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 Thus, longer fibers are more likely to remain in the lungs for an extended period of time. The macrophages die in the process of trying to engulf the fibers and release inflammatory cytokines and other chemicals into the lungs [3]. This and other cellular interactions with the fibers appear to trigger the collagen buildup in the lungs known as fibrosis or asbestosis and, over a longer period, produce cancer as well. Fiber diameter is also important because fiber aerodynamic behavior indicates that only small diameter fibers are likely to reach into and deposit in the airways of the lungs. The smaller the fiber diameter, the greater its likelihood of reaching the gas exchange regions. Finally, fibers that dissolve in lung fluid in a matter of weeks or months, such as certain glass fibers, appear to be somewhat less toxic than more insoluble fibers. The surface properties of fibers are also thought to have an effect on toxicity. Asbestos is one of the most widely studied toxic materials and there have been many symposia dedicated to and reviews of its behavior in humans and animals [4-8]. Several techniques were used for asbestos measurement up until the late 1960s [8]. Earlier than this, it was not widely recognized that the fibrous nature of asbestos was intimately related to its toxicity, so many techniques involved collection of airborne particles and counting all large particles at low magnification by optical microscopy. Thermal precipitators, impactors (konimeters), impingers, and electrostatic precipitators were all used to sample asbestos. Perhaps the primary technique in the United States (US) and the United Kingdom (UK) during this early period was the liquid impinger, in which particles of dust larger than about 1-:m aerodynamic diameter were sampled at 2.7 L/min and impacted into a liquid reservoir [8]. After sampling, an aliquot of the liquid was placed on a slide in a special cell, particles larger than 5-:m size were counted, and the results were reported in millions of particles per cubic foot. Dissatisfaction with this approach stemmed from lack of correlation between measured particle concentration and disease in the workplace. Various indices of exposure have been developed that attempt to relate a portion of the fiber size distribution to the toxic effects. The appropriate indices for each of the asbestos related diseases as a function of fiber length and diameter (Figure 2) were suggested by Lippmann [9]. 2.

FIBER DIMENSIONS Fibers are particles that have one dimension significantly larger than the other two. Fibers are often characterized or selected according to their aspect ratio, i.e., the ratio of the large dimension to one of the small dimensions. If no other criteria are used, then materials that might not normally be considered fibrous may contain a fraction of particles that meet the criteria for fibers. The distribution of fiber dimensions in a sample can usually be characterized by assuming a cylindrical geometry (i.e., the two small dimensions are identical) and measuring the length and diameter of individual fibers. The distribution of airborne fiber sizes generated by grinding bulk material or by mechanically releasing particles into the air often results in a two-dimensional (bivariate) lognormal distribution. Such a distribution is characterized by five parameters: the geometric mean length, the geometric mean diameter, the length and diameter geometric standard deviations, and a correlation term that relates length to diameter [10]. In addition, several other parameters that are a function of length and diameter, such as aerodynamic diameter, can also be characterized by a lognormal distribution [11].

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NIOSH Manual of Analytical Methods