This European Standard provides guidelines for the measurement of the fraction of aerosols, especially for the determination of the numerical concentration of ultrafine aerosols and nanoaerosols at workplaces using condensation particle counters (CPC). This European Standard describes the CPC principle of operation, problems with air sampling in the working environment, guidance on selecting the correct instrument, application limitations, the use of various working fluids and technologies, calibration, operation of measuring equipment, as well as measurement uncertainty and presentation of measurement results. Possible problems and limitations that should be taken into account when taking measurements at workplaces have been indicated.