Implications of the introduction of a replacement method for asbestos fibre counting in South Africa

Phase contrast microscopy (PCM) is an internationally accepted method for the evaluation of occupational exposure to asbestos fi bres present in breathed air. Despite the widespread use of this method, which is based on counting fi bres under a microscope, it has some inherent limitations which may impact on counting results and consequently affect exposure evaluations. An asbestos guide (Health and Safety Guidance 248) that has been published by the Health and Safety Executive (HSE) in the UK during 2005, has suggested several changes to the previous method (Methods for the Determination of Hazardous Substances 39/4), to align it with the World Health Organization’s methodology for asbestos fi bre counting (WHO, 1997). This article explains the main differences between the MDHS 39/4 and the WHO / HSG248 methods and outlines the possible implications on fi bre counting. Since the MDHS 39/4 is still prescribed by South African legislation the article also motivates the move towards the adoption of the more recent method. 1960s3 and involves the collection of an air sample on a cellulose membrane fi lter, followed by a count of the fi bres in randomly selected areas on the fi lter using a phase contrast microscope (Figure 1). The count is then adjusted for the total exposed area of the fi lter and the number of fi bres per millilitre of air sampled is calculated. The MDHS 39/4 evolved from the ‘Recommended technical method number 1(AIA-RTM1): Airborne asbestos fi bre concentrations at workplaces by light microscopy (membrane fi lter method)’, which was developed by the Asbestos International Association (AIA) and was incorporated into a European Directive on asbestos in 1983.4 The revised MDHS 39/4 was published by the UK Health and Safety Executive (HSE) in 1995.2 In an attempt to establish a unifi ed and more accurate methodology for the evaluation of airborne asbestos fi bres in the work environment, the World Health Organization (WHO) published the ‘Determination of airborne fi bre number concentration, a recommended method by phase contrast microscopy’ in 1997.5 An example of this method is shown in Figure 2. The WHO fi bre counting rules came into effect in the UK in November 2006. These rules are described in the Health and Safety Guidance 248: ‘Asbestos: The analysts’ guide for sampling, analysis and clearance procedures’ (HSG248, HSE 2005),6 which replaced the MDHS 39/4. In South Africa, however, Gabriel E Mizan, Occupational Hygienist, National Institute for Occupational Health (NIOH), +27 (0)11 712 6457 Cell: +27 (0)82 776 9686 E-mail: gaby.mizan@ nioh.nhls.ac.za PEER REVIEWED Asbestos fi bre with dust particle attached as seen with a phase contrast microscope (NIOH lab picture)