Sensor system for controlling ventilation systems in vehicles

Sensor system to control ventilation systems in vehicles in recirculation or air input mode dependent on the pollution levels in the air outside the vehicle, with a gas sensor element (52) the electrical resistance of which sinks in the presence of reducing gases and which rises in the presence of oxidizing gases, and an evaluation unit (51), the output of which is connected to the controller of the ventilation unit characterized by the fact that the sensor system is so organized that increase of a signal fed into the evaluation unit following an rise in the concentration of reducing gases is approximately quatitively equivalent to fall of the gas sensor signal fed into the evaluation unit (51) following a rise in the concentration of oxidizing gases, that the evaluation unit (51) calculates the rise or fall per time unit in the gas sensor signals which are fed into it and the evaluation unit (51) generates a switch signal to adjust the ventilation system to recirculation mode as soon as the measured increase or decrease in the gas sensor signal per time unit numerically exceeds a threshold limit.