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 Several field tests have indicated that the fibrous aerosol monitor agrees reasonably well with field measurements of fibers by phase contrast microscopy, though mostly at concentrations above ambient levels. It has been used at abatement sites to provide rapid feedback and ensure acceptable containment of airborne fibers during asbestos removal. 9.

FIBER CLASSIFICATION Several devices have been used to measure or separate fibers by diameter. A spiral centrifuge was used to separate fibers and reference spherical particles to estimate fiber aerodynamic diameter [81]. It was found that the aerodynamic diameter was directly proportional to physical diameter, proportional to the square root of the fiber density, and proportional to fiber length to the 1/6th power. For mineral fibers having a density of about 3 g/cm3, the aerodynamic diameter was approximately three to five times the physical diameter of the fiber. Behavior of glass fibers in a cascade impactor was investigated by Burke and Esmen [82]. A small correction to the aerodynamic diameter was developed to take into account interception of longer fibers with the impaction surface. An inertial spectrometer was used to measure fiber aerodynamic diameter and good diameter separation was achieved [83]. Baron and Deye developed a technique for separating fibers by length using dielectrophoresis [84, 85]. This technique was also shown to be useful for measuring fiber length and diameter distributions [86]. As with most airborne dusts, fiber settling will reduce the number of larger diameter fibers in a distribution as the distance from the source of the dust increases. Esmen et al. showed that average fiber concentration in workplaces decreased exponentially with an increase of fiber diameter, indicating that the larger diameter fibers settled out more quickly than smaller diameter fibers [87]. Cyclones, impactors and porous foam classifiers were evaluated for efficiency of removing airborne fibers not likely to deposit in the lungs [88]. The aerodynamic diameter of fibers is dependent primarily on fiber physical diameter and fiber density, with a minor dependence on fiber length [89]. The diseases caused by asbestos fibers are lung diseases and so it makes sense to measure only fibers that can enter the lungs, i.e., thoracic fibers. Identical conventions for thoracic samplers have been published by ISO, ACGIH [90], and CEN. Baron [89] showed that sampling fibers with a thoracic sampler was approximately equivalent to counting only mineral fibers with a physical diameter smaller than 3 :m. Jones et al. [91] reported that there appeared to be no impediment to using a thoracic sampler for fiber sampling; they found that several samplers matched the thoracic convention, the sample collected by these samplers could be analyzed by standard methods, and that field studies indicated equivalence to the current method. Maynard [92] also found that there appeared to be no variation in penetration through these samplers as a function of fiber length. The advantage to using a thoracic sampler, apart from adhering to conventional sampling practice, is that it would remove larger compact particles and fibers from the sample and result in a cleaner sample. Although current US practice does not use an upper diameter limit for asbestos fibers, such a limit is commonly used for man made fibers. Except for the United States, all national and international organization methods use an upper diameter limit of 3 :m for fiber counting of asbestos fibers.

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