Abstract A comparative study of four commonly used methods for measuring particulate matter in ambient air, viz. the OECD (smokeshade), the AISI tape sampler, the β-attenuation, and the LIB Hi-vol method, is reported. Parallel measurements have been made in two Danish cities. Copenhagen and Aalborg. In the latter, three cement plants are in operation. Linear regression analyses of the results have given a good correlation between the optical methods (OECD and AISI). a fair correlation between the gravimetric methods (Hi-vol and β-attenuation), but only a poor correlation between the optical and gravimetric methods. None of the regression equations derived are of a universal nature, probably because the composition of the aerosol is of great importance. It is concluded that only Hi-vol samplers can provide a reasonably correct measurement of suspended particles in ambient air, that it is of vital importance to agree on an international standard method, and that the OECD-method due to its arbitrary nature is useful only for determining long term changes in dark smoke.