Levels of workplace exposure.

A brief description was presented of the evolution of workplace exposure monitoring. Measurements began in the 1930s and developed through particle counting to the introduction of the membrane filter, light microscope method in the mid-1960s. It is important to be aware that most data from the past were collected in relation to workplace control. The membrane filter phase contrast microscope method provides an index of exposure. Fibres with diameters less than 0.2 microns are unlikely to be counted. The method is not capable of analysing individual fibre types and consequently all fibres are counted. There have been several changes to the detail of the membrane filter method. The possible effect of these changes were presented. The Asbestos International Association (AIA) has implemented a fibre counting performance trial and also an annual return of workplace monitoring data. A WHO project is in progress with the objective of harmonizing the membrane filter method for fibre counting of all fibre types using phase contrast microscopy.