A device for the detection of various pollutant contents in gas streams, preferably in the supply to motor vehicle cabins or protection cabins for operators, with use of one or several pollutant-detecting semiconductor gas sensors. A gas sensor (4) is alternately exposed to the gas streams to be evaluated. The sensor is placed at the separation point between the outer air and the cabin air, whereby, through switching which is effected within seconds, a practially constant comparison between the quality of the inner air and the outer air is performed. For this purpose, a switching arrangement is assigned to the gas sensor (4). The gas sensor (4) and the switching arrangement can be mounted on the fresh air/ambient air valve (3). The output signal of the gas sensor (4) serves for the control of the fresh air/ambient air valve (3).