This International Standard is applicable to transpo rtable or installed equipment for continuous monitoring of radioactive aerosol in the environment for both normal and accident conditions. For the purpose of this standard, monitoring includes the continuous sample collection with, if desired, the capability to automatically initiate sampling. In particular the standard is applicable to equipments designed to fulfil the following functions: - The determination of the activity per unit volume of radionuclides in the form of aerosols, either per unit time together with the variations as a function of time, or integrated over a longer time period, e.g. 24 h and measurement of the volume sampled. - The actuation of an alarm signal when either a predetermined high concentration or a predetermined time integrated concentration of an aerosol activity is exceeded. The aerosol monitoring can be made both by continuous measurements during sampling and by measurement after collection of the sample. In the measurement after collection, the sample is removed from the air sampler and analysed in a laboratory. This procedure of measurement has to be followed in special cases such as the activity assessment of specific radionuclides and is not within the scope of this standard. The continuous method of measurement is the more widely used and consists in collecting the radioactive samples by drawing air through a filter and measuring the aerosol activity accumulated on it with a detector which is close to the filter. The filters are suitably chosen and periodically changed according to the conditions of use. In some cases automatic filter sequences are used to avoid the build-up of radioactivity or excess of dust on the filter. Also the type of detectors depends on the various conditions such as the type of radiation, its energy and the level of activity. The discrimination against natural radioactivity (such as airborne radon and thoron daughters) can be an impo rtant problem in monitoring low-level radioactivity in air especially with transuranic elements.