Interwedge variation in the membrane filter method for airborne asbestos fibers

The poor precision of the current asbestos method (NIOSH P & CAM 139) has been linked to the variation in fiber deposition on the filter, but the cause of this variation had been unknown. A fluidized bed generator was constructed to produce a test atmosphere containing asbestos fibers, and air samples were collected. Fibers were counted at filter positions corresponding to the trench pattern in the cassette directly below the filter. The average fiber counts on wedges taken from above radial trenches were significantly higher (Pr greater than F = 0.0001) than counts on wedges taken from other portions of the filters. The chief cause of variation in asbestos fiber deposition on the filter is the trench pattern in the cassette supporting the filter.