This Technical Report describes the principle underlying evaluation of one or more health related aerosol fractions using an optical particle counter and details its limits and possibilities in the field of occupational hygiene. The method complements conventional long-term aerosol particle sampling and offers possibilities of: - instantaneous (direct reading) measurement; - time-related monitoring; - investigation of space-related aerosol evolution (mapping); - assessment of particle size distribution. The method enables e.g.: - detection and relative quantification of concentration peaks due to specific operations (bagging, sanding, etc.); - identification of most exposed workers with a view to more detailed studies of risks and prevention measures to be applied; - detection of dust emission sources and their relative magnitudes. Basically, OPCs count airborne particles and are therefore suitable for measuring concentrations expressed in number of particles per unit volume of air. The applicability of the method is limited by the particle size and concentration ranges of OPC instruments, usually approximately 10-1 µm to 101 µm and 100 particles/cm3 to 103 particles/cm3, respectively. Depending on specific conditions, the OPC method allows filter collection of an aerosol fraction, in the best case close to a health-related fraction (see EN 481), provided the OPC has the relevant sampling efficiency over its optical particle size range. If this is not the case, at least a sufficient aspiration efficiency is required to cover the size range of particles which can be detected and measured by the OPC optical system. Converting count-based particle number concentrations into mass concentrations based on estimated particle size is indirect and therefore the accuracy of the conversion is limited by several simplifying assumptions: - identical optical parameters for both the calibration aerosol and the measured workplace aerosol; - all counted particles of the workplace aerosol are spherical with a geometric diameter equal to the determined optical diameter and with identical density; - the aspiration and transmission efficiencies of the OPC are known or estimated from engineering models. Therefore confirmation of the estimated mass concentrations from OPC particle size distributions by a conventional sampling method is necessary (see ). The estimated mass concentrations from OPC data are only indicative and cannot be used for a direct comparison with a legally enforced occupational exposure limit.