A system and method for automated analysis of a filter obtained from an air quality monitoring apparatus used for sampling airborne respirable particles such as asbestos fibres, synthetic mineral fibres, pollen or mould particles is described. The system comprises capturing images at a plurality of sample locations. At least one magnified phase contrast image is obtained at each sample location. An automated quality assessment is then performed using a computer vision method to assess one or more quality criteria. Failure may lead to the sample location being ignored for subsequent analysis, or the whole filter slide may be rejected if the overall quality is poor. The quality assessment may performed be in two stages comprising an overall filter quality assessment performed on a series of low power/magnification images captured over the filter and a field of view or graticule level quality assessment performed on high power/magnification images captured at individual sample locations on the filter. Images which pass the quality assessment are then analysed using a computer vision method to identify and count the number of respirable particles.