Comparison of Niosh 7400 “B” Rules and Who Reference Methods for the Evaluation of Airborne Man-Made Mineral Fibres

The purpose of this paper is to compare fibre count estimates for laboratories using the United States (U.S.) National Institute for Occupational Safety and Health (NIOSH) Method 7400 ‘B’ Rules and the World Health Organization (WHO) MRMF Reference Method. These two counting methods were compared by exchanging microscope slides prepared from airborne MMRF samples used for U.S. and European quality control programmes. Five U.S. and five European laboratories experienced with counting MMMF samples participated in the exchange. Fourteen different microscope slides were analysed once by each laboratory; U.S. laboratories analysed the slides by counting fibres according to NIOSH Method 7400 ‘B’ Rules and European laboratories counted using the WHO Reference Method. For all 14 samples, the five laboratories using NIOSH Method 7400 ‘B’ Rules produced lower mean counts than laboratories using the WHO Reference Method; these differences were statistically significant (P < 0.025). Additional data are needed to determine if the differences observed are caused by differences in the methods or by subjective interpretation by the counters.