Phase contrast microscopy asbestos fiber counting performance in the Proficiency Analytical Testing program

This report evaluates 20 years (1972-1992) of asbestos fiber count reporting for the Proficiency Analytical Testing (PAT) program, which is operated by the American Industrial Hygiene Association (AIHA) in cooperation with the National Institute for Occupational Safety and Health (NIOSH). Estimates were obtained for total, intracounter, and intercounter variability. Results show that total variability of counting chrysotile asbestos fibers improved by approximately 35% in recent years when compared with the variability found during 1975-1977, at the lowest filter fiber densities used in the PAT program. Total, intercounter, and intracounter variability for counting amosite and chrysotile asbestos fibers also were compared over a six-year period starting in 1986. PAT program laboratories achieved about one-quarter lower intracounter variability and about one-third lower total and intercounter variability when counting amosite fibers versus chrysotile fibers. In addition, amosite intercounter variability improved by about one-third, with large improvements occurring in the first year that amosite was included in the program. Factors affecting performance, such as changes in phase contrast microscope fiber counting methods, PAT participation, the AIHA Laboratory Accreditation Program, and PAT sample production, are discussed as possible factors affecting variability.