[Inter-laboratory comparison of measuring concentration of asbestos fibers less than 5 microM in length using...

Airborne asbestos fibers at the workplace are usually measured by phase-contrast microscopy (PCM), using the membrane filter method (MFM). Although the MFM has an international standardization, the application of this method can lead to wide differences in results. An inter-laboratory study, involving 19 laboratories, was organized. Two fiber classes were counted: 1) fibers greater than 5 microns in length and less than 3 microns in diameter; 2) fibers less than 5 microns in length and less than 3 microns in diameter. The results of each laboratory were normalised to the mean and their counting performance was compared. The estimated coefficients of variation or relative standard deviation (CV) of the normalised results were fitted assuming a constant component (K) and a component depending on the fiber count (Poisson component: mu-1/2). Fitting showed that the constant K is about 0.36 for counting fibers greater than 5 microns in length and about 0.52 for counting fibers less than 5 microns long. It was shown that counting the latter fiber class is influenced primarily by subjective laboratory-to-laboratory differences.