Background-Airborne fiber size has been shown to be an important factor relative to adverse lung effects of asbestos and suggested in animal studies of carbon nanotubes and nanofibers (CNT/CNF). Materials and Methods-The International Standards Organization (ISO) transmission electron microscopy (TEM) method for asbestos was modified to increase the statistical precision of fiber size determinations, improve efficiency, and reduce analysis costs. Comparisons of the fiber size distributions and

more »

... xposure indices by laboratory and counting method were performed. Results-No significant differences in size distributions by the ISO and modified ISO methods were observed. Small but statistically-significant inter-lab differences in the proportion of fibers in some size bins were found, but these differences had little impact on the summary exposure indices. The modified ISO method produced slightly more precise estimates of the long fiber fraction (>15 μm). Conclusions- The modified ISO method may be useful for estimating size-specific structure exposures, including CNT/CNF, for risk assessment research. 2. In order to accommodate sizing of the large number of fibers needed, diameter and length were recorded into discrete interval categories (rather than precise measurements of each fiber dimension) to increase the efficiency and reduce the cost of the analyses. 3. The ISO stopping rule for dispersed matrices and dispersed clusters was not used and all visible fibers and fiber bundles within these structures were enumerated and sized. This enhancement allowed better resolution of the true diameter/length distribution. Even with this modification, complete enumeration of fibers and fiber bundles within these complex structures is sometimes impossible. 4. The ISO method employs stopping rules of 100 primary structures in the all-sizes count and 100 structures in the PCM-equivalent count (>5 μm in length, >0.25 μm in diameter). In order to increase the count of the less-prevalent longer fibers (>15 μm) and thereby achieve greater statistical precision of the bivariate size distributions, three separate analyses were performed on each sample based on Dement et al.