This International Standard specifies a standard reference method for the determination of PM10 and PM2,5 mass concentrations at stationary emission sources by use of two-stage impactors. The measurement method is especially suitable for measurements of mass concentrations below 40 mg/m3 as half-hourly averages in standard conditions (273 K, 1 013 hPa, dry gas). It is an acceptable method for the measurement in the flue gas of different installations, such as cement and steel production plants, as well as combustion processes. This International Standard is not applicable to the sampling of flue gases that are saturated with water vapour.This International Standard is not applicable where the majority of the particles are likely to exceed PM10, for example, in the case of raw gases or plant operating failures. NOTE 1 Measurements of particulate concentrations higher than 40 mg/m3, as a half-hourly average in standard conditions (273 K, 1 013 hPa, dry gas), can lead to overloading of the collecting plates and backup filters and also could result in shorter sampling times. NOTE 2 The collecting plates and backup filters can be used for further chemical analysis. This International Standard cannot be used for the determination of the total mass concentration of dust. NOTE 3 For data assessment purposes, it can be useful to perform measurements of total particulate matter in parallel to the PM10 and PM2,5 measurements. This International Standard describes the design, use and theory of round-nozzle impactors. It does not exclude other types of impactors, provided these systems meet the performance criteria specified in this International Standard in a validation of the impactor performed by an independent testing laboratory.