"National Institute for Occupational Safety and Health Method 7400 is commonly used to evaluate airborne exposures to man-made mineral fibers (MMMFs). Although recent evidence suggests that current methods for collecting air samples may be biased due to fiber deposition on the inside of the sampling-cassette extension cowl, few data are available on the mechanism of deposition, the number of fibers deposited on the cowl, or potential bias in size selection associated with this deposition. The length and diameter distributions of fiber populations collected on the air-sample filters were compared to those deposited on the inside of the electrically conductive extension cowl. The samples were collected during insulation of residential buildings with pneumatically conveyed loose-fill insulation. Two different filters recommended for sample collection in the World Health Organization Reference Method for MMMFs were used. It was determined that a significantly different fiber-size distribution is derived from analysis of fibers collected on the air-sample filter versus fibers deposited on the extension cowl. In general, fibers deposited on the inside of the extension cowl were longer and thicker than fibers from the air-sample collection filter."

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