Ventilation system for a mobile device, in particular a motor vehicle, comprising: - a variably adjustable filter apparatus, wherein the variably adjustable filter apparatus has one or more filter elements (31a, 31b) whose respective filtering performance is infinitely adjustable, and wherein the filter apparatus has at least one controllable variable filter characteristic relating to a class of pollutants; - a device (26, 28, 29) for acquiring information about the air quality in the surroundings of the mobile device and information about the air quality in an interior space of the mobile device, wherein the device for acquiring information about the air quality in the interior space of the mobile device comprises a further measuring device for measuring at least one indication of the air quality for a class of pollutants; - a control unit which is designed to vary the filter characteristic of the filter apparatus in a variable and infinitely adjustable fashion as a function of the information about the air quality in the surroundings, in such a way that an indication of relevant air quality for a class of pollutants is determined from the at least one indication of the air quality for the respective class of pollutants in the surroundings and the at least one indication of the air quality for the respective class of pollutants in the interior space as a function of a recirculation proportion, wherein the control unit is designed to vary the filter characteristic of the filter apparatus as a function of the indication of the relevant air quality, in such a way that a desired air quality in the interior space is achieved. - a variably adjustable filter apparatus, wherein the variably adjustable filter apparatus has one or more filter elements (31a, 31b) whose respective filtering performance is infinitely adjustable, and wherein the filter apparatus has at least one controllable variable filter characteristic relating to a class of pollutants; - a device (26, 28, 29) for acquiring information about the air quality in the surroundings of the mobile device and information about the air quality in an interior space of the mobile device, wherein the device for acquiring information about the air quality in the interior space of the mobile device comprises a further measuring device for measuring at least one indication of the air quality for a class of pollutants; - a control unit which is designed to vary the filter characteristic of the filter apparatus in a variable and infinitely adjustable fashion as a function of the information about the air quality in the surroundings, in such a way that an indication of relevant air quality for a class of pollutants is determined from the at least one indication of the air quality for the respective class of pollutants in the surroundings and the at least one indication of the air quality for the respective class of pollutants in the interior space as a function of a recirculation proportion, wherein the control unit is designed to vary the filter characteristic of the filter apparatus as a function of the indication of the relevant air quality, in such a way that a desired air quality in the interior space is achieved.