[en] This monitor has to permit detection in the environment, the α and β radioactivity of aerosols, and so among others of plutonium. Its name, the BFSAB, means that it is a monitor with a sequential filter to measure Alpha and Beta aerosols. It is shown that the study of a environment monitor to measure ininterruptedly the radioactivity of aerosols was a necessity. The sampler has been studied to obtain a sampling representative for any wind direction, and, for any velocity of which limit has been fixed at 10m/s. The aim fixed for this study is to sample at least the particles that may be inhaled. The lowest detectable radioactivity has been fixed, for α, lower than or equal to 1,3.10^-2Bq. At the artificial radioactivity which has to be detect, the natural radioactivity (radon and thoron daughters) has to be added; to eliminate it, the pseudo-coincidence technique is used; its principle is briefly recalled. The BFSAB monitor includes two units: the sampler (CFSAB) and the processing unit (UTSE). To simplify the presentation of the monitor, three subsystems are distinguished: the ventilation circuit, the filter changer, and, the detectors, their electronics and the associated processing