Authors: Mizan G

Source: Occupational Health Southern Africa 2009, 15(1), 16-19


Phase contrast microscopy (PCM) is an internationally accepted method for the evaluation of occupational exposure to asbestos fibres present in breathed air. Despite the widespread use of this method, which is based on counting fibres 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 Organisation methodology for asbestos fibre 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 fibre 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.