A study of employee exposures to airborne fibers during production of fibrous glass was conducted to compare fiber exposures at the facilities of Owens-Corning Fiberglass (OCF) Corporation (SIC-2221) using the NIOSH-P/CAM-239 (NIOSH-239) and NIOSH-7400 methods, employing the A-counting rules. The NIOSH-239 method was the standard method for determining airborne concentrations of asbestos (1332214), fiber glass, and other manmade mineral fibers up to 1986. The NIOSH-7400 method included a 50 millimeter (mm) extension cowl in the sampling train along with the 25mm cassette used in the NIOSH- 239 method. Seventy five paired personal and area samples were collected at specific sites along production lines at OCF facilities and were analyzed for total fibers by the two methods. Mean total fiber exposures for the forehearth, line, packer, bagger, rollup, repack cubed, and repack milled operations obtained by the NIOSH-239 method were 0.017, 0.003, 0.028, 0.023, 0.021, 0.024, and 0.040 fibers per cubic centimeter (f/cc), respectively. The corresponding values determined by NIOSH-7400 were 0.025, 0.006, 0.036, 0.030, fiber exposures measured by the NIOSH-239 and NIOSH-7400 methods were 0.024 and 0.03f/cc, respectively. None of the differences were statistically significant. Additional analyses indicated that 70 to 75% of the fibers were fibrous glass. Approximately 60% were of respirable size having diameters less than 3.5 microns (micron) and lengths of 5 to 250micron. The ratios of fibers deposited on the cowls plus filters to fibers deposited on the filters were determined to be 1.7 for the NIOSH-239 method and 1.5 for NIOSH- 7400. The authors conclude that a significant proportion of the fibers is deposited on the cowls used in the sampling trains. This should be considered when assessing total fiber exposures.