APPLICATION OF THE THORACIC SAMPLING DEFINITION TO FIBER MEASUREMENT

As part of a consideration of the sampling method for refractory ceram ic fibers, calculations were carried out at the National Institute for Occupational Safety and Health to evaluate different approaches to fi ber measurement. The most common technique for estimating fibers that can reach the lungs is to use an upper diameter limit of 3 Cim in the phase contrast optical microscope counting rules. Calculations were ca rried out to estimate the aerodynamic diameter of fibers in several lo gnormal size distributions likely to occur in workplaces. Using these size distributions, the use of a 3 mu m fiber diameter upper limit in the counting rules was compared with results expected from a sampler d esigned to collect fibers according to the thoracic definition, which is based on the aerodynamic diameter of compact particles. The other l imits in the optical counting procedure, i.e., counting only fibers lo nger than 5 mu m and thicker than 0.25 mu m, were included in the calc ulations. The calculations indicate that the 3 mu m upper diameter cou nting rule agrees with the thoracic definition within about +/- 25% fo r a wide range of possible fiber size distributions. The advantages of using a sampler designed to coiled the thoracic fiber size fraction i nclude reducing analyst decision making (all fibers collected would be counted) and reducing the nonthoracic particles on the sample, making the sample easier to analyze. Until thoracic samplers are available f or fibrous aerosols, incorporating the 3 mu m upper diameter limit as part of the criteria for counting fibers may serve as a surrogate for thoracic sampling.