Calibration standards for counting asbestos

A laboratory proficiency testing program has been initiated by NIOSH to permit standardization of asbestos counting procedures by various state agencies. This has required preparation of multiple membrane filter samples containing "identical and predictable" concentrations of asbestos fibers, with consistent particulate backgrounds. A technique to filter liquid (toluene) suspensions was developed to provide chrysotile asbestos standards ranging from 200 to 1,500 fibers/mm-2 of filter area, with an aluminum oxide particulate background. Details of the necessary techniques are described. To evaluate variations inherent in preparing these "identical" samples and to estimate variations between different counters, extensive replicate fiber counting was performed. Count data from several counters at a single facility show that this liquid suspension filtration technique provides reproducible asbestos standards as measured by optical microscopy with a coefficient of variation of plus or minus 20% with the maximum individual variations of plus or minus 50%.