Within-laboratory quality control of asbestos counting

Internal quality control schemes for asbestos counting are presented for four laboratories handling different sample types and with different organizations and facilities. The purpose is to present these not as perfect models, but as sources of ideas and experience which other laboratories can use in order to plan schemes best suited to their own needs. Common problems which emerge are the determination of reference counts for low-density slides and the handling of the density dependence of counter performance on such slides. One solution is a stock of permanently mounted reference slides, slowly replaced by random selection from routine slides. The reference result can be determined by multiple counting over a period, and the density dependence can be allowed for by using Ogden's method [Ogden (1982) Health and Safety Executive Research Paper, No. 18. ISBN 0 7176 0101 3] of expressing deviations as multiples of a calculated 'expected standard deviation'. A link to the national mean can be provided through the Regular Inter-laboratory Counting Exchange (RICE) inter-laboratory quality control scheme.