was applied to asbestos sampling in combination with high magnification phase contrast light microscopy (PCM) for counting fibers. This technique involved collection of fibers uniformly over the surface of a cellulose ester filter, placing the filter or a segment of the filter on a microscope slide and making it transparent, and observing the fibers in the sample with a high magnification (~450X) phase contrast light microscope. Over the years, many researchers have endeavored to improve and standardize the PCM method. One researcher, Walton, discussed many aspects of this technique in a review [15]. The high variability of the analysis results and the method’s dependence on operator technique made method improvement and research difficult. The PCM method does not measure all fibers; typically only those >0.25-:m diameter are visible and counted and only those >5 :m length are counted by protocol. Therefore, the PCM method is only an index of exposure and uses the assumption that what is detected is correlated with the fibers actually causing disease (Figure 2). The PCM method does not allow identification of asbestos fibers. This is an important limitation when the method is used in settings where fiber concentrations with a significant non-asbestos fraction may occur. This should be remembered when considering some of the parameters discussed below. The aim of evaluating changes to the PCM technique may depend on whether consistency with other

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