Device for the isokinetic extraction of a sample from a flowing aerosol, comprising a sample collector with an inlet opening or with an intake piece which has an inlet opening and connects the inlet opening to the sample collector in a gas-conducting manner, a vacuum pump connected to the sample collector and via the sample collector or the sample collector and the intake piece to the inlet opening in a gas-conducting manner, a flow sensor for detecting a flow velocity of the flowing aerosol, a flow meter for detecting the flow velocity in the device which flow meter is arranged in a flow channel within the device and a control unit for isokinetic control of an aerosol flow through the inlet opening as a function of the flow velocity respectively detected by the flow sensor and by the flow meter and transmitted to the control unit, characterised in that in that the flow sensor is a flow sensor for detecting a flow velocity of the flowing aerosol in the region of the inlet opening and is arranged at or in the inlet opening or at a distance of at most 7.5 cm from an edge of the inlet opening, wherein an evaluation unit for calculating a particle concentration or activity concentration in the aerosol on the basis of - a period of sample collection in the sample collector, - a detected number or a detected radioactivity of particles collected in the sample collector during this period, and - the flow velocity detected by the flow sensor and transmitted to the evaluation unit in combination with a cross-sectional area of the inlet opening or the flow velocity detected by the flow meter and transmitted to the evaluation unit in combination with a cross-sectional area of the flow channel at the location of the flow meter is provided, wherein at least a probe for detecting the direction of flow of the aerosol is arranged at or in the inlet opening or at a distance of at most 7.5 cm from the edge of the inlet opening in order to enable a simulation of an isokinetic extraction of the sample in the case of an oblique or swirled flow towards the inlet opening by the flowing aerosol, wherein the device comprises a carrier system with a movable bearing to be arranged within the flowing aerosol, wherein the sample collector with the inlet opening or with the intake piece with the inlet opening is held by the bearing. - a period of sample collection in the sample collector, - a detected number or a detected radioactivity of particles collected in the sample collector during this period, and - the flow velocity detected by the flow sensor and transmitted to the evaluation unit in combination with a cross-sectional area of the inlet opening or the flow velocity detected by the flow meter and transmitted to the evaluation unit in combination with a cross-sectional area of the flow channel at the location of the flow meter is provided, wherein at least a probe for detecting the direction of flow of the aerosol is arranged at or in the inlet opening or at a distance of at most 7.5 cm from the edge of the inlet opening in order to enable a simulation of an isokinetic extraction of the sample in the case of an oblique or swirled flow towards the inlet opening by the flowing aerosol, wherein the device comprises a carrier system with a movable bearing to be arranged within the flowing aerosol, wherein the sample collector with the inlet opening or with the intake piece with the inlet opening is held by the bearing.